Bacterial sulfoquinovose cycling across aquatic ecosystems

Sulfoquinovosyldiacylglycerol (SQDG) is a ubiquitous sulfolipid of phototroph membranes, including those of oxygenic phototrophs and some anoxygenic phototrophic bacteria, and its headgroup sulfoquinovose (SQ) represents one of the most abundant organosulfur compounds in the biosphere. Global SQ production is estimated at ∼10 Pg yr-1, with marine systems alone contributing ∼1.5 Pg C yr-1 in SQ-containing compounds. In marine systems, heterotrophic processing of SQ is increasingly well characterized, whereas the pathways governing SQ transformation in freshwater environments remain poorly resolved. SQ enters both marine and freshwater systems primarily through turnover of phototrophic biomass, with freshwater systems also receiving terrestrial plant-derived sulfolipids. Although phototrophs synthesize SQDG for membrane function, cyanobacteria appear to have limited capacity for SQ catabolism, and SQ turnover is largely mediated by heterotrophic microorganisms. This functional partitioning establishes a metabolic relay in which distinct microbial guilds mediate successive steps of SQ processing, from extracellular sulfolipid hydrolysis to intracellular sulfoglycolysis and terminal sulfur mineralization. Here, we integrate current knowledge of the enzymatic pathways and microbial interactions underpinning SQ transformation within their ecological context. We propose that SQ metabolism constitutes an important, yet underappreciated, axis of aquatic sulfur biogeochemistry, particularly in freshwater systems where cyanobacterial production of dimethylsulfoniopropionate is minimal.