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

The globally dominant N-2-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 degrees C), supra-optimal (32 degrees C) and suboptimal (22 degrees 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 degrees C to 34 degrees 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 degrees C), 32 degrees C-selected Crocosphaera cells had an advantage in survival and nitrogen fixation over cell lines selected at 22 degrees C and 28 degrees 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.

Automated summary

Through experimental evolution methods, this study found that the N-2-fixing cyanobacteria Trichodesmium and Crocosphaera have different responses to long-term ocean warming. Trichodesmium can rapidly and reversibly acclimate to temperature shifts, while Crocosphaera shows irreversible phenotypic changes and whole-genome adaptation. These different strategies help maintain fitness and stabilize key ocean nitrogen cycle functions under future warming scenarios.