Organic matter composition and heterotrophic bacterial activity at declining summer sea ice in the central Arctic Ocean

Judith Piontek,Luisa Galgani,Ilka Peeken,Anja Engel,Eva‐Maria Nöthig

doi:10.1002/lno.11639

Abstract

AbstractThe Arctic Ocean is highly susceptible to climate change as evidenced by rapid warming and the drastic loss of sea ice during summer. The consequences of these environmental changes for the microbial cycling of organic matter are largely unexplored. Here, we investigated the distribution and composition of dissolved organic matter (DOM) along with heterotrophic bacterial activity in seawater and sea ice of the Eurasian Basin at the time of the record ice minimum in 2012. Bacteria in seawater were highly responsive to fresh organic matter and remineralized on average 55% of primary production in the upper mixed layer. Correlation analysis showed that the accumulation of dissolved combined carbohydrates (DCCHO) and dissolved amino acids (DAA), two major components of fresh organic matter, was related to the drawdown of nitrate. Nitrate‐depleted surface waters at stations adjacent to the Laptev Sea showed about 25% higher concentrations of DAA than stations adjacent to the Barents Sea and in the central Arctic basin. Carbohydrate concentration was the best predictor of heterotrophic bacterial activity in sea ice. In contrast, variability in sea‐ice bacterial biomass was largely driven by differences in ice thickness. This decoupling of bacterial biomass and activity may mitigate the negative effects of biomass loss due to ice melting on heterotrophic bacterial functions. Overall, our results reveal that changes in DOM production and inventories induced by sea‐ice loss have a high potential to enhance the bacterial remineralization of organic matter in seawater and sea ice of the Arctic Ocean.

Highlights

Special Issue: Biogeochemistry and Ecology across Arctic Aquatic Ecosystems in the Face of Change
Reactive dissolved organic matter (DOM) in the open ocean is mainly derived from phytoplankton primary production and released by phytoplankton exudation, bacterial particle solubilization, viral lysis of cells, and sloppy feeding by zooplankton (Carlson and Hansell 2014)
Our results reveal an inverse relationship between nitrate and DCCHO and dissolved amino acids (DAA) concentrations, respectively, suggesting that the utilization of inorganic nitrogen for primary production was proportional to the release and subsequent accumulation of semi-labile DOM during the summer season

Summary

Introduction

Special Issue: Biogeochemistry and Ecology across Arctic Aquatic Ecosystems in the Face of Change. The availability of fresh, reactive dissolved organic matter (DOM) derived from phytoplankton exudates, sloppy feeding and viral lysis of phytoplankton cells drives the coupling between heterotrophic bacterioplankton and phytoplankton (Carlson and Hansell 2014). Melting sea ice provides a strong seasonal pulse of DOM into seawater that is used efficiently by bacterial communities in the surface Arctic Ocean (Niemi et al 2014; Holding et al 2017; Underwood et al 2019). Enzymatic hydrolysis and subsequent uptake of organic compounds are considered as redundant functions in bacterial communities, Arctic bacterioplankton showed a small spectrum of polysaccharide-cleaving enzymes that may hamper the access to semi-labile DOM (Arnosti et al 2011). The coupling of heterotrophic bacterial activity to the production and accumulation of organic matter in seawater and sea ice is investigated and potential effects of sea-ice loss on bacterial organic matter recycling in a warmer and fresher future Arctic Ocean are discussed

Methods

Results

Conclusion