The rate and fate of N2 and C fixation by marine diatom-diazotroph symbioses

Rachel A Foster,Angelicque E White,Sten Littmann,Daniela Tienken,Marcel M M Kuypers,Martin J Whitehouse

doi:10.1038/s41396-021-01086-7

Abstract

N2 fixation constitutes an important new nitrogen source in the open sea. One group of filamentous N2 fixing cyanobacteria (Richelia intracellularis, hereafter Richelia) form symbiosis with a few genera of diatoms. High rates of N2 fixation and carbon (C) fixation have been measured in the presence of diatom-Richelia symbioses. However, it is unknown how partners coordinate C fixation and how the symbiont sustains high rates of N2 fixation. Here, both the N2 and C fixation in wild diatom-Richelia populations are reported. Inhibitor experiments designed to inhibit host photosynthesis, resulted in lower estimated growth and depressed C and N2 fixation, suggesting that despite the symbionts ability to fix their own C, they must still rely on their respective hosts for C. Single cell analysis indicated that up to 22% of assimilated C in the symbiont is derived from the host, whereas 78–91% of the host N is supplied from their symbionts. A size-dependent relationship is identified where larger cells have higher N2 and C fixation, and only N2 fixation was light dependent. Using the single cell measures, the N-rich phycosphere surrounding these symbioses was estimated and contributes directly and rapidly to the surface ocean rather than the mesopelagic, even at high estimated sinking velocities (<10 m d−1). Several eco-physiological parameters necessary for incorporating symbiotic N2 fixing populations into larger basin scale biogeochemical models (i.e., N and C cycles) are provided.

Highlights

In large expanses of the sunlit open ocean, concentrations of bioavailable nitrogen (N) are low to below analytical detection
The Rhizosolenia-Richelia symbioses have been more commonly reported in the North Pacific gyre [26, 27, 31], and the western tropical North Atlantic (WTNA) near the Amazon and Orinoco River plumes is an area where widespread blooms of the H. hauckiiorientation purposes during the secondary ion mass spectrometry (SIMS) analyses and for post-processing using look@nanoSIMS [38] and WinImage2 (CAMECA) software packages
Richelia symbioses at multiple light levels, we plotted the individual rates of N2 and C fixation as a function of photosynthetically active radiation (PAR) measured at the time of collection (Fig. 3)

Summary

Introduction

In large expanses of the sunlit open ocean, concentrations of bioavailable nitrogen (N) are low to below analytical detection. High rates of N2 and C fixation have been measured in bulk water field incubations when Richelia symbioses are present [26,27,28,29].

Results

Conclusion