Marine BacteriaAlteromonasspp. Require UDP-glucose-4-epimerase for Aggregation and Production of Sticky Exopolymer

Abstract

AbstractThe physiology and ecology of particle-associated marine bacteria are of growing interest, but our knowledge of their aggregation behavior and mechanisms controlling their association with particles remains limited. We have found that a particle-associated isolate,Alteromonassp. ALT199 strain 4B03, and the related type-strainA. macleodii27126 both form large (>500 μm) aggregates while growing in rich medium. A non-clumping variant (NCV) of 4B03 spontaneously arose in the lab, and whole genome sequencing revealed a partial deletion in the gene encoding UDP-glucose-4-epimerase (galEΔ308-324). In 27126, a knock-out ofgalE(ΔgalE::kmr) resulted in a loss of aggregation, mimicking the NCV. Microscopic analysis shows that both 4B03 and 27126 rapidly form large aggregates, whereas their respectivegalEmutants remain primarily as single planktonic cells or clusters of a few cells. Strains 4B03 and 27126 also aggregate chitin particles, but theirgalEmutants do not. Alcian Blue staining shows that 4B03 and 27126 produce large transparent exopolymer particles (TEP), but theirgalEmutants are deficient in this regard. This study demonstrates the capabilities of cell-cell aggregation, aggregation of chitin particles, and production of TEP in strains ofAlteromonas, a widespread particle-associated genus of heterotrophic marine bacteria. A genetic requirement forgalEis evident for each of the above capabilities, expanding the known breadth of requirement for this gene in biofilm-related processes.ImportanceHeterotrophic marine bacteria have a central role in the global carbon cycle. Well-known for releasing CO2by decomposition and respiration, they may also contribute to particulate organic matter (POM) aggregation, which can promote CO2sequestration via the formation marine snow. We find that two members of the prevalent particle-associated genusAlteromonascan form aggregates comprising cells alone or cells and chitin particles, indicating their ability to drive POM aggregation. In line with their multivalent aggregation capability, both strains produce TEP, an excreted polysaccharide central to POM aggregation in the ocean. We demonstrate a genetic requirement forgalEin aggregation and large TEP formation, building our mechanistic understanding of these aggregative capabilities. These findings point toward a role for heterotrophic bacteria in POM aggregation in the ocean and support broader efforts to understand bacterial controls on the global carbon cycle based on microbial activities, community structure, and meta-omic profiling.