High growth potential and activity of 0.2 µm filterable bacteria habitually present in coastal seawater

Abstract. The presence of 0.2 µm filterable bacteria in aquatic environments has been known. Some of these bacteria have been reported to be starvation forms, especially those in oligotrophic oceanic seawater. However, 0.2 µm filterable bacteria have not yet been described in temperate coastal seawater. Here, we report the presence of 0.2 µm filterable bacteria in coastal seawater with their high growth potential that appeared under the absence of grazers. In this study, filtered seawater (FSW) microcosms were prepared with 0.2 µm filtered coastal seawater collected in summer and winter without any nutritional amendment and incubated at the ambient seawater temperature (25 °C in summer, 12 °C in winter) and lower temperature (6 °C). During the first several days of the incubations of FSW, the number of prokaryotes collected on 0.2 µm filters markedly increased especially at the ambient seawater temperatures. The diversity of the regenerated bacteria community was comparable to that of the original bacterial community, and most of the identified bacteria were typical marine bacteria (members of Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes), indicating that the 0.2 µm filterable forms of typical marine bacteria show rapid growth under the no grazing and low competition conditions present in the FSW bottles. These results suggest that 0.2 µm filterable bacteria are habitually present even in coastal water, and that these bacteria are always ready for growing in changeable aquatic ecosystems.