Two co‐dominant nitrogen‐fixing cyanobacteria demonstrate distinct acclimation and adaptation responses to cope with ocean warming.

Summary: The globally dominant N2‐fixing cyanobacteria Trichodesmium and Crocosphaera provide vital nitrogen supplies to subtropical and tropical oceans, but little is known about how they will be affected by long‐term ocean warming. We tested their thermal responses using experimental evolution methods during 2 years of selection at optimal (28°C), supra‐optimal (32°C) and suboptimal (22°C) temperatures. After several hundred generations under thermal selection, changes in growth parameters, as well as N and C fixation rates, suggested that Trichodesmium did not adapt to the three selection temperature regimes during the 2‐year evolution experiment, but could instead rapidly and reversibly acclimate to temperature shifts from 20°C to 34°C. In contrast, over the same timeframe apparent thermal adaptation was observed in Crocosphaera, as evidenced by irreversible phenotypic changes as well as whole‐genome sequencing and variant analysis. Especially under stressful warming conditions (34°C), 32°C‐selected Crocosphaera cells had an advantage in survival and nitrogen fixation over cell lines selected at 22°C and 28°C. The distinct strategies of phenotypic plasticity versus irreversible adaptation in these two sympatric diazotrophs are both viable ways to maintain fitness despite long‐term temperature changes, and so could help to stabilize key ocean nitrogen cycle functions under future warming scenarios. [ABSTRACT FROM AUTHOR]