Sea‐ice microbial communities in the Central Arctic Ocean: Limited responses to short‐term pCO2 perturbations

Anders Torstensson,Walker O Smith,Andrew R Margolin,Elizabeth H Shadwick,Francesco Bolinesi,Jody W Deming,Gordon M Showalter,Shelly D Carpenter

doi:10.1002/lno.11690

Abstract

AbstractThe Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO2 uptake. We studied how sea‐ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreaker Oden, we addressed microbial growth, production of dissolved organic carbon (DOC) and extracellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea‐ice microbial communities were exposed to elevated partial pressures of CO2 (pCO2). We incubated intact, bottom ice‐core sections and dislodged, under‐ice algal aggregates (dominated by Melosira arctica) in separate experiments under approximately 400, 650, 1000, and 2000 μatm pCO2 for 10 d under different nutrient regimes. The results indicate that the growth of sea‐ice algae and bacteria was unaffected by these higher pCO2 levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO2 enrichment. These central Arctic sea‐ice microbial communities thus appear to be largely insensitive to short‐term pCO2 perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO2 and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea‐ice microorganisms even as the loss of Arctic sea ice continues.

Highlights

Associate editor: Lauren Juranek Special Issue: Biogeochemistry and Ecology across Arctic Aquatic Ecosystems in the Face of Change Edited by: Peter J
We observed no detectable change in the bottom-ice communities we examined in terms of biomass, general bacterial assemblage structure, C/N ratio, photosynthetic activity, Particulate EPS (pEPS) or dissolved organic carbon (DOC) concentration after 10 d of pressure of CO2 (pCO2) treatment up to 2000 μatm (Table 3)
Despite differences between Arctic and Antarctic sea ice and irrespective of the different inorganic nutrient levels and growth responses, the general lack of change in response to pCO2 treatment in our experiments is similar to what has been observed in Antarctic sea-ice microbial communities, where no significant changes in photosynthetic activity or algal and bacterial growth were detected after incubations with pCO2 up to 3700 μatm (McMinn et al 2017; Cummings et al 2019)

Summary

Methods

Experimental setup Sea-ice cores and bottom aggregate communities were collected in the central Arctic Ocean at 88–90N during August and September of the Oden Arctic Ocean 2018 expedition (Fig. 1; Table 1). The morning after the acclimation period (Day 0), four treatments were initiated by injecting CO2-saturated seawater (0.2 μm filtered, from either 10 or 1000 m, depending on the experiment; Table 1) into each of the remaining 16 bags to reach pCO2 of approximately 400, 650, 1000, and 2000 μatm, with four replicates for each treatment (Fig. 2) These treatments approximately simulate the present-day pCO2 (400 μatm), the atmospheric pCO2 in 50 years derived using the present rate of increase (650 μatm), the estimate for the year 2100 (1000 μatm), and an extreme value that has already been observed in situ in Arctic sea ice during spring (2000 μatm; Geilfus et al 2012). The carbonate system and seawater salinity were monitored throughout the experiments by evacuating

Sept 2018

Results

Discussion