Differing responses of marine N2 fixers to warming and consequences for future diazotroph community structure

Abstract

The globally distributed colonial cyanobacterium Trichodesmium and unicellular diazotrophs including Crocosphaera together carry out the majority of marine biological nitrogen (N 2 ) fixation. Future sea surface warming is predicted to influence their abundance and distribution, but temperature reaction norms have been determined for very few representatives of each genus. We compared thermal responses within and between the 2 genera Trichodesmium and Crocosphaera by measuring reaction norms for growth, N 2 fixation, carbon fixation, and elemental ratios in 7 strains from a global culture collection. Temperature reaction norms of Trichodesmium and Crocosphaera were remarkably similar for all isolates within each genus, regardless of their geographic origin. Thermal limits of Trichodesmium and Crocosphaera ranged from 18 to 32°C and 24 to 32°C, and optimum growth temperatures were ~26 and ~30°C, respectively. The highest cellular ratios of nitrogen to phosphorus and carbon to nitrogen were found at optimum growth temperatures, and the lowest ratios near their thermal limits. In a mixed competition experiment, Trichodesmium growth rates were ~25% higher than those of Crocosphaera at 24°C, while those of Crocosphaera were ~50% higher at 28°C. Comparison of these results to current and projected seasonal temperature regimes in the subtropical Atlantic and Pacific Oceans suggests that predicted warmer temperatures may favor Crocosphaera over Trichodesmium, but that both genera may be excluded where future temperatures consistently exceed 32°C. Sea surface warming could profoundly alter the community structure and stoichiometry of marine N 2 -fixing cyanobacteria, thus fundamentally changing the biogeochemical cycling of this globally significant source of new nitrogen. Resale or republication not permitted without written consent of the publisher

Individual and interactive effects of warming and CO<sub>2</sub> on <i>Pseudo-nitzschia subcurvata</i> and <i>Phaeocystis antarctica</i>, two dominant phytoplankton from the Ross Sea, Antarctica

“…In the Ross Sea as elsewhere, temperature determines both phytoplankton maximum growth rates (Bissinger et al, 2008) and the upper limit of growth (Smith and Sakshaug, 1990) in a species-specific manner. Thermal functional response curves of phytoplankton typically increase in a normally distributed pattern, with growth rates increasing up to the optimum temperature range, and then declining when temperature reaches inhibitory levels (Boyd et al, 2013;Fu et al, 2014;Xu et al, 2014;Hutchins and Fu, 2017). Specific growth rates of P. subcurvata reached optimal levels at 8 • C, demonstrating that this species grows fastest at temperatures substantially above any temperatures found in the present-day Ross Sea.…”

Ideas and perspectives: climate-relevant marine biologically driven mechanisms in Earth system models

“…In particular, a moderate rise in sea surface temperature (Fu et al, 2014), as well as a decrease in pH, will favor their growth conditions (Hutchins et al, 2007). More cyanobacteria will intensify the biogeophysical feedback mechanisms and possibly the particle shuttle.…”

Phytoplankton adapt to changing ocean environments

“…Phytoplankton species have short generation times and large population sizes, so they may be particularly able to adapt to rapid climate change (20,21). In addition, temperature response curves measured in the laboratory show that phytoplankton usually have the fastest growth rates at or slightly below the mean temperature of the environment they were isolated from, suggesting that natural populations are adapted to their local environment (15,22), although some species have niches that do not reflect the environmental conditions from which they were isolated (23). Evolutionary experiments in the laboratory indicate that phytoplankton species have the capacity to evolve over hundreds to thousands of generations in response to single environmental factors; specifically, changes in CO 2 concentration or temperature (24)(25)(26)(27)(28)(29).…”