Potential ecological role, chemistry and molecular modelling studies of secondary metabolites isolated from the tropical marine sponge Cymbastela hooperi

A single sample of the sponge Cymbastela hooperi yielded 23 sesquiterpenes and diterpenes represented by the hydrocarbon skeltons 1-9. Of these, the majority were functionalised with –NC, -NCS, -NCO, and -NHCHO, while others were substituted with -SH (3), or –OH (R₁ in 8). All of the compounds that contained the –NC functionality were found to have antimalarial and cytotoxic activities. These activities, although very interesting, gave little or no indication of the potential ecological role of any of these compounds. In an attempt to gain some insight into the possible ecological function of these compounds some potentially ecologically relevant assays were undertaken with ones where sufficient material was available.

The assays included a general assessment of antimicrobial properties against Escherichia coli, Ustilago violacea, Bacillus megaterium, Eurotium repens, Mycotypha microspora, Fusarium oxysporum, and Vibrio harveyi, and of their potential antialgal properties against Chlorella fusca. When activity was detected in the antialgal assay compounds were then tested for their ability to inhibit photosynthesis. The results if these assays showed that all except one of the tested compounds demonstrated some antimicrobial properties, and that three of the five compounds found to inhibit the growth of the alga Chlorella fusca also inhibited its photosynthesis. Molecular modelling studies revealed a likely mode of action for the compounds found to inhibit photosynthesis.