Predictability of thermal fluctuations influences functional traits of a cosmopolitan marine diatom

Evolutionary theory predicts that organismal plasticity should evolve in
environments that fluctuate regularly. However, in environments that
fluctuate less predictably, plasticity may be constrained because
environmental cues become less reliable for expressing the optimum
phenotype. Here we examine how the predictability of +5 °C temperature
fluctuations impacts the phenotype of the marine diatom Thalassiosira
pseudonana. Thermal regimes were informed by temperatures experienced by
microbes in an ocean simulation, and featured regular or irregular
temporal sequences of fluctuations that induced mild physiological stress.
Physiological traits (growth, cell size, complexity, pigmentation) were
quantified at the individual cell level using flow cytometry. Changes in
cellular complexity emerged as the first impact of predictability after
only 8-11 days, followed by deleterious impacts on growth on days 13-16.
Specifically, cells with a history of irregular fluctuation exposure
exhibited a 50% reduction in growth compared with the stable reference
environment, while growth was 3-18 times higher when fluctuations were
regular. We observed no evidence of heat hardening (increasingly positive
growth) with recurrent fluctuations. This study demonstrates that
unpredictable temperature fluctuations impact this cosmopolitan diatom
under ecologically-relevant time frames, suggesting shifts in
environmental stochasticity under a changing climate could have widespread
consequences amongst ocean primary producers.