The biogeochemical implications of the contrasting responses of iron-limited N2-fixing cyanobacteria to ocean warming

Nina Yang1, Carlin A Merkel2, Yu-An Lin2, Naomi Marcil Levine3, Sara Rivero-Calle4, Hai-bo Jiang5, Nicholas Hawco6, Pingping Qu2, Feixue Fu7 and David A Hutchins7, (1)Woods Hole Oceanographic Institution, Marine Policy Center, Woods Hole, United States, (2)University of Southern California, Los Angeles, CA, United States, (3)University of Southern California, Los Angeles, United States, (4)University of North Carolina at Wilmington, Wilmington, NC, United States, (5)Ningbo University, China, (6)University of Southern California, Earth Sciences, Los Angeles, CA, United States, (7)University of Southern California, Department of Biological Sciences, Los Angeles, United States
Abstract:
Climate change threatens the stability and resiliency of marine ecosystems and will alter the distribution and activity of important microbial communities. Marine nitrogen fixers (N2-fixers) are distributed throughout the iron (Fe) and nitrogen-limited, subtropical and tropical ocean gyres, where they convert inert N2 gas into bioavailable forms essential to supporting primary productivity. Both temperature and Fe availability are recognized constraints on marine N2-fixation, and both are changing in the current ocean due to anthropogenic forcing. However, the interactions between these two controls on N2-fixers remain unclear. We experimentally examined the physiological responses of the two marine N2-fixers, Trichodesmium and Crocosphaera, to a warmer future ocean in concert with growth-limiting Fe availability. Fe-limited Trichodesmium exhibited a ~5°C shift in optimum growth temperature from 27°C to 32°C, with a corresponding increase in N2-fixation compared to Fe-replete cultures. Conversely, cellular Fe content decreased, driving increase in Iron Use Efficiencies (IUEs, mol N2 fixed hour-1 mol Fe-1) that enable Trichodesmium to more efficiently leverage Fe under warmer temperatures. In contrast, Crocosphaera maintained the same optimal growth temperature of 27°C across Fe-replete and Fe-limited conditions, but exhibited growth and N2-fixation enhancement at the lower end of its thermal range, from 22°C to 27°C. Additionally, the IUEs of Fe-limited Crocosphaera increased by 66% relative to Fe-replete cultures, suggesting it may be better adapted to low-Fe conditions than Trichodesmium. Modeling these results in the context of the IPCC RCP 8.5 warming scenario predicts that IUEs of N2-fixers could increase 44%-76% by 2100, alleviating prevailing Fe-limitation across large parts of the oligotrophic ocean. Resulting increases in future global marine N2-fixation may profoundly transform existing paradigms of Fe and nitrogen biogeochemistry in open ocean regimes.
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