*2.2. Anthraquinones*

Thirteen different anthraquinone isolates were obtained from different marine microbial co-cultures; the co-cultures of marine fungi–bacteria represented the majority, 69% (9/13 isolates; Figures 2B and 11).

**Figure 11.** Anthraquinones isolated from the co-cultures of marine fungi–fungi, fungi–bacteria and bacteria–bacteria.

#### 2.2.1. Anthraquinones Derived from the Co-Cultures of Di fferent Marine Fungi

In the recent study, the combination of cultures from two di fferent developmental stages of marine alga-derived *Aspergillus alliaceus* (teleomorph: *Petromyces alliaceus*) drastically changed the metabolite profile and resulted in the production of allianthrone A (**81**) and two diastereomers, allianthrones B (**82**) and C (**83**) (Figure 12) [53]. **81**–**83** exhibited cytotoxic activity against SK-Mel-5 melanoma cell lines with IC50 (11.0, 12.2, and 19.7 μM) and HCT-116 colon carcinoma cells with IC50 (9.0, 10.5 and 13.7 μM), respectively. This study presented the first example of elicitation of novel fungal chemical diversity by a co-existing strategy of two di fferent developmental phenotypes of *Aspergillus* species. For several Aspergilli, e.g., *A. alliaceus*, asexual and sexual life developmental stages were known. However, rarely did they co-cultivate at the same time. Even more surprising was the presence of novel bianthrones when the sclerotial and asexual morphs of the same species co-existed. There were only a few examples that showed di fferences in secondary metabolites in fungi based on their distinct developmental stages or chemical profiles for the two mating types of heterothallic fungi. However, none of these compounds displayed any activity against *P. aeruginosa*, *E. faecium*, *S. aureus*, *E. coli*, *C. albicans* and *B. subtilis.* Furthermore, non-significant results were obtained against lung (A549), prostate (PC3) and breast (MCF-7) human cancer cells compared with the positive control, etoposide [53].

**Figure 12.** Chemical structures of **81**–**83**.

#### 2.2.2. Anthraquinones Derived from the Co-Cultures of Marine Fungi and Bacteria

Two novel anthraquinones, (*z*)(11*S*,12 *R*)-versicolorin B (**84**) and 6,8-O-dimethylbipolarin (**85**), along with seven known substances bipolarin (**86**), versiconol (**87**), versiconol acetate (**88**), versicolorin B (**89**), 8- *O*-Methylversicolorin B (**90**), averufin (**91**) and endocrocin (**92**) (Figure 13) were isolated and identified from the mixed fermentation broth of the marine fungus *A. versicolor* and *B. subtilis* [38].

**Figure 13.** Chemical structures of **84**–**92**.

Versiconol (**87**) was characterized as an inhibitor of protein tyrosine kinases against EGF-R and v-abl protein tyrosine kinases that were responsible for catalyzing phosphorylation of tyrosine residues of protein substrates, and suppression of MK-cells [54]. **89** displayed inhibitory activity against the Gram-positive *S. aureus* with MIC value of 50 μM and antifungal activity against *Fusarium solani* with MIC values of 16–32 μg/mL [38,55]. The cytotoxic bioassay of **90** was recorded against mouse lymphoma cell line L5178Y with an IC50 value of 21.2 μM. Moreover, **91** displayed antibacterial activity against *B. subtilis* (MIC = 8–16 μg/mL) and the Gram-positive *S. aureus* (MIC = 25 μM) and four Gram-positive microbes, including two *E. faecalis* and two *E. faecium* (MIC = 12.5–25 μM) [38,55]. Neither **89** nor **91** had cytotoxicity against L5178Y cell line, which implied that their antimicrobial activities were not associated with their respective general toxicities. Besides, **90** also displayed mild cytotoxic activity against human lung cancer cells H460 and the human prostate cancer cells PC-3 with IC50 values of 27.2 and 19.5 μM, respectively [56]. Other compounds did not exhibit distinct cytotoxic activity against L5178Y cell line and antibacterial activity against five Gram-positive microbes, including one *S. aureus*, two *E. faecalis* and two *E. faecium*.

#### 2.2.3. Anthraquinones Derived from the Co-Cultures of Di fferent Marine Bacteria

A new antibiotic, keyicin (**93**) (Figure 14), was purified and identified from a co-culture of two marine invertebrate-associated bacteria *Micromonospora* sp. WMMB-235 and *Rhodococcus* sp. WMMA-185 [57]. It showed selective inhibitory activity against Gram-positive bacteria and could inhibit the growth of *B. subtilis* and Methicillin Sensitive *Staphylococcus aureus* (MSSA) with MIC values of 9.9 μM and 2.5 μM, respectively. In contrast to many anthracyclines, **93** might modulate fatty acid metabolism and exhibit antibacterial activity without nucleic acid damage that is explained by keyicin's mechanism of action (MOA) based on *E. coli* chemical genomics studies [57].

**Figure 14.** Chemical structures of **93**.
