The natural products chemistry of marine microorganisms is an emerging area of organic chemistry with the aim of discovering novel secondary metabolites exhibiting both biomedical and ecological activities. While marine bacteria have proven to be a productive source of new natural products, there are many groups of marine microorganisms which have not been fully investigated. In particular, marine fungi represent an untapped and potentially novel source of bioactive secondary metabolites. While the biomedical potential of marine microbial natural products is being investigated, it is the ecological role of these metabolites in the marine environment which has been completely ignored. The goal of this thesis research was to investigate the ecological role of secondary metabolites involved in the association of fungi with marine plants and to continue the development of marine fungi as a source of novel natural products.

The ecological study of chemical interactions between fungi and marine plants had two main foci: 1) the study of fungi as producers of chemical signals through the investigation of phytotoxic metabolites produced by algal associated fungi, and 2) to examine fungi as receivers of chemical messages by studying the mechanisms by which marine plants chemically control the distributions of fungi on their surfaces. The first objective was accomplished by showing that marine fungi produce potent phytotoxic metabolites which can inhibit the growth of marine algae, and thus may be important in the pathology of marine plants. The second goal was attained by demonstrating that the seagrass Thalassia testudinum is chemically defended against zoosporic marine fungi.

The second part of the thesis, the development of marine fungi as a source of new natural products, was investigated through the chemical study of fungal metabolites from both filamentous and zoosporic marine fungi. These studies led to the isolation of a large number of metabolites including a series of glycosphingolipids isolated from a group of previously unexplored marine microorganisms, zoosporic fungi. This work illustrates the potential for examining unusual or previously unexplored marine microbes for new secondary metabolites and further develops marine microorganisms as a source of novel natural products.