A specific tripartite tricarboxylate transporter is involved in alginate utilization by <i>Vibrio</i> sp. C42

ABSTRACT Alginate is an important component of the cell wall of brown algae and is mainly utilized by bacteria in the oceans. Although many marine Vibrio strains are known alginate utilizers, their mechanisms for alginate utilization have not been fully understood. Here, we investigated the alginate utilization pathway of the marine bacterium Vibrio sp. C42, an alginate lyase-excreting strain collected from the surface of Sargassum . Based on genomic, transcriptomic, and proteomic analyses, an alginate utilization locus (AUL) responsible for alginate utilization was identified in Vibrio sp. C42. In the alginate utilization pathway, eight extracellular alginate lyases break down alginate into oligosaccharides. These oligosaccharides are then internalized through an outer membrane porin and two inner membrane transporters: sodium-solute symporter (SSS) and tripartite tricarboxylate transporter (TTT). Biochemical and structural analyses demonstrated that the TTT is novel and specific to alginate oligosaccharides. Moreover, bioinformatic analysis suggests that TTT is widely involved in alginate utilization of marine bacteria. These findings reveal a new TTT-involved alginate utilization pathway in marine Vibrio and other bacteria, shedding light on alginate metabolism in the sea. IMPORTANCE Due to the huge quantity of brown algae in global oceans, alginate is an important organic carbon source in marine ecosystems. Alginate is utilized mainly by bacteria, becoming an important component of marine carbon cycling. Marine Vibrio strains play an important role in marine alginate utilization. Here, we reveal a novel TTT (tripartite tricarboxylate transporter) involved in the alginate utilization pathway in marine Vibrio sp. C42. Moreover, the TTT was identified as a novel protein specific to the transport of alginate oligosaccharides. Bioinformatic analysis further indicated its universality in the AULs of marine Vibrio and other bacteria, suggesting that TTT is widely involved in alginate utilization by marine bacteria. Therefore, this TTT-dependent alginate utilization pathway may have significance in marine alginate metabolism and cycling.