Phylogenetic Identification of Fungi Isolated from the Marine Sponge Tethya aurantium and Identification of Their Secondary Metabolites

Fungi associated with the marine sponge Tethya aurantium were isolated and identified by morphological criteria and phylogenetic analyses based on internal transcribed spacer (ITS) regions. They were evaluated with regard to their secondary metabolite profiles. Among the 81 isolates which were characterized, members of 21 genera were identified. Some genera like Acremonium, Aspergillus, Fusarium, Penicillium, Phoma, and Trichoderma are quite common, but we also isolated strains belonging to genera like Botryosphaeria, Epicoccum, Parasphaeosphaeria, and Tritirachium which have rarely been reported from sponges. Members affiliated to the genera Bartalinia and Volutella as well as to a presumably new Phoma species were first isolated from a sponge in this study. On the basis of their classification, strains were selected for analysis of their ability to produce natural products. In addition to a number of known compounds, several new natural products were identified. The scopularides and sorbifuranones have been described elsewhere. We have isolated four additional substances which have not been described so far. The new metabolite cillifuranone (1) was isolated from Penicillium chrysogenum strain LF066. The structure of cillifuranone (1) was elucidated based on 1D and 2D NMR analysis and turned out to be a previously postulated intermediate in sorbifuranone biosynthesis. Only minor antibiotic bioactivities of this compound were found so far.


Introduction
Natural products are of considerable importance in the discovery of new therapeutic agents [1]. Apart from plants, bacteria and fungi are the most important producers of such compounds [2]. For a long time neglected as a group of producers of natural products, marine microorganisms have more recently been isolated from a variety of marine habitats such as sea water, sediments, algae and different animals to discover new natural products [3,4]. In particular, sponges which are filter feeders and accumulate high numbers of microorganisms have attracted attention [5,6]. Though the focus of most of these investigations was concerned with the bacteria, a series of investigations identified marine sponges also as a good source of fungi [7][8][9][10][11][12][13][14][15][16][17][18]. Due to the accumulation of microorganisms, it is no surprise that sponges account for the majority of fungal species isolated from the marine realm [19]. However, the type of association and a presumable ecological function of accumulated fungi in sponges remain unclear and little evidence is available on fungi specifically adapted to live within sponges. One example is represented by fungi of the genus Koralionastes, which are known to form fruiting bodies only in close association with crustaceous sponges associated with corals [20].
Consistently, fungi isolated from sponges account for the highest number (28%) of novel compounds reported from marine isolates of fungi [19]. Marine isolates of fungi evidently are a rich source of chemically diverse natural products which has not been consequently exploited so far. Among a number of metabolites from sponge-associated fungi with promising biological activities are the cytotoxic gymnastatins and the p56 lck tyrosine kinase inhibitor ulocladol [21,11]. In view of these exciting data and our own previous work on the bacterial community associated with Tethya aurantium [22], we have now isolated and identified a larger number of fungi from this sponge. In European waters, Tethya aurantium is commonly found in the Atlantic Ocean, the English Channel, the North Sea as well as the Mediterranean Sea, where our specimens originated from [23]. Except for a single report from Indriani [24], fungi associated with Tethya sp. have not been investigated so far. The formation of natural products by these fungi and their biotechnological potential has not been evaluated yet.
For the identification of the fungal isolates from Tethya aurantium, we combined morphological criteria and phylogenetic analyses based on the sequence of the internal transcribed spacer (ITS) regions 1 and 2. On the basis of their classification, strains were selected for analysis of their ability to produce natural products. In addition to a number of known compounds, the new cyclodepsipeptides scopularide A and B were produced by a Scopulariopsis brevicaulis isolate [25]. Because of their antiproliferative activities against several tumor cell lines, these peptides and their activities have been patented [26]. During the present study, we have isolated four so far undescribed substances. Structure and properties of the new cillifuranone, a secondary metabolite from Penicillium chrysogenum strain LF066 are reported here.

Identification of the Fungal Strains Isolated from T. aurantium
In most studies on fungi associated with sponges the taxonomic classification of the fungi was based exclusively on morphological characteristics and in many cases identification was possible only at the genus level [17]. This can be attributed to the fact that taxonomic identification of fungi at the species level is not always easy. It is impaired by the fact that under laboratory conditions many fungi do not express reproductive features like conidia or ascomata, which represent important traits for identification. These fungi are classified as "mycelia sterilia". Therefore, morphological criteria as well as sequence information and the determination of phylogenetic relationships are considered to be necessary for the identification of fungi. Consequently, we have combined the morphological characterization with a PCR-based analysis using ITS1-5.8S-rRNA-ITS2 gene sequences to identify 81 fungi isolated from Tethya aurantium. Based on these criteria the strains could be identified to the species level ( Figure 1, Table 1).     First of all, morphological criteria enabled the identification of most of the fungal isolates at the genus level (Table 1). However, under the culture conditions applied, 11 strains did not produce spores and were designed as "mycelia sterilia". A morphological classification of these strains was not possible. Despite the formation of spores, another four strains could not be classified on the basis of morphological characteristics.
In order to verify the results of the morphological examination and identify the strains at the species level, they were subjected to ITS1-5.8S-ITS2 gene sequence analysis. The results obtained from this sequence analysis corresponded well with those from the morphological identification (Table 1, Figure 2) and in addition allowed identification of those strains not identified microscopically. In most cases, the sequence data and the phylogenetic relationships allowed the identification at the species level. Results from BLAST search are depicted in Table 1. Taking together morphological and genetic characteristics, most isolates belong to the Ascomycotina with representatives of the fungal classes Dothideomycetes (24 isolates), Eurotiomycetes, (25 isolates), Sordariomycetes (30 isolates) and Leotiomycetes (1 isolate). Only a single isolate (strain LF538) was classified as belonging to the Mucoromycotina.    (Figure 2), 10 isolates of the Dothideomycetes closely affiliated to fungal species of the order Capnodiales (Cladosporium spp. and its teleomorph Davidiella). Within the order Pleosporales (13 isolates) 5 isolates were closely related to Alternaria sp., including Lewia infectoria as teleomorph. From the same order we also isolated 2 strains of the species Epicoccum nigrum and 1 Paraphaeosphaeria strain. Furthermore, 5 isolates of the genus Phoma were found, of which one isolate (strain LF258) shows only 91% sequence similarity to Septoria arundinacea as next relative according to BLAST results (Table 1) and presumably represents a new species in the Phoma lineage. The low similarity of this isolate to known sequences is also reflected in the position in the phylogenetic tree, clustering distinct from other Phoma species. A single isolate (strain LF241) was assigned to Sphaeropsis sapinea/Botryosphaeria sp. within the order Botryosphaeriales in the class Dothideomycetes.
The 30 isolates affiliated to the class Sordariomycetes were grouped into the orders Hypocreales (26 isolates), Microascales (2 isolates) and Xylariales (1 isolate). Within the Hypocreales, one isolate each was closely related to Beauveria bassiana (strain LF256), Tritirachium album (strain LF562), Verticillium sp. (strain LF496), and Volutella ciliata (strain LF246). The most frequent genera within this order were Fusarium (13 isolates), Trichoderma (5 isolates, including Hypocrea teleomorphs) and Clonostachys (4 isolates, including teleomorphs of Bionectria). The Microascales were represented by two isolates of the genus Scopulariopsis. One of these (strain LF064), morphologically classified as Scopulariopsis murina, was distantly related to the next cultured relative (89% according to BLAST search) and appears as a sister group to the Scopulariopsis lineage in the phylogenetic tree. The order Xylariales was represented by only a single isolate (strain LF550), Bartalinia robillardoides. As the only representatives of the order Helotiales within the class Leotiomycetes strain LF248 was closely related to Botryotinia fuckeliana.
The combination of microscopic and genetic analyses has proven to be a reliable method for the identification of fungal isolates and results from both approaches corresponded quite well (Table 1, Figure 2). The reliable identification of isolates is a fundamental prerequisite in order to characterize the producers of marine natural products [3], to determine the occurrence of fungal species in different habitats, and to correlate distinct secondary metabolite patterns to fungal species. Therefore, we highly recommend that morphological identification of fungal isolates consequently should be verified by molecular means and thus raise the number of reliably identified species in public databases.
A number of investigations, mainly with the aim of finding novel natural products, showed that most marine sponges harbor a plethora of cultivable fungi within their tissue [10][11][12][13][14][15]18]. Despite the large number of fungi isolated from sponges, a selective accumulation of specific taxa within sponges and a truly marine nature of these fungi is doubted, which is why they are commonly referred to as "marine-derived" [27,28]. In fact, it could be shown that the phylogenetic diversity of fungi isolated from different sponges varies [17]. It has also been stated that the taxa frequently isolated from sponges resemble those described from terrestrial habitats [11,16,28]. This is in good accordance with our results, showing representatives of Acremonium, Aspergillus, Fusarium, Penicillium, Phoma, and Trichoderma to be abundant in Tethya aurantium and with those of Wang (2006) demonstrating that they also are widely distributed among different sponges from various locations [29]. Although it appears that the marine environment indeed provides various habitats for fungi, this has rarely been demonstrated. Nonetheless, due to the accumulation of fungi within sponges a large number of strains can be isolated which increases the probability to find representatives of less common taxa which might produce unprecedented secondary metabolites. For example, fungi belonging to the genera Beauveria, Botryosphaeria, Epicoccum, Tritirachium, and Paraphaeosphaeria have rarely been obtained from marine sponges [11,18,24] and, to the best of our knowledge, we have isolated Bartalinia sp. and Volutella sp. from a marine sponge for the first time.
Although evidence is presented that some bacterial symbionts of sponges are the producers of metabolites originally assumed to be produced by the sponge [4], equivalent evidence for fungi is lacking. There is actually little evidence for sponge-specific fungal associations and the only reports on this matter deals with the above mentioned Koralionastes species and a yeast living in symbiosis with the sponge Chondrilla sp. [30]. In fact, most of the studies had the biotechnological potential of sponge-derived fungi in mind but not the ecological role. Our culture-dependent approach was not considered to approach the aspect of specificity of the association with the sponge and molecular-based studies would be more suited to identify specifically sponge-associated fungi.

Secondary Metabolite Analyses
With the cultivation-based approach used in this study, we obtained a variety of strains from a surprisingly broad range of phylogenetic groups of fungi. A selection of the isolated and identified fungi was subjected to analysis of their secondary metabolite profiles. Strains were selected in order to represent a wide spectrum of genera, representatives of a variety of different genera, some known to include strains and species known for the production of secondary metabolites others from less common taxa. Some of the strains selected according to systematic criteria did not produce detectable amounts of secondary metabolites under the applied culture conditions. Unraveling their potential of secondary metabolite production will require more intense studies. Those extracts which did contain at least one compound in significant amounts were analyzed by HPLC with DAD (UV)-and MS-detection and the metabolites which could be identified are listed in Table 2. A high percentage of the substances identified this way could be verified by 1 H NMR spectroscopy (see Table 2). The majority of these metabolites have been reported from fungi before. Two of our previous reports on metabolites from fungi isolated from Tethya aurantium deal with the antiproliferative scopularides A-B [25,26] and the sorbifuranones A-C [31].    For four compounds, database searches [33,[42][43] did not lead to a hit (B, C, D) or no publication was available (A). The structure elucidations of compounds A and B, metabolites with modified diketopiperazine substructures, and compound D, a new azaphilone derivative, are in progress. Compound C was identified as the new metabolite cillifuranone and its structure is described in the following.
Penicillium strain LF066, the producer of cillifuranone, was singled out for further investigations, because first surveys proved it to be a very potent producer of secondary metabolites. From the same strain, sorbifuranone B and C as well as 2′,3′-dihydrosorbicillin have already been described by Bringmann et al. [31] and it was obvious that the full potential of the strain had not been exploited, yet. Further analysis led to the detection of xanthocillines and sorbifuranones and the isolation of sorbifuranone B, meleagrin, roquefortin C, a couple of ergochromes as well as the new cillifuranone (1) whose structure was elucidated based on 1D and 2D NMR experiments.
The 13 C NMR spectrum of 1 displayed 10 clearly distinguishable carbon signals which was in good agreement with the molecular formula C 10 H 12 O 4 , deduced from the result of a HRESI-MS measurement (calculated for C 10 H 12 O 4 Na 219.0628, measured 219.0627). The carbon signals included resonances belonging to three carbonyl or enol carbons (δ C 170.9, 196.7 and 201.7), three sp 3 hybridized methylene carbons (δ C 20.9, 31.6 and 76.3), one methyl group (δ C 14.0), two olefinic methines (δ C 118.5 and 133.0) and finally one quaternary olefinic carbon (δ C 112.6). The structure of the molecule could be delineated from 1D ( 1 H, 13 C and DEPT) and 2D NMR ( 1 H-13 C HSQC, 1 H-1 H COSY and 1 H-13 C HMBC) spectra. From the 1 H-1 H COSY spectrum two separate spin systems could be identified. The first one consisted of the olefinic methine groups CH-9 (δ C 133.0, δ H 7.32) and CH-10 (δ C 118.5, δ H 6.83), forming an E-configured double bond as proven by their 3 J coupling constant of 16 Hz. The corresponding protons H-9 and H-10 both showed 1 H-13 C HMBC correlations to the carboxyl carbon C-11 (δ C 170.9) as well as to the quarternary carbons of the furanone ring, C-3 (δ C 201.7) to C-5 (δ C 196.7). C-3 was the ketone carbonyl group included in the furanone substructure which was in accordance with its chemical shift. C-4 (δ C 112.6) and C-5 were also part of the furanone and formed a tetrasubstituted double bond in which C-5 was located adjacent to an oxygen atom. Compared to an unsubstituted enol, the resonance of C-5 was shifted further downfield due to the conjugation of the double bond Δ 4,5 with the carbonyl carbon C-3. Apart from C-9 of the double bond Δ 9,10 , the carbonyl carbon C-3 and the oxygen atom of the furanone ring, Δ 4,5 was also connected to C-6 of the second spin system consisting of the methylene groups CH 2 -6 (δ C 31.6, δ H 2.76) and CH 2 -7 (δ C 20.9, δ H 1.76) as well as the methyl-group CH 3 -8 (δ C 14.0, δ H 1.03). Thus, the second spin system evidently was an n-propyl-chain. The furanone ring was completed with the methylene group CH 2 -2 (δ C 76.3, δ H 4.67). Its 1 H and 13 C shifts proved it to be linked to an oxyen atom, the 1 H-13 C HMBC correlations to C-3 and C-5 secured its exact position. Thus, the structure of cillifuranone (1) could be unambiguously determined (Figure 3, Table 3).  170.9, C Cillifuranone (1) was tested in a panel of bioassays evaluating the compound with respect to cytotoxic, antimicrobial and enzyme inhibitory activity. Very low activity was only found against Xanthomonas campestris (24% growth inhibition) and Septoria tritici (20% growth inhibition) at a concentration of 100 µM.
Strain LF066 was identified as Penicillium chrysogenum, a species that in our experience often produces metabolites deriving from sorbicillinol as a biosynthetic precursor (sorbicillinoids). The detection of bisvertinolone and the sorbifuranones in culture extracts of the fungus was consistent with this experience. Furanone substructures are abundant in natural products and can be found in metabolites from bacteria, fungi and plants [42] and presumably are products from different biosynthetic pathways [44][45][46]. From the genus Penicillium a number of furanone containing compounds has been described, including simple small molecules like penicillic acid, but also more complex ring structures such as the rotiorinols [47] or rugulovasines [48]. In most cases the furanone ring is a furan-2(5H)-one, whereas in cillifuranone (1) we have a furan-3(2H)-one. With that differentiation being made the number of related compounds gets fewer, furan-3(2H)-ones do not seem to be as ubiquituous as the furan-2(5H)-ones. From Penicillium strains, apart from the above mentioned sorbifuranones, berkeleyamide D [49] and trachyspic acid [50], both spiro compounds like sorbifuranone C, are examples. The new cillifuranone represents a substructure of the sorbifuranones, albeit with a different configuration of the exocyclic double bond, and represents the substructure which makes the sorbifuranones unique in the compound class of the sorbicillinoids. Just recently Bringmann et al. [31] published the structures of the sorbifuranones and postulated 1 to be an intermediate in their biosynthesis ( Figure 4) which makes the isolation of 1 a very interesting result. The only difference between the postulated intermediate and our structure is, as stated above, the configuration of the double bond. However, in the crude extract of the strain, we detected two isomers with the same molecular weight and a very similar UV-spectrum, so that we assume that both isomers were present, but after the isolation process only the E-isomer was obtained, so that it might be the favoured configuration under the applied conditions.

Sampling Sites
The Limsky kanal (Canal di Lemme or Limsky channel) is a semi-closed fjord-like bay in the Adriatic Sea nearby Rovinj (Istrian Peninsula, Croatia). It is situated along an east-west axis, with an approximate length of 1 km and a maximum width of about 650 m and reaches a maximum depth of 32 m [51]. The sampling site was located at N45°7972′ and E13°43,734′.

Sponge Collection
Several specimens (13)  The sponges were collected into sterile plastic bags, cooled on ice and transported immediately to the laboratory, where they were washed three times with sterile filtered seawater (0.2 µm). The sponge tissue was then cut into small pieces of approximately 0.1 cm 3 each, which were either placed directly onto GPY agar plates (LF236 to LF255) or homogenized and diluted with membrane-filtered seawater (all other isolates).

Isolation, Cultivation and Storage of Fungi
Fungi were isolated on a low nutrient GPY agar, based on natural seawater of 30 PSU, containing 0.1% glucose, 0.05% peptone, 0.01% yeast extract and 1.5% agar. Small pieces of sponge tissue or 50 µL of the homogenate (undiluted or 1:10 or 1:100 diluted with sterile seawater) were used as inoculum. The agar plates were incubated for periods of 3 days to 4 weeks and were checked regularly for fungal colonies, which were then transferred to GPY agar plates. Pure cultures were used for morphological identification by light microscopy and for scanning electron microscopy. Fungal isolates were stored as agar slant cultures at 5 °C, and additionally were conserved at −80 °C using Cryobank vials (Mast Diagnostica).

Morphological Identification of Fungal Isolates
The morphology of GPY agar grown fungal isolates was studied using a stereo microscope (10-80× magnification) and a phase-contrast microscope (300-500× magnification). By this method, the majority of spore-producing isolates could be identified up to the generic level using the tables of Barnett and Hunter [52] and, for more detailed descriptions, the site of MycoBank [53]. Morphological identification of the selected pure cultures was supported by light microscopy and scanning electron microscopy.

Genetic Identification of Fungal Isolates and Phylogenetic Analysis
DNA-extraction was performed using the Precellys 24 system (Bertin Technologies). To one vial of a Precellys grinding kit with a glass beads matrix (diameter 0.5 mm, peqlab Biotechnologies GmbH) 400 µL DNAse-free water (Fluka) were added. Cell material from the fungal culture was then transferred to this vial and homogenized two times for 45 s at a shaker frequency of 6500. The suspension was centrifuged at 6000 g for 10 min and 15 °C. The supernatant was stored at −20 °C until further use in PCR.
PCR products were sequenced using the ABI PRISM ® BigDye™ Terminator Ready Reaction Kit (Applied Biosystems) on an ABI PRISM ® 310 Genetic Analyzer (Perkin Elmer Applied Biosystems). The ITS1 primer was used for sequencing. Sequence data were edited with ChromasPro Version 1.15 (Technelysium Pty Ltd.). Sequences from fungal strains obtained during this study were submitted to the EMBL database and were assigned accession numbers (FR822769-FR822849). Closest relatives were identified by sequence comparison with the NCBI Genbank database using BLAST (Basic Local Alignment Search Tool) [55]. Sequences were aligned using the ClustalX version 2.0 software [56] and the alignment was refined manually using BioEdit (version 7.0.9.0) [57]. For alignment construction, ITS1-5.8S-ITS2 gene fragments from closest cultured relatives according to BLAST as well as type strains were used, whenever possible. However, not from all fungal species, ITS sequences from type strains were available in NCBI/Genbank. The ITS1-5.8S-ITS2 gene sequence of Saccharomyces boulardii strain UOA/HCPF EM10049 (acc. No. FJ433878) was used as outgroup sequence for phylogenetic calculations. Phylogenetic calculations were performed with all closest relatives according to BLAST results (data not shown). For clarity, not all of these sequences were included in Figure 2. Phylogeny was inferred using MrBayes version 3.1 [58,59], assuming the GTR (general time reversible) phylogenetic model with 6 substitution rate parameters, a gamma-shaped rate variation with a proportion of invariable sites and default priors of the program. 1,000,000 generations were calculated and sampled every 1000th generation. Burn-in frequency was set to 25% of the samples. The consensus tree was edited in Treeview 1.3 [60].

Fermentation and Production of Extracts
The fungi were inoculated in 2 L Erlenmeyer flasks containing 750 mL modified Wickerham-medium [61], which consisted of 1% glucose, 0.5% peptone, 0.3% yeast extract, 0.3% malt extract, 3% sodium chloride (pH = 6.8). After incubation for 11-20 days at 28 °C in the dark as static cultures, extracts of the fungi were obtained. The mycelium was separated from the culture medium and extracted with ethanol (150 mL). The fermentation broth was extracted with ethyl acetate (400 mL). Both extracts were combined. Alternatively, cells and mycelium were not separated and the culture broth was extracted together with the cells of some cultures using ethyl acetate. After evaporation of the solvents, the powdery residue was reextracted with ethyl acetate (100 mL). The resulting residues were dissolved in 20 mL methanol and subjected to analytical HPLC-DAD-MS.

Chemical Analysis
UV-spectra of the identified metabolites were obtained on a NanoVue (GE Healthcare). NMR spectra were recorded on a Bruker DRX500 spectrometer (500 and 125 MHz for 1 H and 13 C NMR, respectively), using the signals of the residual solvent protons and the solvent carbons as internal references (δ H 3.31 ppm and δ C 49.0 ppm for methanol-d4). High-resolution mass spectra were acquired on a benchtop time-of-flight spectrometer (MicrOTOF, Bruker Daltonics) with positive electrospray ionization. Analytical reversed phase HPLC-DAD-MS experiments were performed using a C 18 column (Phenomenex Onyx Monolithic C18, 100 × 3.00 mm) applying an H 2 O (A)/MeCN (B) gradient with 0.1% HCOOH added to both solvents (gradient: 0 min 5% B, 4 min 60% B, 6 min 100% B; flow 2 mL/min) on a VWR Hitachi Elite LaChrom system coupled to an ESI-ion trap detector (Esquire 4000, Bruker Daltonics).
Preparative HPLC was carried out using a VWR system consisting of a P110 pump, a P311 UV detector, a smartline 3900 autosampler and a Phenomenex Gemini-NX C18 110A, 100 × 50 mm, column or a Merck Hitachi system consisting of an L-7150 pump, an L-2200 autosampler and an L-2450 diode array detector and a Phenomenex Gemini C18 110A AXIA, 100 × 21.20 mm, column.
For the preparation of cillifuranone (1), the same solvents were used as for the analytical HPLC, with a gradient from 10% B, increasing to 60% B in 17 min, to 100% B from 17 to 22 min. 1 eluted with a retention time of 6.8 min and the amount of 23.2 mg of 1 could be obtained from a culture volume of 10 L.

Conclusion
The marine sponge T. aurantium was found to be a valuable source of secondary metabolite producing fungi. In addition to a variety of known substances, several new natural products were found and it is likely that additional ones can be identified during further studies. The antiproliferative active scopularides [25,26] were the first new metabolites from a Scopulariopsis species isolated from T. aurantium. The new cillifuranone (1) is a second example of a natural product produced by a fungal isolate from T. aurantium. Additional compounds were detected of which the chemical structures are not yet described. The application of alternative cultivation methods, which have not been used so far, are expected to further increase the spectrum of produced metabolites of our isolates obtained from T. aurantium.
The combination of morphological criteria and the results of the ITS1-5.8S-ITS2 fragment sequencing have been proven to be a valuable tool for the identification of fungal isolates. Apart from representatives of genera, which are widely distributed in terrestrial samples and in addition also reported from different sponges, we also identified members of taxa which so far have not been described to be associated with sponges. These strains distantly affiliated to Bartalinia sp. and Votutella sp. and one strain most likely is a new Phoma species.