Oceans cover more than 70% of the Earth’s surface and provide 99% of the living space (Subramani and Sipkema, 2019). Animals, plants, and other organisms that live in the oceans, coastal areas, or brackish coastal water can harbor millions of bacteria, fungi, and many other organisms. Some of these marine species live in stressful habitats: high salinity, low temperature, high pressure, and low light conditions (Li et al., 2021; Wu et al., 2022b). Importantly, a large number of species with high diversity survive under such conditions and produce fascinating and structurally complex natural products that are not found in terrestrial organisms. This may be due to the bioaccumulation of metabolites in marine microorganisms, as they adapt and survive in harsh conditions that differ from other habitats (Bhakuni and Rawat, 2005; Debbab et al., 2010). Therefore, the associated microorganisms could produce bioactive substances with unique structural skeletons and bioactivity. These metabolites also act as chemical defense mechanisms for marine organisms. This is especially true for soft-bodied organisms which lack structural defense mechanisms (Jensen and Fenical, 1994). For example, the sequence of malate dehydrogenase in mollusks in the Conch is produced by the organisms to adapt to high temperatures, while the Mariana lionfish genome species undergoes mutations in genes that regulate bone development and ossification, leading to an early termination of the calcification process of its bones, making most of its bones cartilage, in response to deep-sea pressure. The chlorine-secreting cells of barracuda gills can be flexibly adjusted to adapt to different salinities (Kobayashi and Yuki, 1954; Skinner, 1982; Liao et al., 2017).

Many deep-sea organisms need to coexist with microorganisms to survive in these harsh environments, such as sulfide-oxidizing bacteria symbiosis with tubular worms near deep-sea cold springs. Thus, it is known that bioactive metabolites discovered from marine organisms such as sponges and sea slugs are the products of microbial metabolism (Gerwick and Moore, 2012).

Studies on marine-derived microorganisms were intensively initiated in the nineties. Marine microorganisms quickly proved to be prolific sources of novel bioactive compounds that are of considerable interest as new drug leads in the pharmaceutical industry (Marmann et al., 2014). Although studies on marine microbial natural products are fascinating, there are some limitations as well. For example, only 5% of the marine microbes can be cultured with normal laboratory microbiological conditions and techniques, taxonomy is very poorly defined, and fermentation yields are very low. Also, the complexity of the molecule makes it difficult to resynthesize (Jimenez et al., 2009; Ameri, 2014). Even with these disadvantages, the development of microbial natural products with good bioactivity is of great interest.

Recently published research articles showed that most of the marine microbial metabolites are potent anticancer compounds and most of them are clinically used or under clinical trials. Already, 23 marine-inspired agents are in clinical use, of which 16 are of anticancer antitumor interest (Ameri, 2014; Almaliti and Gerwick, 2023). The most famous example of a marine microbial compound is salinosporamide A (marizomib), which is isolated from marine Salinispora bacteria and is a proteasome inhibitor that is currently undergoing clinical studies as a potential new anticancer drug (Marmann et al., 2014). Didemnin B and thiocoraline are two drug candidates that were identified from marine microorganisms and show potent antitumor anticancer activities. They are used for the treatment of different cancers at present (Ameri, 2014). Recently, in 2018, the novel diketopiperazine derivative plinabulin was discovered in marine Aspergillus sp. and is now in a worldwide phase III clinical trial for non-small cell lung carcinoma (Blayney et al., 2016).

Studies on microbial natural products have become a famous research area due to their ability to produce biologically active, biosynthetically rare, structurally diverse, and similar metabolites as the host. The ability to produce metabolites on a large scale without damaging the hosts, consumption of minimal period, and reproducibility are also advantageous qualities in this field of study. Thus, the secondary metabolites produced by marine microorganisms have unique and diverse structures, and significant biological activities, which have great potential for traditional Chinese medicine (Yin et al., 2023).

As an important component of the deep-sea microbial community, deep-sea fungi have attracted much attention from researchers due to their rich types of secondary metabolites, high emergence rates, and significant biological activity (Gozari et al., 2020). More and more researchers are pinning their hopes for developing new drugs on the discovery of secondary metabolites from marine fungi. This article reviews the literature on deep-sea fungal active substances during the years 2020-2022 and describes 184 new compounds first reported from marine sources in 47 papers. In this review, compounds are classified into several broad categories based on their structure. This review also summarizes the biological activities of these compounds and the source of bacteria.

Three new hybrid sorbicillinoids, bisorbicillchaetones A-C(37-39), were discovered from Penicillium sp. SCSIO06868. Compounds 37 and 38 showed inhibitory activity towards NO production in LPS-activated RAW264.7, with IC₅₀ values of 80.3 ± 3.6 μM and 38.4 ± 3.3 μM, respectively (Pang et al., 2022). Purpureocillium sp. SCSIO 06693 produced (E)-2-(8,9-dihydroxy-6,8-dimethyldec-4-en2-yl)-methylfuran-3(2H)-one (40), which inhibits the pancreatic lipase enzyme at 50 μg/mL (Song et al., 2022).

Two new polyketides fischerins A (41) and B (42) were obtained from Aspergillus fischeri FS452, among which 42 showed cytotoxicity against A549, HepG-2, MCF-7, and SF-268 ranging from 7 to 10 μM (Liu et al., 2021). Aspergillus puniceus SCSIO z021 produced three new metabolites, (±)-puniceusone A (43), (±)-2,3-dihydroxypropyl 6-ethyl-2,4-dihydroxy3-methylbenzoate (44), and (5-ethyl-3-hydroxy-2-methyl) phenyl-b-D-glucoside (45)(Liu et al., 2022a). Phomalichenones E-G (46-48) were isolated from Alternaria sp. MCCC 3A00467, and 47 showed cytotoxic activity against U266 cells (Zhong et al., 2022). Two new macrolides cyclopiumolides A (49) and B (50) with a spiculisporic acidic moiety were isolated from Penicillium cyclopium SD-413 and showed cytotoxicity against TE-1, FaDu, and SF126 cells with IC₅₀ values ranging from 5.86-17.05 µM (Li et al., 2022). A study of Paraconiothyrium hawaiiense FS482 resulted in the identification of hawatides A-G (51-57). Compound 54 inhibited the production of NO and cytotoxicity against HepG-2, A549, SF-268, and MCD-7 cell lines (Chen et al., 2021b). Ochroconis sp. FS449 yielded a series of usnic acid derivatives ochuscins A(58), B(59), F(60), and G (61), of which compounds 59 and 61 inhibited the action of acetylcholinesterase (Chen et al., 2020d). Lithocaldehydes A (62) and B (63), were isolated from Phomopsis lithocarpus FS508, which displayed antifungal and antibacterial activity at micromolar levels (Liu et al., 2020).

A series of polyketides asperochratides A-J (64-73) were produced by Aspergillus ochraceus, and compounds 68-71 displayed cytotoxicity toward BV-2 cells (Zou et al., 2020). Cultivation of Penicillium chrysogenum SCSIO 07007 produced new α-pyrone derivatives chrysopyrones A and B (74 and 75) which inhibit human protein tyrosine phosphatases (Han et al., 2020). A chemical investigation of a culture extract of Phomopsis tersa FS441 led to the discovery of chlorinated azaphilones tersaphilones A-E (76-80). Compounds 79-80 demonstrated cytotoxicity against cancer cell lines A549, MCF-7, and SF-268 (Chen et al., 2021a). Spiromastol L-N (81-83) and spiromastixin (84) were isolated from an extract of Spiromastix sp. SCSIO F190, which displayed inhibitory activity against methicillin-resistant bacterial strains of Staphylococcus aureus, Enterococcus faecalis ATCC 29212, and Bacillus subtilis BS01 (Cai et al., 2023).

Steckwaic acids A-D (85-88), 11-ketotanzawaic acid D (89), 6,15-dihydroxytanzawaic acid M (90), (15R)-methoxytanzawaic acid M (91), 15S-methoxytanzawaic acid M (92), 8-hydroxytanzawaic acid M (93), and 8-hydroxytanzawaic acid B (94) were obtained from Penicillium steckii AS-324, which was isolated from Acanthogorgiidae sp. coral in the Western Pacific Ocean. Compounds 85, 89, and 93 displayed antibacterial activity with Vibrio anguillarum, Micrococcus luteus, and Pseudomonas aeruginosa (Hu et al., 2022b). Peniterphenyls A-C (95-97) was discovered in Penicillium sp. SCSIO4103, which displayed activity against HSV-1/2 with EC₅₀ values in the range of 1.4 ± 0.6 to 9.3 ± 3.7 μM (Chen et al., 2021c). Penithoketone (98) and penithochromones A-L (99-109) were produced by Penicillium thomii YPGA3. Compound 98 displayed cytotoxicity against MCF-7, MDA-MB-468, C4-2B, and C4-2B/ENZR (Li et al., 2020). Aspergillus terreus CC-S06-18 afforded a dihydrobenzofuran-phenyl acrylate hybrid compound, aspeterreurone A (110), which displayed cytotoxic activities against cell lines BGC823, MGC803, HGC27, and AGS and inhibited the STAT3 phosphorylation (Wang et al., 2020).

Hypoxone A (111), 4,8-dimethoxy-1-naphthol (112), and 1’-hydroxy-4’,8,8’-trimethoxy[2,2’]binaphthalenyl-1,4-dione (113) were produced by the fungus Hypoxylon rubiginosum FS521, and were cytotoxic against the tumor cell lines SF-268, MCF-7, HepG-2, and A549 (Zhang et al., 2020). The dibenzodioxocinone canescenin A (114) produced by the Penicillium canescens SCSIO z053 showed weak antibacterial and antibiofilm activities toward Bascillus amyloliquefaciens and Pseudomonas aeruginosa (Dasanayaka et al., 2020). Two phenols, insphenol A (115) and acetylpeniciphenol (116), were isolated from the cold sea-derived Aspergillus insuetus SD-512. Compound 116 inhibited Edwardsiella tarda, Vibrio alginolyticus, and Vibrio vulnificus (Chi et al., 2021a).

In summary, we reported 184 new compounds derived from 46 deep-sea-associated fungi during 2020-2022 ( Table 1 ). The categories of the 184 new compounds discovered during the review period is shown in the following Figures 1 , 2 . In the past three years, researchers have discovered 80 polyketides, 68 terpenoids, and 36 Nitrogen-containing compounds ( Figure 1 ). It can be hypothesized that the content of polyketides and terpenoids in the metabolites of deep-sea fungi is higher than that of Nitrogen-containing compounds, but it remains to be verified. According to the biological activity profile of these new compounds ( Figure 1 ), only 70 new compounds have detected biological activity. Of the 70 active new compounds, 20 were associated with cytotoxicity. Almost half of the compounds have anti-infective activity. Meanwhile, 11 compounds have shown enzyme inhibitory activity, and a few have anti-inflammatory activity. These activities will lead the research culture to develop new drug leads for crucial diseases like cancer. Analysis from the environmental source of strains ( Figure 2 ) suggested that mud was the main source of the deep-sea fungi, contributing 34 out of all 46 strains. Analysis of the microbial origin of these compounds found that Aspergillus fungi were the main contributors to these new compounds, contributing a total of 82 new compounds derived from 17 different strains. In terms of culture medium ( Figure 3 ), 36 out of 46 strains were cultured using the rice medium and 10 strains using liquid culture medium. All of these media are rich in nutrients and differ significantly from the oligotrophic environment in the deep sea. No habitat-based cultivation strategies according to the deep-sea environmental factors, such as low oxygen and high pressure, were explored and applied to activate the silent gene cluster of deep-sea fungi, although these strategies have been successfully applied to other deep-sea microorganisms.

In summary, these structurally novel active compounds reveal an astonishing unique secondary metabolic system of deep-sea fungi. However, deep-sea fungi are still a special resource with infinite potential of biologically active and structurally new compounds that has not been fully recognized, explored, and utilized. In the cultivation of deep-sea fungi, it is necessary to try to activate their silent gene clusters using the deep-sea habitat-simulating strategy, so as to further tap their secondary metabolic potential. This requires the joint efforts of researchers dedicating their efforts to deep-sea-derived fungi.

ZW drafted the manuscript. MQ reviewed and corrected the manuscript. CW, YW, and FK conceived, guided, and revised. QW revised the manuscript. All authors have read and agreed to the published version of the manuscript.