Swift thermal reaction norm evolution in a key marine phytoplankton species

Abstract

Temperature has a profound effect on the species composition and physiology of marine phytoplankton, a polyphyletic group of microbes responsible for half of global primary production. Here, we ask whether and how thermal reaction norms in a key calcifying species, the coccolithophore Emiliania huxleyi, change as a result of 2.5 years of experimental evolution to a temperature ≈2°C below its upper thermal limit. Replicate experimental populations derived from a single genotype isolated from Norwegian coastal waters were grown at two temperatures for 2.5 years before assessing thermal responses at 6 temperatures ranging from 15 to 26°C, with pCO 2 (400/1100/2200 μatm) as a fully factorial additional factor. The two selection temperatures (15°/26.3°C) led to a marked divergence of thermal reaction norms. Optimal growth temperatures were 0.7°C higher in experimental populations selected at 26.3°C than those selected at 15.0°C. An additional negative effect of high pCO 2 on maximal growth rate (8% decrease relative to lowest level) was observed. Finally, the maximum persistence temperature (T max) differed by 1–3°C between experimental treatments, as a result of an interaction between pCO 2 and the temperature selection. Taken together, we demonstrate that several attributes of thermal reaction norms in phytoplankton may change faster than the predicted progression of ocean warming.

Thermal niche evolution of functional traits in a tropical marine phototroph

“…The data presented here are central to describing and understanding the physiological and evolutionary responses to temperature change among phytoplankton (Reusch and Boyd ), and are consistent with evolutionary experiments performed on the coccolithophore Emiliania huxleyii and the green alga Chlamydomonas reinhardtii , where T opt also shifted upwards (Listmann et al. , Schaum et al. ).…”

“…For example, while Listmann et al. () report an increase in the CT max with long‐term warming in a temperate species of coccolithophore, we report an increase in the CT min for a tropical dinoflagellate. These differences highlight that the mechanisms underlying the different modes of TPC evolution remain unknown and should be further investigated in future long‐term experiments.…”

“…, coccolithophores, Listmann et al. ), we observed a horizontal shift in the TPC for growth, centered at a warmer T opt as well as enhanced thermal resistance at extreme high temperatures. These results suggest that adaptation has the capacity to alter the current physiological limits of species and to some extent, track the expected increase in average SST across the global ocean (Boyd et al.…”

“…, Listmann et al. ). However, an important factor determining high‐temperature evolution, is the adaptability of functional traits, as these are the underlying elements of a phenotype that mediate growth, reproduction, survival and ecological function (Litchman and Klausmeier ).…”

Rapid thermal adaptation in a marine diatom reveals constraints and trade‐offs

“…Rising temperatures may negatively impact phytoplankton productivity, biomass and species diversity (Boyce, Lewis, & Worm, ; Thomas et al., ). However, rapid adaptation to warming may mitigate some ecological impacts of climate change (Listmann et al., ; Litchman, Edwards, Klausmeier, & Thomas, ). For example, evolutionary change in the thermal optimum for per‐capita population growth ( T opt ) and the maximum temperature at which positive growth is possible ( CT max ) in response to ocean warming may reduce heat‐induced mortality, allowing some species to persist at low latitudes where they might otherwise go regionally extinct (Thomas et al., ).…”