Diatom mannan – a sulfated glycan shapes surface ocean microbiomes

Abstract

Diatoms account for 20% of global photosynthesis and secrete up to 50% of their primary products as glycans. The fate of these glycans influences global ocean carbon budgets and exerts substantial selection pressure on the surrounding microbiome. However, current knowledge about secreted diatom glycans is restricted to monosaccharide composition analyses and lacks information regarding glycan configuration, connectivity, and additional modifications such as sulfation. In this dissertation, I describe an anion-exchange chromatography-based approach to isolate a pure anionic mannan from the culture supernatant of a marine diatom. A combination of techniques solved the structure of a sulfated mannan. Microbial growth analyses showed that the mannan serves as a food source for specialized marine bacteria that were identified and isolated during diatom blooms. The genomes of these isolates contained a previously identified gene cluster rich in mannanases and sulfatases. Using heterologous expression, I solved the bacterial degradative cascade for the mannan, which comprised three enzymes that hydrolyzed the mannan in four catalytic steps. The enzymes within the cascade included a novel glycoside hydrolase family and activities for a previously undescribed sulfatase subfamily. Furthermore, an α-mannosidase is the first member within its family that degrades sulfated mannan oligosaccharides. I further exploited the specificity of these enzymes to develop a tool to quantify the glycan in complex mixtures such as seawater. Metagenomic analyses of the TARA Oceans circumnavigation dataset revealed a global abundance of mannan PUL-harboring bacteria, indicating that the mannan modulates microbiomes globally.