Use of organic exudates from two polar diatoms by bacterial isolates from the Arctic Ocean

Global warming affects primary producers in the Arctic, with potential consequences for the bacterial community composition through the consumption of microalgae-derived dissolved organic matter (DOM). To determine the degree of specificity in the use of an exudate by bacterial taxa, we used simple microalgae–bacteria model systems. We isolated 92 bacterial strains from the sea ice bottom and the water column in spring–summer in the Baffin Bay (Arctic Ocean). The isolates were grouped into 42 species belonging to Proteobacteria, Bacteroidetes, Actinobacteria and Firmicutes. Forty strains were tested for their capacity to grow on the exudate from two Arctic diatoms . Most of the strains tested (78%) were able to grow on the exudate from the pelagic diatom Chaetoceros neogracilis , and 33% were able to use the exudate from the sea ice diatom Fragilariopsis cylindrus . 17.5% of the strains were not able to grow with any exudate, while 27.5% of the strains were able to use both types of exudates. All strains belonging to Flavobacteriia ( n = 10) were able to use the DOM provided by C. neogracilis , and this exudate sustained a growth capacity of up to 100 times higher than diluted Marine Broth medium, of two Pseudomonas sp. strains and one Sulfitobacter strain. The variable bioavailability of exudates to bacterial strains highlights the potential role of microalgae in shaping the bacterial community composition. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'.

Physiology of sea ice diatoms. II. Dark survival of three polar diatoms

Three polar sea ice diatoms were exposed to 5 months of darkness at −2 °C to simulate polar winter conditions. Cells were tested monthly for the ability to initiate growth of a population. A critical percentage (~1–100%) of a nonaxenic population of each diatom remained viable after 5 months of dark incubation. Survival was generally enhanced by preconditioning cells with a simulated summer–winter transition (decreasing light and temperature, increasing salinity) or a simple light–dark transition; however, preconditioning was not essential to the survival of sea ice diatom cultures under winter conditions.