*2.7. Steroids*

Steroids contain a characteristic arrangemen<sup>t</sup> of four cycloalkane rings that are joined together. They represent a large family of compounds that play important roles as chemical messengers, and the scaffold is present in many FDA-approved drugs [82–84]. A total of five steroidal metabolites were reported; four of them were isolated from the co-culture of marine fungi–bacteria (80%); only one isolate was identified from the co-culture of marine fungi–fungi (20%). No isolates were obtained from the co-culture of marine bacteria–bacteria.

#### 2.7.1. Steroids Derived from the Co-Cultures of Different Marine Fungi

To the best of our knowledge, the only one steroid, ergosterol (**136**), was found from the co-culture broth of two marine mangrove epiphytic fungi, *Aspergillus* sp. FSY-01 and FSW-02 (Figure 25) [21,85]. It was an essential component of fungal cell membrane with strong specificity and stable structure. Therefore, **136** was widely applied to detecting fungal containment as an indicator of fungal biomass [86].

**Figure 25.** Chemical structures of **136**.

#### 2.7.2. Steroids Derived from the Co-Cultures of Marine Fungi and Bacteria

An unprecedented steroid, 7β-hydroxycholesterol-1β-carboxylic acid (**137**), together with three known steroidal metabolites, 7β-hydroxycholesterol (**138**), <sup>7</sup>α-hydroxycholesterol (**139**) and ergosterol-5<sup>α</sup>,8<sup>α</sup>-peroxide (**140**) (Figure 26), have been confirmed from the co-culture of two marine alga-derived microbes, *Aspergillus* sp. BM05, and an unidentified bacterium (BM05BL), isolated from the brown alga of the genus *Sargassum* collected off Helgoland, North Sea, Germany [87].

**Figure 26.** Chemical structures of **137**–**140**.

Compounds **137**–**140** showed moderate activities against four human tumor cell lines, A2780, HCT116, K562 and A2780 CisR with the IC50 values of 10.0–100.0 μM. At the same time, the total extract of co-culture of *Aspergillus* sp. BM05 and BM05BL showed obvious antiproliferative activity compared with its single steroidal compounds. This implied a synergistic role of these steroidal metabolites in the extract. Furthermore, **140** was reported as a promising new candidate that could overcome the drug-resistant property of malignant cancer cells through abolishing miR-378, a microRNA involved in new tumor initiation, unlimited self-renewal and recurrence of tumor cells after chemotherapy [88].
