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

Abstract

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 C), supraoptimal (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 longterm temperature changes, and so could help to stabilize key ocean nitrogen cycle functions under future warming scenarios.

The ongoing need for rates: can physiology and omics come together to co-design the measurements needed to understand complex ocean biogeochemistry?

“…multi-year) adaptation by Trichodesmium to high carbon dioxide levels demonstrated that genetic assimilation may be driving adaptation, but also identified the metabolic pathways that were initially altered and then maintained during this adaptation. In another lab-based diazotroph study, Qu et al . (2022) provide a mechanistic understanding of how the diazotrophs Crocosphaera and Trichodesmium acclimate or adapt to ocean global change using phenotypic metrics in conjunction with whole-genome sequencing and variant analysis.…”

“…Pivotal events in the omics timeline (lower green line) include problem solving ( Zehr and Montoya, 2007 ) and discovery of diazotroph diversity including in unicellular cyanobacteria group A (UNCYN-A) and diverse uncultured heterotrophic bacteria ( Martínez-Pérez et al ., 2016 ). Recent examples of more integrated physiological and omics co-designed studies ( Walworth et al ., 2016 ; Qu et al ., 2022 ) offer an important way forward.…”

Integrating the impact of global change on the niche and physiology of marine nitrogen‐fixing cyanobacteria

“…However, biology is highly dynamic, with both evolution and adaptation likely to occur. A recent experimental evolution study comparing Trichodesmium with Crocosphaera under sustained thermal selection suggested that the former showed little capacity to adapt to warming, but instead relied on non‐genetic plasticity to meet temperature challenges (Qu et al, 2022 ). Crocosphaera however exhibited a limited ability to adapt to supraoptimal warming supported by a suite of specific genetic changes, suggesting that evolutionary capacity may need to be considered at least for this diazotroph (Qu et al, 2022 ).…”

The ongoing need for rates: can physiology and omics come together to co-design the measurements needed to understand complex ocean biogeochemistry?

“…Pivotal events in the omics timeline (lower green line) include problem solving (Zehr and Montoya, 2007) and discovery of diazotroph diversity including in unicellular cyanobacteria group A (UNCYN-A) and diverse uncultured heterotrophic bacteria (Martínez-Pérez et al, 2016). Recent examples of more integrated physiological and omics co-designed studies (Walworth et al, 2016;Qu et al, 2022) offer an important way forward. can complicate the estimates of community N 2 fixation rates (Church et al, 2005), and measurements of nif expression are not well correlated with 15 N-based rates of N 2 fixation (Turk et al, 2011).…”