| Summary: | This thesis presents results from investigations of secondary metabolites from marine organisms. The first part pursued the isolation of new compounds from soft bodied marine organisms, the study of aqueous extracts from the different algae and soft bodied marine organisms permitted the selection of one extract from the red alga <i>Laurencia </i>sp., from which six new sesquiterpenes cyclic ethers were isolated. The investigation of the sponge <i>Stylotella aurantium </i>collected from Fijian waters, resulted in isolation of seven previously reported pyrrole alkaloids. The second part focused on the exploration of three novel actinomycete microbes isolated from Mariana Trench sediment, collected at depth 10,898 m (Challenger Deep; 11°19’911’’ N; 142°12’372’’E) for the production of bioactive compounds. Seven novel phenazine alkaloids, dermacozines (A-I), were isolated from the fermentation broth of two Mariana Trench microbes <i>Dermacoccus abyssi</i> strain MT1.1 and <i>Dermacoccus </i>strain MT1.2. The investigated cytotoxic activity against leukaemia cell line (K562) showed that dermacozine F and G have moderate activity with IC<sub>50s</sub> of 9 and 7 <i>μ</i>M, respectively, while the highest radical scavenger activity was observed with dermacozine C with an IC<sub>50</sub> value of 21 <i>μ</i>M. Investigation of <i>Micromonspora</i> sp. isolated from Mariana Trench sediment led to the isolation of six compounds including: one unusual depeptide and the known deferoxamine together with four known diketopiprazines. Finally, the last part of work focused on the investigation of molecular basis of cold temperature and high pressure adapted growth in extremophilic microbes such as <i>Photobacterium profundum sp.</i> The overall aim of this project was to understand more about the high pressure and cold-adapted growth of two novel <i>P. profundum</i> strains SS9R (Piezophile) and 3TCK (Piezosensitive) by studying the surface of polysaccharides which play an important role in the cold-adapted growth of <i>P. profundum</i> <i>SS9.</i>
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