Antarctic marine bacterioplankton subpopulations discriminated by their apparent content of nucleic acids differ in their response to ecological factors

Abstract

Bacterial abundances determined in Drake Passage and Bransfield and Gerlache Straits (Antarctica) in the Austral summer ranged from 0.78 to 9.4·105 cells ml 1 , and were positively correlated with standing stocks of Chl a. Two bacterial subpopulations were discriminated based in their different levels of green fluorescence and wide angle light scatter (SSC) per cell after SYTO-13 staining for the first time in Antarctic waters. High nucleic acid (HNA) and low nucleic acid (LNA) subpopulations differed considerably in their response to changes in environmental variables. The apparent content of nucleic acids per cell for the HNA subpopulation (FL1-HNA) showed vertical profiles similar to those of Chl a, including the presence of a maximum at the subsurface chlorophyll maximum. FL1- HNA was positively correlated with Chl a. No similar trends were observed for the LNA fraction. HNA and LNA subpopulations differed in the response of the wide angle light scatter signal to environmental factors as well. SSC-HNA decreased strongly with depth and was positively correlated with Chl a. Again, no similar trends were observed for the LNA subpopulation. The percentage of HNA cells (%HNA) ranged between 35.0 and 76.7% and showed a general tendency to increase with depth. This increase seemed to be larger when the stratification of the water column was higher. Differences in grazing pressure could be responsible of the unexpected vertical distribution of HNA cells. Our results shows that in situ LNA and HNA bacterioplankton subpopulations are under different ecological controls and likely to play different trophodynamic roles in Antarctic waters.