A MULTI-YEAR INVESTIGATION OF THE SPECIALIZED METABOLITE COMPOSITION OF TRICHODESMIUM SPP.

The specialized metabolite composition of members of the genus Trichodesmium is understudied in comparison to what is known about its ecological role as a dominant nitrogen fixer in oligotrophic tropical and subtropical waters. The occurrence of large surface accumulations, or blooms, comprised of these filamentous cyanobacteria is increasing globally and these events contain potentially toxic metabolites. Currently there is a lack of a systematic field and laboratory studies with respect to Trichodesmium spp. metabolic capacity. This study aims to determine the specialized metabolite profile of two members of the Trichodesmium genus examining cultivated specimens and environmental collections (from the Gulf of Mexico and North Atlantic Ocean) of both T. erythraeum and T. thiebautii colonies. Chapter one introduces marine natural products with an emphasis on the halogenated specialized metabolites produced by marine cyanobacteria. In chapter two I utilized bioassay- and mass spectrometry-guided isolation to characterize specialized metabolites from an environmental collection of Trichodesmium thiebautii. Identifying the unique isotopic patterns in molecules containing halogen atoms, I isolated and characterized the structure of a dichlorinated metabolite named trichothiazole A. In Chapter three, I used an untargeted metabolomics approach, namely LC-MS/MS-based molecular networking to visualize the chemical space available in Trichodesmium biomass and to guide the isolation of chlorovinylidene-containing compounds from the same Trichodesmium collection examined in Chapter two. These networks showed the large abundance of highly functionalized metabolites, and two mass spectrometry library hits included the previously characterized smenamides A and B. This led to the isolation and complete characterization of additional smenamide analogs C, D, and E. In Chapter four, I continued to utilize LC-MS/MS-based molecular networking along with other targeted and untargeted mass spectrometry approaches to compare the specialized metabolite composition of several Trichodesmium collections. These collections include a time-series collection from the Gulf of Mexico, as well as cultured and field collections of another major Trichodesmium species, Trichodesmium erythraeum. This comparison showed distinct differences amongst species and intriguing similarities in secondary metabolites over time and geographic location in T. thiebautii collections. Overall, the results of this dissertation project showed that T. thiebautii is a prolific producer of specialized metabolites and that most of these metabolites contain the chlorovinylidene moiety that has been only identified in cyanobacterial natural products. The T. thiebautii “metabolome” is consistent over time and space, but intriguingly does not appear to be shared by T. erythraeum