Secondary Metabolites from the Marine Sponges of the Genus Petrosia: A Literature Review of 43 Years of Research

Sponges are prolific sources of various natural products that have provided the chemical scaffolds for new drugs. The sponges of the genus Petrosia inhabit various regions and contain a variety of biologically active natural products such as polyacetylenes, sterols, meroterpenoids, and alkaloids. This review aims to provide a comprehensive summary of the chemical structures and biological activities of Petrosia metabolites covering a period of more than four decades (between 1978 and 2020). It is also described in this review that the major groups of metabolites from members of the genus Petrosia differed with latitude. The polyacetylenes were identified to be the most predominant metabolites in Petrosia sponges in temperate regions, while tropical Petrosia species were sources of a greater variety of metabolites, such as meroterpenoids, sterols, polyacetylenes, and alkaloids.


Introduction
The oceans represent the largest habitat on earth and contain organisms with high biological and chemical diversity. The secondary metabolites of diverse marine organisms allowed these species to develop mechanisms that would enable them to survive in the ocean. Over 30,000 natural products have been isolated from marine organisms [1], and 1490 and 1554 new compounds were discovered recently as 2017 and 2018, respectively [2,3].
Marine invertebrates, such as sponges, cnidarians, bryozoans, echinoderms, and tunicates, exclusively inhabit aquatic environments [4]. They are either sessile or slow moving, have soft bodies, and lack morphological defense structures such as shells or thorns [5]. Therefore, it is not incidental that these organisms evolved chemical defense mechanisms against predation and overgrowth of fouling organisms [6,7]. The abundance and diversity of secondary metabolites synthesized for chemical defense provide an opportunity to examine the chemical entities with potential therapeutic applications.
Marine sponges (Porifera) have been the central focus of the research for the discovery of biologically-active secondary metabolites. As sponges are highly effective filter feeders, microorganisms in the surrounding water are actively swirled by the sponge-driven currents. While some of the microorganisms are immediately digested, others are retained within the sponge body. It was reported that associated microorganisms can account for up to 60% of the fresh weight of marine sponges, and are more diverse than can be estimated using current technology [8,9]. It is believed that these sponge-associated microorganisms such as bacteria, fungi, cyanobacteria, and unicellular algae may be involved in the biosynthesis of natural products that are isolated from the sponges, although the relationship between sponge and associated microorganisms are highly complex [10].
Several statistical studies on marine natural products have revealed that sponges are outstanding in number of isolated biologically-active compounds. Hu et al. have analyzed the temporal trend, chemical structure distribution, bioactivity groups, and species distribution of biologically-active compounds from marine organisms discovered during the 28 years from 1985 to 2012 [11]. In this study, the authors found out that approximately 75% of the compounds were isolated from marine invertebrates, and the highest proportion of bioactive compounds had been isolated from sponges. Another report, which covers 9812 marine natural products isolated from invertebrates discovered during 1990-2009, also showed that the highest proportion of metabolites could be traced to sponges (48.8%) [12].
It is generally believed that there is a higher chance of discovering biologically-active sponge metabolites in habitats characterized by intense competition for survival due to their high biological diversity and density, such as coral reefs in tropical regions [13,14]. Following the principle that sponge chemical defense is mainly driven by predation pressure [15], it was hypothesized that chemical diversity is higher in the tropical region than the temperate region. This hypothesis is only partly supported, as it was reported that tropical and temperate benthic systems differ with regard to dominant consumption regimes. While fishes are dominant predators on reefs in tropical regions, temperate systems are characterized by mesopredators such as crustaceans, sea urchins, and gastropods [16,17].
Geographical location is clearly one of the most influential factors related to the variation in sponge metabolites; however, few studies have investigated the geographical variation in sponge metabolites in terms of yield or biological activity. The first study regarding this issue claimed the inverse relationship between latitude and toxicity in sponges [18]. The authors examined the toxicity of 78 sponge species collected in temperate (San Juan Island and Santa Catalina Island) and tropical regions (La Blanquilla reef and Zihuatanejo Bay) on fishes by measuring the time from exposure to death. They found that 9% of the sponge species in the regions of 48 degrees north were toxic and 75% of the sponges studied in the region of 19 degrees north were toxic. A study by Ruzicka et al. also demonstrated that sponge species on the temperate reefs in the South Atlantic Bight are a smaller chemical deterrent to fish predators than their counterparts at lower latitudes in the Florida Keys by measuring the palatability of sponge crude extracts quantitatively and comparing the results with those obtained by Pawlik et al. [14,19].
Results that are contradictory to those above have also been reported. A feeding assay with 17 pairwise comparisons conducted by Burns et al. showed no significant differences in deterrence between Red Sea and Caribbean sponges [20]. The field-based feeding experiments with diverse fish predators conducted by Becerro et al. showed similar results [21]. When 20 sponge species from tropical Guam and temperate Northeast Spanish coasts were compared, no significant difference was observed based on habitat. It is worthwhile to mention, however, that the percentage yield of crude organic extracts tends to be proportional to latitude, although the authors did not provide any comment on this. When a total of 20 sponge species in 10 species pairs were extracted, the tropical sponges were found to provide up to a 3.4 times higher yield than their temperate counterparts in nine pairs.
More recent studies focused on the intra-specific variability in secondary metabolites in terms of concentration. One such study examined Aplysina aerophoba, a common Mediterranean sponge known to contain large concentrations of brominated alkaloids [22]. When the concentrations of bromotyrosine alkaloids such as aerophobin-1, aerophobin-2, aplysinamisin-1, and isofistularin-3 were evaluated using HPLC according to the geographic location of its habitat, significant variation was observed at the largest (Canary Islands and Mediterranean, over 2500 km) and the smallest (two sites less than 500 m apart) geographic scales. Another study evaluating the variability in metabolites of Stylissa massa came to a similar conclusion [23]. The concentrations of the bromopyrrole alkaloidse.g., hymenidine analogs, sceptrin, and oroidin-were measured using LC-MS analysis. Concentrations varied geographically across the Pacific basin, with American Samoa and Pohnpei exhibiting the greatest differences, and Guam and Saipan being the most similar to each other.
A very recent metabolomics study revealed the relationship between sponge metabolites and the geographic location. Total 139 specimens of Xestospongia spp. were collected in four locations-Martinique, Curacao, Taiwan, and Tanzania-and the extracts were analyzed by 1 H NMR and LC-MS, followed by statistical analysis (OPLS-DA); the collected samples were clearly grouped according to their location [24].
Experimental evidence supports this geographical trend of the sponge metabolites, although it is scattered throughout the literature. This review aims to compile this information and determine variations in metabolites according to geographical location using the genus Petrosia. Petrosia is one of the four genera (Acanthostrongylophora, Neopetrosia, Petrosia, Xestospongia) of the family Petrosiidae belonging to the order Haplosclerida, which is known as the most prolific source of secondary metabolites among sponges [12]. As of 2020, Petrosia genus includes 122 species belonging to two subgenera of Petrosia and Strongylophora, according to the Word Register of Marine Species (WoRMS). They are widely distributed throughout tropical and temperate waters, from intertidal zones to deep waters.
This review collects and compares information from peer-reviewed articles on secondary metabolites isolated from the sponges of the genus Petrosia. The articles were retrieved from the following databases: PubMed, Chemical Abstracts ® , ISI Web of Knowledge, Google Scholar. Based on the assumption that the latitude of sponge habitat plays an important role in the metabolite, the regions were divided into polar (above the Arctic Circle (66.5 • N) and below the Antarctic Circle (66.5 • S)), temperate (between the Tropic of Cancer (23.5 • N) and the Arctic Circle and between the Tropic of Capricorn (23.5 • S) and the Antarctic Circle), and tropical area (between the Tropic of Cancer and the Tropic of Capricorn). Mixtures of high-molecular weight polyacetylenes with 46-55 carbons were isolated from both the sponge P. ficiformis and its predator nudibranch Peltodoris atromaculata [25] in the Mediterranean Sea. These were the first reported polyacetylenes from Petrosia sponges, although some of the structural features of these-e.g., the aliphatic carbon chain length between the characteristic functional groups and stereochemistry-were not revealed. Five years later, the same research group isolated small amounts of two additional polyacetylenes, but their structural assignment remained incomplete [26]. Mixtures of five additional polyacetylenes up to 52 carbons long, isolated from P. ficiformis found in dark caves in the Mediterranean Sea, were also reported [27].
Isolation of polyacetylenes from Petrosia sp. inhabiting temperate regions of Japan was initiated from the discovery of the C30 polyacetylenes, petrosynol (26) and petrosynone (27) (Figure 2) [33,34]. In the case of petrosynol (26), the stereochemistry of hydroxymethine was revealed by measuring the Cotton effect of benzylated derivatives. This was the first report to completely elucidate the structure of polyacetylenes, including stereochemistry. Four more petrosynol derivatives (28)(29)(30)(31) were reported by another research group [35]. These compounds inhibited the cell division of fertilized ascidian (Styela partita) eggs with IC 50 values of 5.0-30.0 µg/mL and displayed toxicity in the brine shrimp lethality bioassay with LC 50 values from 0.1-30.0 µg/mL. Petroacetylene (32), a tetraketone derivative of 29 and 30, was also reported by a different research group [36]. The petroformyne analogs were also isolated from a Japanese Petrosia sponge; petroformynes 1 and 4 (1, 4) [37], in addition to the novel derivatives, neopetroformynes (33)(34)(35)(36), were isolated [38]. The functional groups were identified based on NMR spectroscopy data including those from 2D NMR experiments such as COSY, HMBC, and TOCSY. The length of the alkyl chains was determined by analyzing FAB-MS/MS data and stereochemistry, except for that of neopetroformyne D (36), was revealed by the modified Mosher method. In this report, the authors suggested that neopetroformyne A (33) and petroformyne 1 (1) might share the same structures, as their NMR data were indistinguishable. The structure of 1, however, could not be reexamined by FAB-MS/MS due to a lack of remaining material. Neopetroformynes A-D (33-36) exhibited cytotoxicity against P388 murine leukemia cells with an IC 50 values in ranging from 0.09 to 0.45 µg/mL.
The related analogs, named durynes (37)(38)(39)(40)(41)(42), were reported by the same research group [39]. Compound 37 ((−)-duryne) was found to be an enantiomer of (+)-duryne, which has previously been isolated from the marine sponge Cribrochalina dura collected off the shore of Staniel Cay (24.2 • N) in the Bahamas [40]. The taxonomy of Cribrochalina dura was later revised to be Petrosia, and the absolute configuration of each enantiomer was confirmed by the synthesis of both enantiomers [41].
Miyakosynes are also allyl propargyl alcohols or ketones like petroformyne derivatives, but they have a branched methyl group in the aliphatic chain in the middle. Six miyakosynes (A-F, 43-48) were isolated from the sponge Petrosia sp. collected at Miyako sea knoll in Japan [42]. The locations of methyl branches were determined by tandem FAB-MS/MS analysis, and the stereochemistry was assigned using the modified Mosher's method. Petrosynes (49,50) are polyacetylenic enol ethers isolated from a Petrosia sp. collected near Ishigaki Island ( Figure 3) [43]. Followed by the deduction of the plane structure by spectroscopic analysis, all possible stereoisomers of 49 and 50 were synthesized enantioselectively, which showed both to consist of mixtures of 7R and 7S diastereomers. Polyacetylene carboxylic acids are another group of compounds isolated from Japanese Petrosia sponges. Five corticatic acids (51)(52)(53)(54)(55), which show antifungal activity against Mortierella ramanniana or Candida albicans, have been isolated from P. corticata [44,45]. Petroformynic acids B and C (56, 57)-analogous to petroformynes 4 and 3 (4, 3), respectivelywere isolated along with petroformynes 1 and 4 (1, 4) [37]. Compounds 56 and 57 inhibited the growth of P388 cells with an IC 50 value of 0.4 µg/mL.

Polyacetylenes Isolated from Tropical Petrosia Sponges
Several polyacetylenes have been isolated from tropical Petrosia sponges, although not as many as those from their counterparts in temperate regions. These compounds can be categorized into two groups: one comprised of compounds bearing a diynol group and the other comprised of enyne carboxylates.
The same type of acetylenic alcohols (123-131) were additionally isolated from an Okinawan Petrosia sp. [61]. In the regarding report, it is described that the position of the central double bonds in compounds was revealed by chemical transformation-the oxidative cleavage of the olefin, followed by the transformation of resulting aldehyde into the corresponding 2,4-dinitrophenylhydrazone. The analysis of the HRMS and 13 C-NMR spectral data of each resulting hydrazones clarified the position of the central double bonds in 123-131.
Enyne carboxylates (141-144), comprising the second group of petrosiacetylenes in tropical Petrosia, were isolated from the sponge mentioned directly above (Figure 8) [63].  In an earlier study on tropical Petrosia sp., aztèquynols A and B (145, 146) were isolated from the cytotoxic extract of a Petrosia sp. collected in New Caledonia [64]. The structures were assigned by NMR spectroscopy, FAB MS/MS, and chemical transformation; however, neither of the compounds accounted for the cytotoxicity of the sponge extract against KB tumor cell lines (less than 20% cytotoxicity at 10 µg/mL).

Sterols Isolated from Tropical Petrosia Sponges
Unique sterols with an extended side chain have been isolated from Petrosia sponges. Sterols were isolated from tropical Petrosia more frequently. There are 12 papers reporting the isolation of 40 sterols from tropical Petrosia sp., while four papers regarding five sterols from temperate counterparts.
The first Petrosia-derived sterol was isolated from Indo-Pacific S. durissima [65] (Figure 9). The structure of this sterol (147), named strongylosterol, was completely resolved later by chemical synthesis [66]. Subsequently, additional minor sterols (148-151) were isolated from the extract of the same species, and the structures were elucidated by comparing spectroscopic data with those obtained for synthesized derivatives [67,68].
Oxygenated cholestane derivatives were isolated from a Petrosia sp. collected off the Saudi Arabian Red Sea coast (21 • N) [69]. The compounds in this series-containing a tetracyclic skeleton of 3,7,9-trihydroxycholastane (152)-are isolated in the form of oxidized derivatives, i.e., formate (153), epoxide (154), peroxide (155), and diene (156). Compounds with the reduced C-7 (157) or a carbonyl group at C-17 (158) were also isolated. For these compounds, cytotoxicity against human hepatocellular carcinoma (HepG2) and human breast adenocarcinoma (MCF7) was measured; however, none of the compounds showed a meaningful level of activity. The C29-steroids with a cyclopropane ring at C-25-C-27 (159-174) were isolated from the Thai sponge of Petrosia genus ( Figure 10) [70]. Based on the assumption that compounds with 3-dimethyl ketal functionality (161, 162, 169, 170, 172) might have been artificially formed during isolation and purification, compounds with a carbonyl group at C-3 (165, 173) were subjected to conditions similar to those during isolation and purification. No change in the thin layer chromatography analyses was observed, even after one month, suggesting that isolated dimethyl ketal derivatives occur naturally. Compounds-except for 162, 167 and 169-were evaluated for cytotoxicity against six human cancer cell lines and a normal cell line. Compound 173 was the most potent, with IC 50 values of 7.1 and 6.1 µM against HepG2 and HeLa cell lines, respectively. All the other compounds exhibited minimal to weak cytotoxicity, with IC 50 values in the range of 11.2-103.5 µM.
Xetobergsterol A (175) and contignasterol (176), which contain a highly oxygenated cyclic skeleton and a ketone moiety at C-15, were isolated from P. cf. contignata and P. contignata Thiele, respectively [71,72]. Both compounds contain a 14β hydrogen, which is very rare in naturally occurring steroids, although steroids with a 14β-hydroxyl functionality (i.e., digitoxin) are well-known in nature.
Lembehsterols A and B (185, 186) were isolated from P. strongylata collected at Lembeh Island, Indonesia [76]. Both showed inhibitory activity against thymidine phosphorylase (IC 50 = 41.0 and 45.0 µM, respectively) which is related to angiogenesis in solid tumors. Desulfated derivatives obtained by the treatment of 185 with acid showed no inhibition at 230.0 µM, suggesting the importance of the sulfate group for the activity against thymidine phosphorylase.

Sterols Isolated from Temperate Petrosia Sponges
Sterols have rarely been isolated from Petrosia sponges in temperate regions. Only two species, P. ficiformis and S. corticata, were found to contain sterol compounds. All the isolated sterols from both contain cyclopropane in the branch at C-17 ( Figure 12). Petrosterol (187), a steroid with a cyclopropane ring at C-25 and C-26, was isolated from P. ficiformis collected in the Bay of Naples, and its structure was confirmed by the X-ray crystallography of a synthesized p-bromobenzoate derivative [77,78]. Subsequent studies tracing the minor component of the same species resulted in the isolation of ficisterol (188) [79]. 7-Oxo and 7-hydroxy derivatives of petrosterols (189, 190) were isolated from S. corticata collected off the coast of Tokushima (33 • N) [80] along with petrosterol (187) and dihydrocalysterol (191). Compound 191 has been isolated from the sponge Calyx niceaensis, previously [81]. The biological activity of the petrosterol analogs (187-191) have not been reported yet.

Meroterpeonids Isolated from Tropical Petrosia Sponges
Meroterpenoids are a group of compounds that are partially derived from the terpenoid biosynthetic pathway. In the marine environment, meroterpenoids have been isolated most frequently from brown algae and microorganisms, but another important source is marine invertebrates, such as sponges and tunicates [82]. All the meroterpenoids isolated from Petrosia sponges are from sponges collected in tropical areas.
A dimeric strongylophorine (217), composed of two equivalents of strongylophorine 3 (194), was isolated from a Philippine marine sponge of the genus Strongylophora (Petrosia), along with strongylophorines 2-4 (193-195) [91]. In this study, compounds 194 and 195 showed marginal activity against Micrococcus luteus and Salmonella typhi, respectively. Compound 194 was also active against the phytopathogenic fungus Cladosporium cucumerinum and the neonate larvae of the polyphagous insect pest Spodoptera littoralis (EC 50 69.0 µg/mL). Compound 217 was the most active in the brine shrimp lethality assay, with an LC 50 value of 10.5 g/mL.

Saponins Isolated from Indonesian Petrosia sp.
Lanostane-type triterpene oligoglycosides, sarasinosides (245-248), were isolated from the Indonesian Petrosia sponge (Figure 17) [101]. Sarasinosides were originally isolated from marine sponges of the genera Asteropus, Melophlus, and Lipastrotethya [102][103][104], which belong to the same subclass (Heteroscleromorpha) as Petrosia. Compound 245, named sarasinoside S, was isolated for the first time and its structure was elucidated based on its spectroscopic data. None of these isolated sarasinosides exhibited cytotoxicity against human solid cancer cell lines, Huh-7 and A549, although some sarasinoside derivatives have been reported to exhibit weak cytotoxicity against several cancer cell lines.   Isoquinoline alkaloids have been isolated from Petrosia sponges. Mimosamycin (250) and related derivatives (251-254) were isolated from Petrosia sp. and P. similis collected in India [106,107]. Isoquinoline quinones analogous to 254 (255-257) were isolated from two Philippine Petrosia sponges [108]. These compounds exhibited low cytotoxicity with an IC 50 ranging from 24.0 to 45.0 µg/mL against the HCT116 human colon carcinoma cell line.
Petrosamine (258), a pyridoacridine alkaloid, was isolated from the Petrosia sp. collected in Belize [109]; it was confirmed to exist as a keto form by X-ray crystallography. However, a signal corresponding to that of a C-5 carbonyl group (δ C 161 ppm) was detected, which suggests that it takes an enol form in solution. 2-Bromoamphimedine (259) was isolated later from a Thai marine sponge along with 258 [110]. Compound 258 showed strong acetylcholinesterase inhibitory activity approximately six times higher than that of galantamine that used as a reference (IC 50 0.09 vs. 0.59 µM), while 259 was totally inactive.
Purine derivatives (260-263) conjugated with a β-amino acid were isolated from P. nigricans collected in Indonesia, and named nigricines [111]. None of these compounds showed cytotoxicity against the murine lymphoma cell line (L5178Y) at 10.0 µg/mL, and no other activities could be measured due to the limited quantities obtained.
Phenethylguanidine alkaloids isolated from Indo-Pacific marine sponge P. cf. contignata, (264, 265) are close derivatives of tubastrine [71]. Tubastrine was originally reported as a metabolite of the soft coral Tubastrea aurea, and compound 264 has previously been synthesized by the hydrogenation of tubastrine [112].

Peptides Isolated from a Korean Petrosia sp.
So far, only one article has reported the isolation of peptides from Petrosia sponge. This article described the isolation of five halicylindramides (266-270) from the Petrosia sp. collected in Korea ( Figure 19) [113]. The structures of newly reported compounds (268-270) were elucidated based on their spectroscopic and mass data, and the stereochemistry was assigned by Marfey's method and ECD spectroscopy. The article points out that the producers of these compounds might be sponge-associated cyanobacteria, as all of the previously reported halicylindramides were discovered from the sponge Halichondria cylindrata. Compounds 266-268, bearing indole moiety at R 2 position, showed antagonistic activity against human farnesoid X receptor (hFXR) with IC 50 values of 6.0, 0.5, and 5.0 µM, respectively, while 269 and 270 were inactive even at 100.0 µM. Figure 19. Depsipeptides isolated from a Korean Petrosia sp.

Fatty Acid Derivatives
A cyclitol derivative (271) was reported as a metabolite of Korean Petrosia sp. (Figure 20) [114]. This compound exhibited minimal cytotoxicity against a panel of human solid tumor cells with IC 50 values around 10.0 µg/mL; however, HeLa extractpromoted in vitro DNA replication was inhibited by the treatment of 271 in a dosedependent manner. Sphingolipid 272 was isolated from Red Sea Petrosia sp. and exhibited minimal cytotoxicity against MCF-7 and HepG2 cell lines with IC 50 values around 20.0 µg/mL [69]. Brominated fatty acids (273-277) were isolated from the Caribbean Petrosia sp., and its structure was elucidated by mass spectrometry and chemical transformations, including deuteration with Wilkinson's catalyst [115].

Discussion
This review addressed 277 compounds isolated from Petrosia sponges from 87 peerreviewed articles (Figure 21). The type of predominant metabolite depended on the geographical location of the source sponge. Polyacetylenes were the most frequently found in Petrosia sponges inhabiting temperate regions. Of the 36 research articles regarding the metabolites isolated from sponges collected in the Mediterranean Sea, Korean waters, or the temperate regions of Japan, 29 reported the isolation of polyacetylenes. More than 100 polyacetylenes have been isolated from temperate Petrosia sponges, and 44 polyacetylenes were reported as the metabolites of tropical Petrosia sponges. Meroterpenoids and sterols were the most frequently found metabolites in tropical Petrosia sponges; 15 and 12 articles of the 50 papers on the metabolites of tropical Petrosia sponges report a total of 53 meroditerpenoids and 40 steroids, respectively. There are only five reported sterols isolated from Petrosia collected in Italy and Japan, and no reported meroterpenoids have been isolated from temperate Petrosia sponges. Alkaloids have also only been isolated from tropical Petrosia sponges. Seventeen alkaloids have been reported in eight publications regarding the Petrosia sponges collected in the Philippines, Indonesia, Papua-New Guinea, India, Belize, and Saudi Arabia.
Saponins and peptides have also been isolated from Petrosia sponges, but they are not very common. Four noriterpene glycosides of the sarasinoside class were isolated from the Indonesian Petrosia sp., and five depsipeptides were isolated from a Korean Petrosia sponge. Considering the scarcity of these compounds, their origins might be the sponge-associated microbes.
All metabolites isolated from Petrosia sponges from temperate and tropical regions are summarized in Tables 1 and 2, respectively.    [115] Considering that the actual producers of natural products isolated from sponges are the microbes associated therein in many cases [8,10], the latitudinal variation in metabolite composition of Petrosia sponges might be correlated with their associated microbial community. Genes involved in the biosynthesis of Petrosia metabolites have never been deduced until now; however, the biosynthetic gene clusters for analogous polyacetylenes, meroterpenoids, and terpenoids have been found from fungi, cyanobacteria, and actinomycetes. In addition, it has been reported for P. ficiformis that microbial symbiotic populations were more similar in genetically distinct individuals from the same location, than in genetically similar individuals from distant regions [116]. The factors which affects the structure of associated microbial community might be diverse and complex, as tropical and temperate seas have completely different ecosystems, not to mention physical factors, such as temperature and salinity. To better understand the factors that govern the composition of sponge metabolites, research to analyze the structure of associated microbial community of the sponges using various metagenomics tools should be conducted.
The biological activity most frequently investigated for Petrosia metabolites was cytotoxicity; most of the petrosiacetylenes showed growth inhibitory activity against human cancer cell lines. Research to better understand the ecological and pharmacological role of these compounds should be conducted. Biogenesis of Petrosia metabolites would be another significant topic in this field. This research would reveal the symbiotic relationships between marine sponges and microorganisms, and the strategies in biosynthesis that can be applied to the production of biologically-relevant molecules. In addition, the discovery of novel genes involved in the biosynthesis would provide potential bioengineering applications.

Conflicts of Interest:
The authors declare no conflict of interest.