Dominant marine bacterioplankton species found among colony-forming bacteria

TLDR

The study focused on diverse marine bacteria spanning alpha, beta, and gamma Proteobacteria and the Cytophaga–Flexibacter group. The authors isolated 48 bacteria from the northern Baltic Sea over a year and quantified their abundance using whole‑genome DNA hybridization against community DNA. Colonies on solid media represented 7–69 % of total bacteria (29–111 % of nucleoid cells), indicating that a substantial fraction of bacterioplankton can form colonies and that their seasonal dynamics mirror phytoplankton, while 16S rRNA analysis revealed low similarity to known sequences, underscoring the unexplored diversity.

Abstract

The density of specific aquatic bacteria was determined by use of whole-genome DNA hybridization towards community DNA. From a coastal marine environment (northern Baltic Sea), 48 specific bacteria were isolated on solid media over a 1-year period. Based on the presented hybridization protocol, the total density of the isolates ranged between 7 and 69% of the bacteria determined by acridine orange direct counts. When compared to the number of nucleoid-containing cells, the range increased to 29 to 111%. Thus, our results showed that bacteria able to form colonies on solid media accounted for a large fraction of the bacterioplankton. There were significant changes in the density of the different bacteria over the year, suggesting that bacterioplankton exhibit a seasonal succession analogous to phytoplankton. The bacteria studied were of diverse phylogenetic origin, being distributed among the alpha, beta, and gamma subdivisions of the class Proteobacteria and the cytophaga-flexibacter group. Partial 16S rRNA gene sequence analysis of 29 Baltic Sea isolates as well as of 30 Southern California Bight isolates showed that a majority of the isolates had low similarity (0.85 to 0.95) to reported sequence data. This indicated that the diversity of marine bacteria able to grow on solid media is largely unexplored.