Comparative proteomics of Alteromonas macleodii cultured with sulfoquinovose

Sulfoquinovose (SQ) and sulfoquinovosyl glycerol (SQGro), derived from abundant membrane sulfolipids termed sulfoquinovosyl diacylglycerols (SQDG) produced by photosynthetic organisms, serve as sources of carbon and sulfur for bacteria. The conversion processes of these sulfoquinovosyl compounds (SQ, SQGro, and SQDG) within marine ecosystems, and their quantitative contributions to the marine organic matter pool, are poorly understood. We have identified Alteromonas macleodii, a cosmopolitan marine bacterium, as a novel organism capable of metabolizing SQ and SQGro. A. macleodii possesses a sulfoquinovosidase that converts SQGro to SQ, and is a member of a distinct clade within glycoside hydrolase family 31 distinct from other sulfoqinovosidases. The ubiquitous presence of sulfoquinovosidases and their transcripts throughout marine environments suggests active metabolism of sulfoquinovose glycosides, particularly in the sunlit surface ocean. Complementing these observations, we demonstrate that marine algae and cyanobacteria produce significant quantities of SQGro, and field samples from coastal and open ocean environments enabled estimation of the annual turnover of SQGro in the teragram range. Together with SQDG and SQ, these sulfoquinovosyl compounds constitute a substantial portion of the marine organic sulfur, estimated at around 1.5 petagrams of carbon turnover per annum. These findings reveal a vast, previously unappreciated pool of organosulfonates within the microbial food web that contributes significantly to the marine carbon and sulfur cycles.