Seasonal changes in planktonic bacterivory rates under the ice-covered coastal Arctic Ocean

Bacterivory was determined in surface waters of Franklin Bay, western Arctic, over a seasonal ice-covered period (winter-spring, 2003-2004). The objectives were to obtain information on the functioning of the microbial food web under the ice, during winter (from 21 December 2003 to 21 March 2004) and during spring (from 22 March 2004 to 29 May 2004), and to test whether bacterial losses would increase after the increase in bacterial production following the spring phytoplankton bloom. Chl a concentrations ranged from 0.04 to 0.36 mu g L(-1), increasing in March and reaching a peak in April. Bacterial biomass showed no consistent trend for the whole period, and protist biomass followed a pattern similar to that of Chl a. Bacterial production increased 1 week after Chl a concentrations started to increase, while bacterivory rates increased very slightly. Average bacterivory rates in winter (0.16 +/- 0.07 mu g C L(-1) d(-1)) were not significantly different from those in spring (0.29 +/- 0.24 mu g C L(-1) d(-1)). Average bacterial production, on the other hand, was similar to bacterivory rates in winter (0.19 +/- 0.38 mu g C L(-1) d(-1)), but higher than bacterivory in spring (0.93 +/- 0.28 mu g C L(-1) d(-1)). Therefore, bacterial production was controlled by grazers during winter and by substrate concentration in spring

Functional diversity of bacterioplankton assemblages in western Antarctic seawaters during late spring

Functional diversity and aminopeptidase activity (AMA) in bacterial assemblages were determined in western Antarctic waters during late spring 2002. Functional diversity was assayed by the patterns of sole carbon source utilization in Biolog-ECO Microplates(TM) and AMA with the fluorogenic substrate leucine 7-amido-4-methylcoumarin. N-acetyl-D-glucosamine and D-cellobiose were the most used carbohydrates. This suggested that used dissolved organic carbon (DOC) was mostly of either zoo-or phytoplankton origin. Principal component analysis of the sole carbon source utilization profiles separated the samples according to salinity and temperature. This separation corresponded roughly with the 3 areas of study: Bransfield Strait (BR), Gerlache Strait (GE) and Belling-shausen Sea (BE). AMA was higher in the upper 40 m, probably associated with the higher organic matter load. Phytoplankton biomass was the factor that accounted for the highest variance in AMA, but did not have a clear influence on functional diversity of bacterioplankton. Our findings indicate that differences in functional diversity of bacterioplankton populations in western Antarctic waters are not directly related to phytoplanktonic abundance. This suggests that bacteria could utilize other carbon sources than DOC freshly released by phytoplankton

Bacteria-flagellate coupling in microcosm experiments in the Central Atlantic Ocean.

The coupling between planktonic bacteria and bacterivorous protozoans was examined in microcosm experiments at several oligotrophic and ultra-oligotrophic sites in the subtropical and tropical Atlantic Ocean. Bacterial concentrations at these stations were in the range 2.2-8.1 X 10⁵ cells ml⁻¹, heterotrophic nanoflagellates (HNF) in the range 100-800 cells ml⁻¹, bacterial doubling times (estimated from leucine incorporation) in the range 1-100 days, and chlorophyll a levels in the range 0.03-0.36 μg l⁻¹. The experimental uncoupling of the microbial loop by differential filtrations did not result in an increased growth and grazing by nanoflagellates despite a stimulation and increase of bacterial abundance and mean cell volume due to the bottle incubations. A strong response of the grazer population occurred after increasing bacterial numbers about 10-fold by the addition of a complex substrate source (yeast extract). Bacteria responded immediately to the substrate enrichment with an increase in mean cell size and abundance, and reached stationary phase already after about 24 h. In contrast, HNF development showed a pronounced lag phase, and it needed between 3 and 7 days until grazers reduced bacterial numbers to about the initial values. The grazing impact on the bacterial assemblage in the bottles resulted in feed-back effects that resembled those known from other, more productive systems: protozoan size-selective grazing removed preferentially larger sized bacteria and shifted the size-distribution towards the initial, natural situation with a dominance of small cocci. Grazing-resistant morphotypes consisted of bacterial aggregates embedded in a polysaccharide matrix whereas filamentous forms did not develop. These experiments provide evidence that bacterial assemblages have the capacity to respond to enhanced substrate availability (for example in micropatches) and to utilise these substrates without significant grazer control

Differential response of grazing and bacterial heterotrophic production to experimental warming in Antarctic waters

Narrow annual ranges of temperature characterize polar waters. Consequently, small increases in temperature could significantly affect the metabolic processes of marine microorganisms. We investigated the response of bacterial heterotrophic production (BHP) and grazing rates to small temperature changes in 3 zones near the western Antarctic Peninsula-Bransfield and Gerlache Straits, and Bellingshausen Sea-during December 2002. We performed 8 grazing experiments with water samples collected from depths where chlorophyll a (chl a) concentration was maximum, and incubated the samples at ambient temperature and at -1, 1, 2 and 5 degrees C. We expected that grazing would increase in parallel with BHP at increasing temperatures; however, temperature differentially affected these 2 microbial activities. Thus, grazing rates increased maximally at temperatures <= 2 degrees C, except in 1 station in the Gerlache Strait, while BHP increased maximally at temperatures <= 2 degrees C, except in 1 station in the Bellingshausen Sea. The percentage of grazed bacteria to BHP at the highest experimental temperatures was low (56 +/- 19%) in the Gerlache Strait, high (395 +/- 137%) in the Bransfield Strait and approximately balanced (97 +/- 24%) in the Bellingshausen Sea. This suggests that differential microbial processes in each zone at increasing temperatures will also depend on the autochthonous community. The present study contributes to the understanding of the variability of polar biogeochemical fluxes, and may aid in predicting the response of microorganisms in future scenarios with local and seasonal changes in temperature

Tipping Elements in the Arctic Marine Ecosystem

The Arctic marine ecosystem contains multiple elements that present alternative states. The most obvious of which is an Arctic Ocean largely covered by an ice sheet in summer versus one largely devoid of such cover. Ecosystems under pressure typically shift between such alternative states in an abrupt, rather than smooth manner, with the level of forcing required for shifting this status termed threshold or tipping point. Loss of Arctic ice due to anthropogenic climate change is accelerating, with the extent of Arctic sea ice displaying increased variance at present, a leading indicator of the proximity of a possible tipping point. Reduced ice extent is expected, in turn, to trigger a number of additional tipping elements, physical, chemical, and biological, in motion, with potentially large impacts on the Arctic marine ecosystem

Antarctic sea ice region as a source of biogenic organic nitrogen in aerosols

Climate warming affects the development and distribution of sea ice, but at present the evidence of polar ecosystem feedbacks on climate through changes in the atmosphere is sparse. By means of synergistic atmospheric and oceanic measurements in the Southern Ocean near Antarctica, we present evidence that the microbiota of sea ice and sea ice-influenced ocean are a previously unknown significant source of atmospheric organic nitrogen, including low molecular weight alkyl-amines. Given the keystone role of nitrogen compounds in aerosol formation, growth and neutralization, our findings call for greater chemical and source diversity in the modelling efforts linking the marine ecosystem to aerosol-mediated climate effects in the Southern Ocean