Environmental and structural drivers of algal glycan persistence in the ocean

Despite the diverse biological roles of glycans as structural, signalling, and protective molecules, environmental glycobiology has historically focussed on the energy storage glycans produced by eukaryotic photosynthesizers. Structure-function-location relationships dictate that energy storage glycans are simple, intracellular molecules, while at the other end of the spectrum are the highly complex extracellular matrix glycans (ECM) with protective functions. The structural simplicity of energy storage glycans means that bacterial degradation is rapid. Complex ECM glycans on the other hand are forced by evolution of symbiotic and antagonistic interactions to remain difficult for bacteria to degrade; these glycans therefore have the highest environmental stability, and therefore the greatest potential for carbon sequestration. The accumulation of glycans in the ocean as quantitatively important components of dissolved and particulate matter supports that glycans sequester carbon in the ocean. However, detailed understanding of their production by algae and turnover by heterotrophic microorganisms remains lacking, in part due to the challenges inherent to analysis of complex glycans. This thesis developed approaches for the extraction of glycans from seawater and macroalgae (Chapters 4, 6, and 8), as well as for their analysis with mass spectrometry (Chapters 3 and 7) in combination with other analytics. Application of these approaches constrained the secretion rates (Chapters 5 and 6) and structural variation (Chapter 8) of the complex fucose-containing sulfated polysaccharides (FCSPs) found in the cell wall and ECM of brown algae. Inter- (Chapter 5) and intra-species (Chapter 8) differences in the structures of FCSPs were found to challenge microbial degradation. These knowledge advancements are critical given the important role that nature-based solutions will play in mitigating the climate crisis, and pave the way for future studies of ECM glycans of other key photosynthesizers.