High growth potential of transiently 0.2-μm-filterable bacteria with extracellular protease activity in coastal seawater

Abstract

Bacteria that pass through a 0.2-μm pore-size filter (0.2-μm-filterable bacteria) have been reported from various aquatic environments. It has been recognized that some 0.2-μm-filterable bacteria are “transiently” small/thin cells, which are considered as starvation forms of heterotrophic bacteria, especially in oligotrophic environments. However, whether these “transiently 0.2-μm-filterable bacteria” have roles in organic matter processing in aquatic environments has not yet been well described. Here we found high potential of the transiently 0.2-μm-filterable bacteria with extracellular protease activity in their natural nutritional condition. We conducted microcosm experiments using 0.2-μm-filtered coastal seawater (FSW) without any nutritional amendment, and monitored changes of the cell numbers trapped on a 0.2-μm filter (not 0.2-μm-filterable cells anymore) and their community structure in the FSW microcosms, with extracellular protease activities as indicators of heterotrophic microbial activity. We observed a rapid increase in the 0.2-μm-trapped cell number in the FSW microcosms that originally included only 0.2-μm-filterable microbes. The regenerating 0.2-μm-trapped cells were typical marine bacteria (Alphaproteobacteria, Gammaproteobacteria, Flavobacteriia). Extracellular aminopeptidase activities increased with increasing numbers of 0.2-μm-trapped cells. These results suggest that transiently 0.2-μm-filterable-form bacteria in the original coastal seawater recovered their size and were producing extracellular proteases, which might catalyze organic matter processing in seawater. Since organic nutrients were not added, the size increase might be caused by the reduction in competition from larger bacteria and/or the absence of grazing pressure. Our results demonstrate the growth potential and extracellular protease activity of “transiently 0.2-μm-filterable” bacteria in seawater, usually obscured due to the coexistence of grazers and other bacteria.