Nitrogen Fixation in a Changing Arctic Ocean: An Overlooked Source of Nitrogen?

Lisa W Von Friesen,Lasse Riemann

doi:10.3389/fmicb.2020.596426

Abstract

The Arctic Ocean is the smallest ocean on Earth, yet estimated to play a substantial role as a global carbon sink. As climate change is rapidly changing fundamental components of the Arctic, it is of local and global importance to understand and predict consequences for its carbon dynamics. Primary production in the Arctic Ocean is often nitrogen-limited, and this is predicted to increase in some regions. It is therefore of critical interest that biological nitrogen fixation, a process where some bacteria and archaea termed diazotrophs convert nitrogen gas to bioavailable ammonia, has now been detected in the Arctic Ocean. Several studies report diverse and active diazotrophs on various temporal and spatial scales across the Arctic Ocean. Their ecology and biogeochemical impact remain poorly known, and nitrogen fixation is so far absent from models of primary production in the Arctic Ocean. The composition of the diazotroph community appears distinct from other oceans – challenging paradigms of function and regulation of nitrogen fixation. There is evidence of both symbiotic cyanobacterial nitrogen fixation and heterotrophic diazotrophy, but large regions are not yet sampled, and the sparse quantitative data hamper conclusive insights. Hence, it remains to be determined to what extent nitrogen fixation represents a hitherto overlooked source of new nitrogen to consider when predicting future productivity of the Arctic Ocean. Here, we discuss current knowledge on diazotroph distribution, composition, and activity in pelagic and sea ice-associated environments of the Arctic Ocean. Based on this, we identify gaps and outline pertinent research questions in the context of a climate change-influenced Arctic Ocean – with the aim of guiding and encouraging future research on nitrogen fixation in this region.

Highlights

The Arctic Ocean (AO) covers only ~4% of the global ocean surface, but accounts for up to 10–14% of the total oceanic carbon dioxide sink (Bates and Mathis, 2009; Manizza et al, 2019)
We argue that biological nitrogen fixation (BNF) is a hitherto overlooked process and acquisition of basic knowledge on distribution, activity, and ecological drivers of diazotrophy is imperative for analyses of N and carbon biogeochemistry in the current and future AO
We provide a set of pertinent research questions aiming to guide and inspire future research on diazotrophy in the AO, in particular in the light of climate change (Box 1)

Summary

Introduction

The Arctic Ocean (AO) covers only ~4% of the global ocean surface, but accounts for up to 10–14% of the total oceanic carbon dioxide sink (Bates and Mathis, 2009; Manizza et al, 2019). Diazotrophs have been detected in pelagic and sympagic environments of the AO under wide-ranging environmental conditions (Figure 1; Supplementary Table S1), reaching from ice-free surface waters (e.g., Harding et al, 2018), estuaries (Blais et al, 2012; Sipler et al, 2017), and aphotic mesopelagic waters (Salazar et al, 2019), to sea ice brine (Díez et al, 2012), frost flowers (Bowman et al, 2014), sea ice melt-ponds, and algal aggregates (Fernández-Méndez et al, 2016).

Results

Conclusion