Marine Natural Products from Flora and Fauna of the Western Australian Coast: Taxonomy, Isolation and Biological Activity

Marine natural products occurring along the Western Australian coastline are the focus of this review. Western Australia covers one-third of the Australian coast, from tropical waters in the far north of the state to cooler temperate and Antarctic waters in the south. Over 40 years of research has resulted in the identification of a number of different types of secondary metabolites including terpenoids, alkaloids, polyketides, fatty acid derivatives, peptides and arsenic-containing natural products. Many of these compounds have been reported to display a variety of bioactivities. A description of the compound classes and their associated bioactivities from marine organisms found along the Western Australian coastline is presented.


Introduction
Natural products have long played an important role both as direct agents and as molecular scaffolds providing inspiration for novel pharmaceuticals [1]. Notably, over 60% of all agents used currently in the treatment of cancer can be traced back to a natural product source [1]. Similarly, nearly 50% of all anti-bacterial agents and all anti-parasitic small molecules are either natural products or natural-product-derived compounds [2], highlighting the importance of natural product discovery as a source of many different pharmaceutical agents.
Historically, marine natural product research has lagged behind its terrestrial counterpart due to the inaccessibility of samples, as well as the absence of ethnobotanical knowledge in guiding the selection of taxa for investigation [3]. The development of the field in the 1950s coincided with the mass natural product screening campaigns conducted by the National Cancer Institute (NCI) in the United States, as well as the development of SCUBA (Self-Contained Under-water Breathing Apparatus) technology, and later remotely operated vehicles that allowed natural product chemists unprecedented access to unfamiliar benthic biomes. More recently, significant advances in the field have been propelled by developments in tools for identifying small molecules such as high-resolution mass spectrometry (HR-MS) coupled to high-performance and ultra-high-performance liquid chromatography, as well as advances in high-resolution nuclear magnetic resonance (NMR) spectroscopy [3]. The past decades have also seen significant pharmaceutical interest in the discovery of novel drug entities from marine sources [2,4].
Western Australia covers one-third of Australia's coast, from tropical waters in the far north of the state to cooler temperate and Antarctic waters in the state's south [5]. The state has a topographically diverse continental margin, with features of the continental shelf including coarse sediments in the south of the state around Point Hillier and Bald Island with large rocky banks in the central western region around Houtman Abrolhos and a deep continental shelf in the north of the state. The central continental shelf features a number of deep submarine canyons off Perth, Two Rocks and Kalbarri [5]. The sponge A Spongia sp. (Order: Dictyoceratida; Family: Spongiidae) collected from Exmouth gulf afforded the novel linear furanoditerpenes 12-hydroxy ambliofuran (19) and 12-acetoxyambliofuran (20) (Figure 2) [18]. Mosher's ester analysis revealed the compounds to be a scalemic mixture of 3:1, predominantly S configured enantiomers: the authors note that the isolation of enantiomers in non-racemic proportions is unusual in the field of marine natural products. Additional investigations of the sponge extract unearthed the new tetracyclic furanoditerpenes 21-25 and the linear furanosesterpene 26 bearing an epoxide at C-12, as well as the known compounds 27 and 28 ( Figure 2) [18].
The  A Spongia sp. (Order: Dictyoceratida; Family: Spongiidae) collected east of Gun Island, South Abrolohos Group, was the source of a new C-21 bisfuranoterpene bearing a tertiary hydroxyl at position C-8 as an unstable oil (12a) [14]. The structure of the natural product was subsequently revised to 12b following two-dimensional NMR analysis [15]. The former publication also reports the revised stereochemistry via the Horeau method of another bisfuranoterpene 13 isolated from a Leiosella sp. collected by dredge off Rottnest Island ( Figure 1) [14].
An investigation into the chemistry of a Clathria sp. (Order: Poecilosclerida; Family: Microcionidae) collected off the Great Australian Bight yielded the novel compounds clathrins A, B and C (35)(36)(37) (Figure 2). Clathrin A (35) is postulated to provide support for the biosynthetic origins of other marine meroterpenoids derived via a mixed shikimate-terpenoid biosynthetic pathway. Attempts to elucidate the stereochemistry of clathrin B (36) were thwarted by the facile atmospheric oxidation of 36 to compound 37; furthermore, the absolute configuration of 35 remains unresolved [20].  Chemical investigations of a Darwinella australensis (Order: Dendroceratida; Family: Darwinellidae) collected by SCUBA in the East Timor Sea afforded three new sesterpene sulfates, halisulfates 8-10 (49-51) (Figure 3) [25], isolated as their sodium salts. The relative configurations of the decalin moiety were elucidated using combined spectroscopic methods and via comparison to the known halisulfates 1-7. The relative configuration at C-13 remains unresolved. Halisulfates 9 (50) and 10 (51) exhibited inhibition of cell division of sea urchin eggs (Strongylocentrotus intermedius) in moderate concentration (IC 50 = 50 µg/mL and 35 µg/mL, respectively) [25].
Sarasinosides A 4 (52) and A 5 (53) (Figure 3) were isolated from a marine sponge Melophlus sarasinorum (Order: Tetractinellida; Family: Geodiidae) collected near Scott Reef, along with five known sarasinosides [26]. The compounds were elucidated on the basis of extensive nuclear magnetic resonance experiments and density functional theory calculations, as well as MALDI-TOF-MS and GC-MS analysis. The compounds isolated bear the same oligosaccharide moiety and differ only in the composition of the aglycone. Compound 52 is unusual in the composition of its bis-enol ether moiety [26].
Molecules 2023, 28  A sample of Stelletta sp. (Order: Tetractinellida; Family: Ancorinidae) collected by trawling operations in the Great Australian Bight was the source of the terpenyl-pyrrolizidine conjugates bistelletazines A-C (54)(55)(56) and the cyclic terpenyl-imidazole conjugate macrocycle bistelletazole A (57) (Figure 4) [27]. The authors note that despite extensive two-dimensional nuclear magnetic resonance experiments performed, the data acquired did not allow for the unambiguous assignment of stereochemistry for the pyrrolizidine portion of the molecule. The authors propose the compounds to share a convergent biosynthesis, the unique carbon scaffold arising from a presumed Diels-Alder reaction between two polyene sesquiterpene precursors [27]. A sample of Stelletta sp. (Order: Tetractinellida; Family: Ancorinidae) collected by trawling operations in the Great Australian Bight was the source of the terpenylpyrrolizidine conjugates bistelletazines A-C (54)(55)(56) and the cyclic terpenyl-imidazole conjugate macrocycle bistelletazole A (57) (Figure 4) [27]. The authors note that despite extensive two-dimensional nuclear magnetic resonance experiments performed, the data acquired did not allow for the unambiguous assignment of stereochemistry for the pyrrolizidine portion of the molecule. The authors propose the compounds to share a convergent biosynthesis, the unique carbon scaffold arising from a presumed Diels-Alder reaction between two polyene sesquiterpene precursors [27].
The ethanolic extracts of an Echinodictyum sp. (Order: Axinellida; Family: Raspailiidae) collected in the Great Australian Bight afforded four novel compounds, echinosulfone A (81a) and the echinosulfonic acids A-C (82a-84a) ( Figure 5) [33]. The proposed structures were assigned based on extensive two-dimensional NMR analysis. The compounds were found to account for the antibacterial activity of the crude extract but not the reported nematocidal activity. The structures of echinosulfone A and the echinosulfonic acids A-C were subsequently revised by three independent research groups contemporaneously to structures 81b-84b ( Figure 5), respectively, on the basis of synthetic efforts, as well as single-crystal X-ray diffraction and density functional theory analysis [34][35][36]. Subsequent bioassay-guided fractionation of the sponge extract unearthed the novel betaine alkaloids (−)-echinobetaine A (85) [37] and (+)-echinobetaine B (86) ( Figure 5) [38] as the principal nematocidal components responsible for the bioactivity of the sponge crude extract against the commercial livestock parasite Haemonchus contortus with echinobetaine B (86) exhibiting an LD 99 of 8.3 µg/mL. The structures of racemic echinobetaines A and B have also been confirmed via total synthesis [38].
Seven novel zwitterionic indole-2-carboxylic acids, trachycladindoles A-G (119-125) (Figure 7), were isolated from a Great Australian Bight sponge Trachycladus laevispirulifer (Order: Trachycladida; Family: Trachycladidae). Structures were elucidated based on comprehensive spectroscopic analysis. However, due to the paucity of material obtained, the relative configurations of trachycladindoles E (123) and F (124) remain unresolved; furthermore, the absolute configurations of 119-125 also remain unknown [47]. The authors postulate a biosynthetic scheme for the isolated trachycladindoles and related discodermindole family of alkaloids. Compounds 119-124 exhibited specific cytotoxicity against lung (A549), colorectal (HT29) and breast (MDAMB-231) cancer cell lines with GI 50 and TGI values revealing sub µM potency. In addition to this, preliminary structure-activity relationship studies performed on compounds 119-125 highlighted an unusual bioactive molecular motif in favour of N-10 and N-12 dimethylation, as evidenced by the reported activity of compounds 120 and 122-124 [47].
A Western Australian Axinella sp. (Order: Axinellida; Family: Axinellidae) collected in the gulf of Exmouth was the source of the compounds herbindoles A (126), B (127) and C (128) (Figure 7) [48]. The structures of 126 to 128 were determined spectroscopically. The authors postulate that the biogenesis of the compounds is unlikely derived from tryptophan given the lack of substitution at position C-3 of the indole core. Compounds 126-128 exhibited cytotoxic activity against KB cells with an MIC of 5 µg/mL, >10 µg/mL and 10 µg/mL, respectively, and the combined extract also possessed significant fish feeding deterrent properties [48]. Two samples of Trikentrion flabelliforme (Order: Axinellida; Family: Raspailiidae) collected near Port Hedland yielded the new alkaloids trikentramides A-D (129-132) [49]. The planar structures and relative configurations of 129 to 132 were determined spectroscopically via comparison to prior literature reports [49]. Further evidence for the structures assigned was provided by quantum mechanical modelling and simulation of 13 C NMR data as well as application of the DP4 algorithm pioneered by Goodman and co-workers [50]. Six new trikentrin-like natural products, (+)-trans-herbindole A (133) and trikentramides E-I (134-138) (Figure 7), have been recently reported from a sample of Trikentrion flabelliforme collected near Exmouth Gulf [51]. The relative and absolute configurations of 133 to 138 were determined by comparative analysis of optical rotation, computationally aided electronic circular dichroism spectroscopy (ECD) and chemical interconversion of the metabolites. The authors advance a plausible biosynthetic hypothesis for the formation of the trikentrin and herbindole classes of compounds beginning with the incorporation of a pyrrole-carboxylate thioester into a polyketide synthase. The authors also formulate an empirical mnemonic for the determination of the absolute stereochemistry of trikentrin and herbindole analogues dependant on the configuration of Me-C-8 [51]. Two new bromotyrosine alkaloids, pseudoceratinamides A (139) and B (140), as well as an artefact of extraction (141) and the enantiomer of a known compound (148), were isolated from a Pseudoceratina cf. verrucosa (Order: Verongida; Family: Pseudoceratinidae) collected off the Dampier Peninsula [52]. The sponge specimen also afforded the known compounds 142 to 147 ( Figure 8). The planar and relative configurations of the compounds were determined spectroscopically. Absolute configurations of all the compounds were determined using specific rotation and ECD measurements. The authors note that the original depiction of araplysin I (145) depicted the wrong absolute configuration, despite no work being conducted towards the absolute configuration of the molecule. Promulgation of this mistake throughout the literature means that at least some of the com- Two new bromotyrosine alkaloids, pseudoceratinamides A (139) and B (140), as well as an artefact of extraction (141) and the enantiomer of a known compound (148), were isolated from a Pseudoceratina cf. verrucosa (Order: Verongida; Family: Pseudoceratinidae) collected off the Dampier Peninsula [52]. The sponge specimen also afforded the known compounds 142 to 147 ( Figure 8). The planar and relative configurations of the compounds were determined spectroscopically. Absolute configurations of all the compounds were determined using specific rotation and ECD measurements. The authors note that the original depiction of araplysin I (145) depicted the wrong absolute configuration, despite no work being conducted towards the absolute configuration of the molecule. Promulgation of this mistake throughout the literature means that at least some of the compounds assigned in relation to araplysin I will have to be revised. More importantly, the authors note that the isolation of enantiomers of previously isolated compounds highlights the possibility of enantiodivergence in the biosynthesis of the bromotrosine spirooxazoline alkaloids at the epoxidative dearomatisation step [52]. All compounds isolated exhibited moderate activity against Staphylococcus aureus strains. Biological testing revealed that pseudoceratinamide A (139) and pseudoceratinamide B (140) exhibited significant activity (MIQ = 0.31 µg) against methicillin-sensitive S. aureus. Compounds 140, 141, 143-145 and 147 exhibited comparable activity to vancomycin (MIQ = 0.63 µg) against methicillin-resistant S. aureus [52]. authors note that the isolation of enantiomers of previously isolated compounds highlights the possibility of enantiodivergence in the biosynthesis of the bromotrosine spirooxazoline alkaloids at the epoxidative dearomatisation step [52]. All compounds isolated exhibited moderate activity against Staphylococcus aureus strains. Biological testing revealed that pseudoceratinamide A (139) and pseudoceratinamide B (140) exhibited significant activity (MIQ = 0.31 µg) against methicillin-sensitive S. aureus. Compounds 140, 141, 143-145 and 147 exhibited comparable activity to vancomycin (MIQ = 0.63 µg) against methicillin-resistant S. aureus [52]. A sample of Monanchora viridis (Order: Poecilosclerida; Family: Crambeidae) collected off Cape Mentelle in the southwest of the state yielded the known compound crambescidin 800 (149) (Figure 8). Compound 149 exhibited cytotoxic activity in a panel of breast cancer cell lines, with triple-negative breast cancer (TNBC) cells showing more significant differences in cell viability than immortalised fibroblasts. Additionally, 149 was shown to cause cell cycle arrest at G2/M phase in T11 and SUM159PT cells, as well as inhibit the phosphorylation of the Akt/mTOR, MAPK and NF-κB pathways, which are responsible for tumour relapse and metastasis [53].  A sample of Monanchora viridis (Order: Poecilosclerida; Family: Crambeidae) collected off Cape Mentelle in the southwest of the state yielded the known compound crambescidin 800 (149) (Figure 8). Compound 149 exhibited cytotoxic activity in a panel of breast cancer cell lines, with triple-negative breast cancer (TNBC) cells showing more significant differences in cell viability than immortalised fibroblasts. Additionally, 149 was shown to cause cell cycle arrest at G2/M phase in T11 and SUM159PT cells, as well as inhibit the phosphorylation of the Akt/mTOR, MAPK and NF-κB pathways, which are responsible for tumour relapse and metastasis [53].

Polyketides
Bioassay-guided fractionation of a Phorbas sp. (Order: Poecilosclerida; Family: Hymedismiidae) collected by hand using SCUBA near Muiron Island afforded the potent cytotoxins phorboxazoles A (150) and B (151), epimeric at position C-13, as pale yellow amorphous solids [7]. The planar structures of 150 and 151 were determined based on extensive COSY and HMBC experiments, and the relative configurations of all stereocentres on the macrolide hemisphere of the molecule were assigned with the aid of ROESY spectroscopy [7]. The authors note that assignment of the macrolide ring was facilitated by the conformational restrictions imposed by the three oxane rings and one oxazole ring present on the scaffold. Subsequent work established the relative configuration of the hemiketal ring system via synthesis of a model compound and the assignment of absolute configuration via Mosher's ester analysis [54]. Finally, the stereochemistry of methoxy C-43 was assigned by chemical conversion to dimethyl methoxysuccinate and comparison to an authentic sample of the R-enantiomer by chiral GC-MS [55] (Figure 9). Phorboxazoles A (150) and B (151) exhibited antifungal properties against Candida albicans, as well as inducing cell growth inhibition across a spectrum of cancer cells (leukemia, CCRF-CEM, GI 50 = 0.25 nM; HCT-116, GI 50 = 0.44 nM), and displayed extraordinary cytostatic activity (mean panel GI 50 < 7.9 pM) in the NCI 60 cancer cell line panel [7]. Re-examination of the same Phorbas sp. extracts using highly sensitive cryo-probe NMR experiments yielded two new chlorocyclopropane macrolides, phorbasides A (152) and B (153) (Figure 9) [56]. The assignment of absolute configuration was achieved via empirical comparison of ECD data obtained to that of synthesised model systems, taking advantage of the vibronic fine structure associated with an asymmetrically perturbed ene- Re-examination of the same Phorbas sp. extracts using highly sensitive cryo-probe NMR experiments yielded two new chlorocyclopropane macrolides, phorbasides A (152) and B (153) (Figure 9) [56]. The assignment of absolute configuration was achieved via empirical comparison of ECD data obtained to that of synthesised model systems, taking advantage of the vibronic fine structure associated with an asymmetrically perturbed ene-yne chromophore [56]. Subsequent work afforded phorbasides C-E (154-156) [57], the highly chlorinated muironolide A (157a) [58], differing in the absolute configuration of the chloro-cyclopropane ring, along with the nitrile-bearing hemi-phorboxazole A (158) [59], and most recently, phorbaside F (159) [60] and phorbasides G-I (160-162) (Figure 9) [61]. The structure of muironolide A was subsequently revised to 157b following total synthesis [62]. Biological evaluation of compounds 152-157 revealed modest cytotoxicity exhibited by the metabolites towards colon tumour cells (HCT-116; IC 50 = 2-30 µM) with phorbaside C (154) exhibiting the most potent cytotoxic activity (IC 50 = 2 µM).
Bioassay-guided fractionation of a Raspailia (raspalia) sp. (Order: Axinellida; Family: Raspailiidae) collected by trawl on the northern Rottnest Shelf afforded the known compounds phorboxazoles A and B (150, 151) ( Figure 9) as the principal nematocidal agents, as well as the known synthetic compound esmodil (163), isolated for the first time as a natural product [63]. The structure of 163 ( Figure 10) was confirmed spectroscopically and via total synthesis. Biological testing revealed that 150 and 151 exhibited nematocidal activity against Haemonchus contortus (LD 99 = 0.5 mg/mL and 1.1 mg/mL, respectively) [63].  (173) and (Z,E)-14,14-dibromo-4,6,13-tetradecatrienoate (174). Compound 174 was characterised as its methyl ester (174a) (Figure 11), and additional fractionation afforded the known ene-amide side chain [8]. Additional work has analysed the spatiotemporal distribution of the metabolites across species of Haliclona collected across the southwest of the state [64]. Compound 164 exhibited highly potent and specific cytotoxicity (mean panel GI 50 = 15 nM) in the NCI 60 cell line human tumour screen. COMPARE pattern-recognition analysis revealed no significant correlations to the profiles of other known antitumour compounds, suggesting that the salicylihalamides represented a potentially important new class of compounds for antitumour lead optimisation [8]. Subsequent work determined the unprecedented mechanism of action of 164 and 165 via Vacuolar-ATPase inhibition [9].
A collection of two Amphimedon species (Order: Haplosclerida; Family: Niphatidae) collected during trawling operations in the Great Australian Bight afforded the novel macro-bicyclic lactones/lactams amphilactams A-D (166-169) ( Figure 10) [65]. The planar structures of 166 to 169 were elucidated on the basis of extensive spectroscopic evidence and comparison to synthetic model compounds. The relative and absolute configurations of 166 to 169 remain unknown. Compounds 166 to 169 were isolated in sufficient amounts to quantify their in vitro LD 99 activities against Haemonchus contortus as 7.5 µg/mL, 47 µg/mL, 8.5 µg/mL and 0.39 µg/mL, respectively [65].
Bioassay-guided fractionation of a Geodia sp. (Order: Tetractinellida; Family: Geodiidae) collected in the Great Australian Bight yielded a new macrocyclic polyketide lactam tetramic acid, as a magnesium salt 170 (Figure 10), as the sole agent responsible for the in vitro nematocidal activity of the extract [66]. The structure of geodin A (170) was determined spectroscopically. The magnesium content of the sample was determined by energy-dispersive spectroscopy and atomic absorption spectroscopy allowing the authors to deduce the presence of one unit of magnesium for every two units of tetramic acid [66]. Geodin A (170) exhibited potent in vitro nematocidal activity (LD 99 = 1.0 µg/mL) [66].
Bioassay-guided fractionation of an Oceanapia sp. (Order: Haplosclerida; Family: Phloeodictyidae) collected off the northern Rottnest Shelf afforded the novel dithiocyanates thiocyanatins A, B and C (176-178) ( Figure 11) [69]. The structures of 176 to 178 were elucidated spectroscopically and confirmed in a seven-to-eight-step total synthesis starting from 8-bromooctanoic acid [69]. Re-analysis of the ethanolic sponge extract afforded thiocyanatins D 1 and D 2 (177, 180) as an inseparable mixture and thiocyanatins E 1 and E 2 (181, 182), also as an inseparable mixture, as well as a number of analogues tentatively identified by 1 H NMR and LC-ESIMS [70]. The structures of the novel metabolites were elucidated with respect to the prior compounds 176 to 178 and via comparison to synthetic model compounds. The thiocyanatins exhibited potent nematocidal activity, and preliminary structure-activity relationship investigations confirmed the key characteristics of the thiocyanatin pharmacophore. Thiocyanatin A (176) exhibited potent nematocidal activity (LD 99 = 1.3 µg/mL) against Haemonchus contortus [69]. An Oceanapia sp. collected by dredge off Scott Reef afforded the hybrid α,ω-bifunctionalised sphingoid tetrahydroisoquinoline β-glycoside oceanalin A (183), as well as the known compound rhizochalin (184) (Figure 12) [71]. The structure of oceanalin A was elucidated on the basis of 1 H NMR and 13 C NMR spectroscopy as well as chemical derivatisation. The authors conclude that, given the absence of optical rotation for the cleaved eastern hemisphere of the molecule, as well as the propensity of tetrahydroisoquinoline compounds to epimerise, compound 183 is likely a 1:1 mixture of epimers at the C-26 position [71]. A number of new cerebrosides of which two representative examples are depicted (185, 186) were isolated from the same Oceanapia sp. collected off Scott Reef [72]. The cerebrosides were isolated as inseparable mixtures of compounds and assigned by NMR spectroscopy, MALDI-MS, chemical derivatisation and GC-MS [72]. Also from the An Oceanapia sp. collected by dredge off Scott Reef afforded the hybrid α,ω-bifunctionalised sphingoid tetrahydroisoquinoline β-glycoside oceanalin A (183), as well as the known compound rhizochalin (184) (Figure 12) [71]. The structure of oceanalin A was elucidated on the basis of 1 H NMR and 13 C NMR spectroscopy as well as chemical derivatisation. The authors conclude that, given the absence of optical rotation for the cleaved eastern hemisphere of the molecule, as well as the propensity of tetrahydroisoquinoline compounds to epimerise, compound 183 is likely a 1:1 mixture of epimers at the C-26 position [71]. A number of new cerebrosides of which two representative examples are depicted (185, 186) were isolated from the same Oceanapia sp. collected off Scott Reef [72]. The cerebrosides were isolated as inseparable mixtures of compounds and assigned by NMR spectroscopy, MALDI-MS, chemical derivatisation and GC-MS [72]. Also from the same collection of Oceanapia sp. was isolated a ceramide fraction characterised by methanolysis and GC-MS analysis [73]. Most recently, the same collection of Oceania sp. afforded the new bolaampiphilic sphingoid bases rhizochalin B (187) and rhizochalinin B (188) (Figure 12) characterised by NMR spectroscopy as their peracetates [74]. The compounds contain an unusual butoxy group, which the authors note is uncommon in natural products. The ethanolic Oceanapia sp. extract exhibited antimicrobial activity against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Candida albicans and cytotoxic properties against the Erlich murine carcinoma. Metabolite 183 exhibited antifungal activity against Candida glabrata with an MIC of 30 µg/mL [71].
Molecules 2023, 28, x FOR PEER REVIEW 22 of 46 same collection of Oceanapia sp. was isolated a ceramide fraction characterised by methanolysis and GC-MS analysis [73]. Most recently, the same collection of Oceania sp. afforded the new bolaampiphilic sphingoid bases rhizochalin B (187) and rhizochalinin B (188) (Figure 12) characterised by NMR spectroscopy as their peracetates [74]. The compounds contain an unusual butoxy group, which the authors note is uncommon in natural products. The ethanolic Oceanapia sp. extract exhibited antimicrobial activity against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Candida albicans and cytotoxic properties against the Erlich murine carcinoma. Metabolite 183 exhibited antifungal activity against Candida glabrata with an MIC of 30 µg/mL [71].  (Figure 14) for the first time as a natural product. The identity of the metabolite was verified spectroscopically and via total synthesis [78]. Metabolite 199 exhibited potential as a bronchodilator [78].
The known nucleoside spongosine (200), previously reported by Bergmann et al. from Cryptotethya crypta and 2′-deoxyspongosine (201) (Figure 14), previously only reported as a synthetic, was isolated from a sponge of the Order Hadromerida (Tethyidae) collected by hand from Exmouth Gulf [79]. 1 H and 13 C NMR spectral data for both compounds were reported for the first time.  (Figure 14) for the first time as a natural product. The identity of the metabolite was verified spectroscopically and via total synthesis [78]. Metabolite 199 exhibited potential as a bronchodilator [78]. determined to be more than 150 mg/kg [87]. Additional investigations revealed that 214-216 exhibited significant cell division inhibition for the fertilised eggs of the sea urchin Strongylocentrotus intermedius with an IC50 = 5.7 µM, 4.8 µM and 1.1 µM, respectively [88].

Cnidaria
A sample of Ctenocella pectinata (Order: Alcyonacea; Family: Ellisellidae), collected in Exmouth Bay, afforded three new sterols, pectinoacetals A-C (221-223) (Figure 15), isolated as their monoacetylated derivatives [89]. The underivatised natural products were found to undergo rapid interconversion of the C-18 hemiacetal chiral centre. The structures of the natural products were elucidated spectroscopically and by chemical derivati-  (Figure 14), previously only reported as a synthetic, was isolated from a sponge of the Order Hadromerida (Tethyidae) collected by hand from Exmouth Gulf [79]. 1 H and 13 C NMR spectral data for both compounds were reported for the first time.
A Dysidea dendyi (Order: Dictyoceratida; Family: Dysideidae) collected by hand from Scott Reef at a depth of 3 m afforded two new tetrabromodibenzo-p-dioxins: spongiadioxins A and B (214, 215) ( Figure 14) [87]. The structures of 214 and 215 were elucidated using a combination of 1D and 2D NMR spectroscopy. Additionally, the structure of 214 was verified by single-crystal X-ray diffraction of a methyl ether obtained from 214. The structure of 215 was secured by chemical interconversion [87]. Re-investigation of the lipophilic sponge extracts afforded spongiadioxin C (216) and its methyl ether 217 as well as the related diphenyl ethers 218-220 [88]. The structures of the metabolites were determined spectroscopically and via semisynthesis. Biological evaluation revealed that compounds 214 and 215 exhibited cytotoxic activity against mouse Ehrlich carcinoma cells (ED 50 = 29 and 15.5 µg/mL, respectively). In contrast, the LD 50 for 214 and 215 in mice was determined to be more than 150 mg/kg [87]. Additional investigations revealed that 214-216 exhibited significant cell division inhibition for the fertilised eggs of the sea urchin Strongylocentrotus intermedius with an IC 50 = 5.7 µM, 4.8 µM and 1.1 µM, respectively [88].

Cnidaria
A sample of Ctenocella pectinata (Order: Alcyonacea; Family: Ellisellidae), collected in Exmouth Bay, afforded three new sterols, pectinoacetals A-C (221-223) (Figure 15), isolated as their monoacetylated derivatives [89]. The underivatised natural products were found to undergo rapid interconversion of the C-18 hemiacetal chiral centre. The structures of the natural products were elucidated spectroscopically and by chemical derivatisation. The relative configuration of the stereocentre at C-16 could not be assigned conclusively by NOE spectroscopy [89].
Bioassay-guided fractionation of a rare Alcyonacean soft coral Eleuthorobia sp. (Order: Alcyonacea; Family: Alcyoniidae), found near Bennett's Shoal, yielded the new diterpene glycoside eleutherobin (224) (Figure 15) [10]. The structure of 224 was assigned spectroscopically. Eleutherobin (224) exhibited significant specific cytotoxicity against a diverse panel of breast, renal, ovarian and lung cancer cell lines with an IC 50 range of 10-15 nM. Compound 224 was found to stabilize microtubules by competing for the paclitaxel binding site on the microtubule polymer [10].

Tunicata
Investigations of an ascidian collected in the Abrolhos Group afforded the deep blue tetra-pyrrole pigment 245 (Figure 16) [94]. The compound had previously been isolated from mutant strains of the Gram-negative bacterium Serratia marcescens, the structure of the pigment having been confirmed by total synthesis [95]. Microanalysis revealed that the ascidian pigment contained both chloride and bromide counter anions. Compound 245 exhibited an ability to increase the contractile force of guinea-pig ilea, with a dose-dependent increase evident [94].
A new dimeric disulfide alkaloid, polycarpine (250) (Figure 16), was isolated as its dihydrochloride salt from the extracts of the ascidian Polycarpa clavata (Order: Stolidobranchia; Family: Styelidae) [97]. Purification of the metabolite on silica afforded the free base 250a which readily decomposed to the monomeric products 251 to 253, arising from nucleophilic addition of water or methanol to position C-5 of the imidazole ring followed by cleavage of the disulfide bond [97]. Dissection of the organism into anatomical parts and fresh extraction in MeOH followed by immediate acquisition of NMR spectra demonstrated that compound 249 was the sole natural product and that it was located entirely in the organism's branchial sac. Polycarpine dihydrochloride (250) exhibited cytotoxic activity against the human colon tumour cell line HCT-116 at 0.9 µg/mL [97].
An undescribed ascidian Didemnum sp. (Order: Aplousobranchia; Family: Didemnidae), collected near Ningaloo Reef, yielded four new aromatic alkaloids ningalins A-D (260-263) (Figure 17), assigned structures 260 to 263 on the basis of spectroscopic analysis [100]. The authors propose a biosynthetic route to compounds 260 to 263 deriving from repeat condensation of DOPA. The absence of optical rotation in compounds 262 and 264 indicated that they are likely present as mixtures of racemates arising from rotamerism and helicity of the scaffolds [100].
Fractionation of an ascidian Aplidiopsis sp. (Order: Enterogona; Family: Polyclinidae) collected near Ningaloo Reef afforded the zwitterionic hydroxyadenine aplydiamine (264) (Figure 17). The structure of 264 was elucidated spectroscopically and by chemical derivatisation. The assignment of 264 as a zwitterion was based on HMBC correlations in d 6 -DMSO and the observed NOE correlation between all three exchangeable protons [101].
Two new cytotoxic macrolides, lobatamides A and B (265 and 266) (Figure 17), structurally related to the salicylihalamide class of macrolides, were isolated following bioassayguided fractionation of an Aplidium lobatum (Order: Aplousobranchia; Family: Polyclinidae) [102]. Critical evidence for the structures of 265 and 266 was provided by analysis of the FAB-MS data. Re-investigation of the A. lobatum afforded the lobatamides C-F (267-270), the structures of which were elucidated spectroscopically [103]. The authors report the isolation of compounds 265 to 270 from three different shallow-water collections of Australian A. lobatum, an Aplidium sp. collected during a trawling expedition at the Great Australian Bight and finally from an unidentified, shallow-water collection of a Philippine tunicate. The authors note the spectral similarities between the lobatamides A-D (265-268) and the aplidites A-D, isolated from a Great Australian Bight Aplidum sp. [104], and propose revising the structures of the latter compounds to structures 265-268, respectively. The authors also propose revising the structures of the related aplidites E-G to structures 271 to 273 and renaming the natural products lobatamides G-I, respectively [103]. Given the reported isolation of lobatamide A from a species of terrestrial pseudomonad and the isolation of the related salicylihalamide macrolides from a marine sponge, the authors postulate a likely microbial origin for the family of compounds [103]. The relative and absolute configuration of lobatamide C (267) was subsequently confirmed following total synthesis of 267 by the Porco group [105]. Biological testing revealed that the lobatamides A-D (265-268) exhibited approximately equipotent specific cytotoxicity in the NCI 60 cell line human tumour screen (mean panel GI 50 s~1 .6 nM). COMPARE pattern-recognition analysis revealed no significant correlations to the profiles of other known antitumour compounds, suggesting that compounds 265-268 may act by a novel mechanism of action. The differential cytotoxicity profiles of the compounds 265-268 did, however, show high (≥0.7) COMPARE correlations among themselves, as well as with the salicylihalamides A and B (164, 165) isolated from the marine sponge Haliclona sp. The authors remark that the result is not surprising, given the structural similarities between the two compound families [103].

Plantae
A new cleistanthene diterpene hydrocarbon (281) was isolated from the leaves of Amphibolis antartica (Order: Alismatales; Family: Cymodoceaceae) collected from Shark Bay [107]. The structure of 281 was assigned spectroscopically. Chemical instability of the compound prevented degradative analysis. Samples of A. antartica collected near Perth contained 281, as well as the known derivatives sandaracopimaradiene (282) and isopunaradiene (283) (Figure 18), identified by GC-MS analysis. Analysis of individual specimens collected from Shark Bay by GC-MS revealed that the n-hydrocarbon content diminishes with maturity of the specimen, whereas concentration of 281 increases with leaf age [107].

Ochrophyta
Chromatography of the CH 2 Cl 2 extracts of the brown algae Cystophora sp. (Order: Fucales; Family: Sargassaceae) collected from the wave-swept rock platforms of Cosy Corner, southwest WA, afforded three new isoprenoid dihydroquinones derived from geranyltoluquinol. The structures of the compounds were deduced as 284 to 286 ( Figure 18) by 1 H and 13 C NMR spectroscopy and chemical interconversion [108]. Earlier isolation attempts had led to isolation of benzoquinone 287. The authors conclude that 287 is not a genuine natural product as acetylation of the Cystophora sp. crude extract led to the isolation of a diacetylated derivative of 284 and the observation that benzoquinone 287 was not present [108].

Rhodophyta
A sample of Laurencia filiformis (Order: Ceramiales; Family: Rhodomelaceae) collected from Point Peron yielded the sesquiterpene metabolites aplysisistatin (297), previously isolated from the sea hare, Aplysia angasi, as well as 6β-hydroxyaplysistatin (298) (Figure 19) [113]. The structures of 297 and 298 were assigned crystallographically. A chance observation led to the discovery that thermal rearrangement of 6β-hydroxyaplysistatin (298) afforded one major decomposition product 299 involving the formal loss of one unit of HBr and two units of water. The structure of the thermolysis product 299 was confirmed by Capon and Ghisalberti in a five-step total synthesis [114].
Chemical investigation of the red alga Vidalia spiralis (Order: Ceramiales; Family: Rhodomelaceae) collected at Yanchep yielded the new halogenated diol 3,4-dibromo-5methylenecyclopent-3-ene-1,2-diol (300) (Figure 19) as a fine crystalline powder [115]. The structure of 300 was determined spectroscopically and by chemical derivatisation. Attempts to monoacetylate the diol failed, precluding the use of Horeau's method to determine the absolute configuration of the natural product. The Vidalia spiralis crude dichloromethane extract exhibited hypotensive activity, and the crude methanol extract exhibited stimulant activity. Neither of these activities was evident, however, in the purified compound [115].
A sample of Caulerpa trifaria (Order: Bryopsidales; Family: Caulerpaceae) collected at Point Peron afforded the new sesquiterpene metabolite 316, the structure of which was deduced spectroscopically. The absolute configuration of the compound remains unknown. Samples of C. brownii, C. pexilis, C. peltata and C. racemosa also collected from Point Peron failed to yield 316. However C. peltata and C. racemosa afforded caulerpin (317) in low yield as red-plate crystals [120].

Dinoflagelatta
Capillary GC-MS analysis of four closely related species of marine dinoflagellate identified dinosterol (320) (Figure 20) as the major sterol constituent of Prorocentrum balticum (Order: Prorocentrales; Family: Prorocentraceae) and Prorocentrum minimum [122]. Cholesterol (321) (Figure 20) was found to be the major constituent of Prorocentrum micans and Prorocentrum mexicanum [122]. Other steroid components were identified and annotated by GC-MS for all four species. The authors propose that the similarity of steroidal

Dinoflagelatta
Capillary GC-MS analysis of four closely related species of marine dinoflagellate identified dinosterol (320) (Figure 20) as the major sterol constituent of Prorocentrum balticum (Order: Prorocentrales; Family: Prorocentraceae) and Prorocentrum minimum [122]. Cholesterol (321) (Figure 20) was found to be the major constituent of Prorocentrum micans and Prorocentrum mexicanum [122]. Other steroid components were identified and annotated by GC-MS for all four species. The authors propose that the similarity of steroidal fractions from members of the same species grown in different laboratories suggests a strong genetic, rather than environmental, influence on the steroidal composition of such species and that the steroidal profiles reported may be used to delineate the species chemotaxonomically [122].
Recently, cultivation of a marine-derived Aspergillus noonimiae collected in waters near Perth afforded the indolic diterpenes noonindoles A-F (331-336) ( Figure 20) as well as a number of minor metabolites putatively assigned via tandem MS analysis. Structures of the major compounds were assigned following detailed spectroscopic analysis and single-crystal X-ray diffraction. Testing of the metabolites against a panel of microorganisms revealed that the compounds were essentially devoid of biological activity, with the exception of mild antifungal activity displayed by 331 against Candida albicans [124].

Arsenic Metabolism in the Marine Food Web
Vapour generation atomic absorption spectrometry guided fractionation of the commercially important western rock lobster, Panulirus cygnus (George) (Order: Decapoda; Family: Paluniridae), afforded arsenobetaine (337) (Figure 21) as the principal arseniccontaining metabolic constituent [125]. The structure of 337 was elucidated crystallographically and confirmed by total synthesis [125].

Conclusions
Here we have reviewed the marine natural products that have been reported from the fauna and flora of Western Australian waters. This review describes the identification of over 350 metabolites representing a diverse array of chemical compounds that have been reported over the past 40 years. Most of the compounds have also been reported to display some biological activity in line with the high rates of bioactivity studies of marine natural products reported elsewhere [132,133].
A statistical analysis of the distribution of marine natural products from various taxonomic sources and the percentage of each structural class among all the marine natural products reported from Western Australia ( Figure 22) reveals that studies on Porifera have by far yielded the largest and most diverse number of compounds with 220 metabolites reported, from all six arbitrary biogenetic groupings. In general terms, it is likely that the quantity and relative percentage of compound classes isolated are, in part, a reflection of the biosynthetic potential of the organisms (and associated microbiota) under investigation and partly attributable to the interests of the lead researchers involved, as well as the chromatographic and analytical technology available at the time. The former observation is likely true of work conducted on Echinoderms, where all the metabolites reported from Western Australian species are polyketidic anthraquinones, a biogenic grouping known to be of chemotaxonomic relevance to the phylum [134]. The latter observation is afforded some support when analysing the extensive proportion of terpenoids isolated from Rhodophytes and Ochrophytes, work that was overwhelmingly conducted in the early 1980s by the Ghisalberti group, using predominately normal-phase column chromatography, compatible with the typically lipophilic metabolites reported. Subsequent research on alternative taxa, conducted from the 1990s to present, shows a trend towards compounds of increasingly varied biosynthetic provenance, including a higher proportion of alkaloids and polyketides, evident when analysing distributions of isolated metabolites from Porifera, Tunicata and Fungi. This trend can be explained when considering the proliferation of high-and ultra-high-performance liquid chromatography instruments, in analytical and preparative modes, as well as the propensity of researchers to operate under typically reversed-phase conditions, facilitating the purification and analysis of increasingly polar metabolites. While there has been extensive research into marine natural products originating from other major marine biodiversity hotspots, such as the Americas, Southeast Asia, Japan, Eastern and Southern Australia and New Zealand, there have been relatively few major studies of marine natural products from Western Australia. This is the largest coast- While there has been extensive research into marine natural products originating from other major marine biodiversity hotspots, such as the Americas, Southeast Asia, Japan, Eastern and Southern Australia and New Zealand, there have been relatively few major studies of marine natural products from Western Australia. This is the largest coastline of Australia, and biodiversity studies suggest that Western Australia marine areas are a source of significant biodiversity with many of the species remaining largely uncharacterised and underexplored [6,[135][136][137]. In recent times, there have been a number of biodiversity expeditions to explore the species richness of the coastline, but chemical studies of these have so far been lacking. These recent expeditions have subsequently led to the advent of the Western Australian Marine Science Library (WAMBL), where collected specimens have been deposited for future genetic, biological and chemical analysis. The WAMBL provides researchers with easier access to specimens that were previously difficult to obtain, such as deep-sea marine sponges (>100 m). The variety of unclassified species within the WAMBL makes it of high interest for chemical and biological studies such as those we have started recently [34,53,67,75].
In an age of superbugs and viral pandemics, the need for discovering new antiinfective agents is paramount [138,139], and marine natural products are well known as a significant source of biologically active compounds [140]. To that end, the relatively underexplored chemical diversity of species occurring along the Western Australian coastline may offer many more opportunities in this area.