UC San Diego

Cyanobacteria are a group of photosynthetic bacteria that play major roles in all major ecosystems and biogeochemical cycles. Cyanobacteria have also been shown to exhibit a remarkable morphological and metabolic diversity. All cyanobacteria also seem to exhibit the rare capacity to make hydrocarbons. The physiological or ecological function of these cyanobacterial hydrocarbons is currently unknown. Interest in finding sustainable sources of hydrocarbons could provide opportunities to replace petroleum derivatives like fuels, plastics and chemicals with sustainably derived molecules with identical properties. Cyanobacterial hydrocarbons are biosynthetically derived from fatty acids. There are two pathways that have recently been described to produce cyanobacterial hydrocarbons. One pathway involves fatty acyl ACP reductase and aldehyde deformylating oxygenase (FAAR/ADO) and is known to produce saturated alkanes one carbon shorter than their fatty acyl ACP substrates. The other is an olefin synthase (OLS) and is a polyketide synthase-type pathway involving elongation/decarboxylation mechanism that is known to produce terminal alkenes that are one carbon longer than their fatty acyl ACP substrate. The evolutionary history of these pathways is unknown. Additionally, the structural variability of the hydrocarbons derived from these pathways is relatively unexplored. This thesis investigates the structural variability, evolutionary history as well as novel biosynthetic pathway variability of cyanobacterial hydrocarbons. Chapter two presents evidence of the involvement of horizontal gene transfer in the evolutionary history of the two hydrocarbon pathways that were identified using a variety of bioinformatic analyses. An investigation of the structural diversity of cyanobacterial hydrocarbons as a consequence of the hydrocarbon pathway from which they are derived is presented in chapter three. Additionally, a variety of biosynthetic mechanisms are considered in the context of newly observed structural features including unique methylation and double bond positions. An effort to identify the methyltransferase responsible for branched hydrocarbon biosynthesis in Anabaena sp. PCC7120 is explored in chapter four. Finally, chapter five frames the major observations from the previous chapters in a broader context and discusses a wide variety of potential future directions for research on cyanobacterial hydrocarbon biosynthesis