Are readily culturable bacteria in coastal North Sea waters suppressed by selective grazing mortality?

Abstract

We studied the growth of six culturable bacterial lineages from coastal North Sea picoplankton in environmental samples under different incubation conditions. The grazing pressure of heterotrophic nanoflagellates (HNF) was reduced either by double prefiltration through 0.8- micro m-pore-size filters or by 10-fold dilutions with 0.2- micro m (pore-size) prefiltered seawater. We hypothesized that those gamma-proteobacterial genera that are rapidly enriched would also be most strongly affected by HNF regrowth. In the absence of HNF, the mean protein content per bacterial cell increased in both treatments compared to environmental samples, whereas the opposite trend was found in incubations of unaltered seawater. Significant responses to the experimental manipulations were observed in Alteromonas, Pseudoalteromonas, and Vibrio populations. No treatment-specific effects could be detected for members of the Roseobacter group, the Cytophaga latercula-C. marinoflava lineage, or the NOR5 clade. Statistical analysis confirmed a transient increase in the proportions of Alteromonas, Pseudoalteromonas, and Vibrio cells at reduced HNF densities only, followed by an overproportional decline during the phase of HNF regrowth. Cells from these genera were significantly larger than the community average in the dilution treatments, and changes in their relative abundances were negatively correlated with HNF densities. Our findings suggest that bacteria affiliated with frequently isolated genera such as Alteromonas, Pseudoalteromonas, and Vibrio might be rare in coastal North Sea picoplankton because their rapid growth response to changing environmental conditions is counterbalanced by a higher grazing mortality.

FIG. 1.

Cell numbers of bacteria and heterotrophic nanoflagellates in the different treatments (mean ± the standard deviation). Hatched bars indicate the daily samples of North Sea picoplankton.

FIG. 2.

Changes in bacterial mean per-cell protein content in the different treatments during the incubation period (mean ± the standard deviation). a.u: arbitrary units. Hatched bars indicate the daily samples of North Sea picoplankton.

FIG. 3.

Relative contributions of the genera Alteromonas, Pseudoalteromonas, and Vibrio to the total microbial assemblages in the different treatments. The solid lines represent the population development as modeled by logistic regression and the dotted lines depict the upper and lower limits of the 95% confidence intervals.

FIG. 4.

Relative contributions of bacteria related to Roseobacter spp., C. latercula-C. marinoflava, and to the NOR5 clade to the total microbial assemblages in the different treatments.

FIG. 5.

Relationship between the cell numbers of heterotophic nanoflagellates and the changes in relative abundances of Alteromonas, Pseudoalteromonas, and Vibrio spp. in the dilution treatments. Negative y-axis values indicate a decrease in community contribution.

FIG. 6.

Mean cell volume (± standard error) of the total FISH-stained bacterial assemblage and of individual populations in the dilution treatments at day 2. The FISH probes were as follows: EUB338 I to III, all bacteria; ALT1413, Alteromonas spp.; PSA143, Pseudoalteromonas spp.; and GV822, Vibrio spp.