Phytoplankton Responses to Bacterially Regenerated Iron in a Southern Ocean Eddy.

In the Subantarctic sector of the Southern Ocean, vertical entrainment of iron (Fe) triggers the seasonal productivity cycle but diminishing physical supply during the spring to summer transition forces microbial assemblages to rapidly acclimate. Here, we tested how phytoplankton and bacteria within an isolated eddy respond to different dissolved Fe (DFe)/ligand inputs. We used three treatments: one that mimicked the entrainment of new DFe (Fe-NEW), another in which DFe was supplied from bacterial regeneration of particles (Fe-REG), and a control with no addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold (Fe-REG) increases in Chlorophyll a were observed. These responses of the phytoplankton community were best explained by the differences between the treatments in the amount of DFe recycled during the incubation (Fe-REG, 15% recycled c.f. 40% Fe-NEW, 60% Fe-NO). This additional recycling was more likely mediated by bacteria. By day 6, bacterial production was comparable between Fe-NO and Fe-NEW but was approximately two-fold higher in Fe-REG. A preferential response of phytoplankton (haptophyte-dominated) relative to high nucleic acid (HNA) bacteria was also found in the Fe-REG treatment while the relative proportion of diatoms increased faster in the Fe-NEW and Fe-NO treatments. Comparisons between light and dark incubations further confirmed the competition between picophytoplankton and HNA for DFe. Overall, our results demonstrate great versatility by microorganisms to use different Fe sources that results in highly efficient Fe recycling within surface waters. This study also encourages future research to further investigate the interactions between functional groups of microbes (e.g. HNA and cyanobacteria) to better constraint modeling in Fe and carbon biogeochemical cycles.

Viral-mediated lysis of microbes and carbon release in the sub-Antarctic and Polar Frontal zones of the Australian Southern Ocean.

Viral production was determined in the sub-Antarctic zone (SAZ) to the southwest and southeast of Tasmania and in the Polar Frontal zone (PFZ) of the Australian sector of the Southern Ocean during Austral summer (January-February 2007). Concentrations of viruses were the lowest (6.6 x 10(9) particles l(-1)) in the PFZ and the highest (2.1 x 10(10) particles l(-1)) in the eastern SAZ where nutrient input from the East Australian Current (EAC) sustained higher concentrations of bacteria and bacterial production relative to the west. Rates of viral production in the PFZ (1.8 x 10(10) viruses l(-1) day(-1)) were lower than those in the western SAZ (2.5 x 10(10) viruses l(-1) day(-1)). Viral production in the eastern SAZ (2.2 x 10(11) viruses l(-1) day(-1)) was the highest recorded and was approximately one order of magnitude higher than at the other sites. In the western SAZ and PFZ, the percentage of available bacterial biomass lysed by viruses was similar (23.5% and 23% respectively) equating to the release of 3.3 and 2.3 microg carbon l(-1) day(-1) respectively (assuming a burst size of 50 viruses host(-1)). In the eastern SAZ the potential bacterial biomass lysed was higher (on average 40%) and corresponded to the release of 26.5 microg carbon l(-1) day(-1). These findings suggest the importance of the viral shunt in carbon cycling within these regions.

Influence of environmental conditions, bacterial activity and viability on the viral component in 10 Antarctic lakes.

The influence of biotic and environmental variables on the abundance of virus-like particles (VLP) and lysogeny was investigated by examining 10 Antarctic lakes in the Vestfold Hills, Antarctica, in the Austral Spring. Abundances of viruses and bacteria and bacterial metabolic activity were estimated using SYBR Gold (Molecular Probes), Baclight (Molecular Probes) and 6-carboxy fluorescein diacetate (6CFDA). Total bacterial abundances among the lakes ranged between 0.12 and 0.47 x 10(9) cells L(-1). The proportion of intact bacteria (SYTO 9-stained cells) ranged from 13.5% to 83.5% of the total while active (6CFDA-stained) bacteria ranged from 33% to 116%. Lysogeny, as determined with Mitomycin C, was only detected in one of the lakes surveyed, indicating that viral replication was occurring predominantly via the lytic cycle. Principal component analysis and confirmatory correlation analysis of individual variables showed that high abundances of VLP occurred in lakes of high conductivity with high concentrations of soluble reactive phosphorus and dissolved organic carbon. These lakes supported high concentrations of chlorophyll a, intact bacteria, rates of bacterial production and virus to bacteria ratios. Thus, it was suggested that viral abundance in the Antarctic lakes was determined by the trophic status of the lake and the resultant abundance of intact bacterial hosts.