Biocatalytic quantification of laminarin : a major carbohydrate polymer in the ocean

Abstract

In this work, I provide evidence that laminarin, the energy storage glucan of marine diatoms and many other algae, is a central bioenergy molecule of the ocean. This conclusion stems from the quantification of this molecule with new technology that I developed during my thesis. This biocatalytic assay is based on specific enzymes from marine microbes. This first quantification yielded an average contribution of laminarin to the carbon in algae derived organic matter of 37- 19% in the environment. The work contributes to our general understanding of the marine carbon cycle in the surface water of the ocean. In this environment, microalgae sustain approximately half of the global primary production and yield significant amounts of organic carbon in the form of polysaccharides, e.g. laminarin. So far, the role of this class of biological macromolecules in the carbon cycle is poorly understood due to the technological challenges in their analysis. The quantification of a single marine polysaccharide on a broad scale has therefore never been done. Our new approach starts to close this gap of knowledge and technology. It makes use of the enzymatic toolkit from marine microbes, that evolved specific enzymes in order to gain energy and carbon from the abundant carbohydrates. I demonstrate the advantages and the further potential of these enzymes for a faster, stereo- and sequence-specific analysis of selected polysaccharides in marine organic matter. In order to promote its further application, the method is now accessible for researchers in all fields of environmental science and can be easily applied to quantify laminarin in particulate organic matter from the marine environment. The characterization of the microbial machinery for the degradation of laminarin was required for the method development. However, it also contributed to the in-depth investigations that were made on two strains of specialized laminarin degrading bacteria. Environmental proteomics and metagenomics in combination with cultivation experiments, uptake visualization and our biochemical and crystallographic characterizations revealed streamlined bacteria that reach high abundances during repeating algal bloom events but also dominate the laminarin turnover in a highly competitive manner. Finally, I applied our new laminarin quantification method to environmental samples. I used particulate organic matter from two environmental time series, in different size fractions, vertical profiles, marine snow particles and a meridional transect in oceanic regions ranging from the Arctic, the North-, Central- and South-Atlantic, the coastal Pacific and the North Sea. Our measurements allow a more accurate estimation of the global annual laminarin production of 18- 9 gigatons. The varying levels of laminarin indicated different bioenergetic states of the oceanic regions and the ecological relevance of this molecule was highlighted by its sheer abundance of more than one third of the particulate organic carbon in surface waters.