Abstract
We expect the structure and functioning of marine ecosystems to change over this century in response to changes in key ocean variables associated with a changing climate. Organisms with generation times from years to decades have the capacity to adapt to changing environmental conditions over a few generations by selecting from existing genotypes/phenotypes, but it is unlikely that evolution through mutation will be a major factor for organisms with generation times of years to decades. However, phytoplankton and other microbes, with generation times of days or less, experience hundreds of generations each year, allowing the possibility for favorable mutations (i.e., those that produce organisms with fitness maxima nearer to the environmental conditions at that time) to dominate existing genotypes and survive in a changing climate. Several laboratories have grown phytoplankton cultures for hundreds to thousands of generations and demonstrated that they have changed genetic makeup. In particular Schluter et al (2014) grew replicates derived from a single cell of Emiliania huxleyi, a coccolithophorid with broad geographical and thermal range, for 3 years (~1250 generations) at 15°C, and then for a year at 26.3°C, near their upper thermal limit. During the last year the intrinsic growth rate increased more or less linearly, which the authors attribute to genetic mutation. Here we simulate genetic mutation of a single trait (intrinsic growth rate), both for the control phase and the warm phase of their study. We consider sensitivities to frequency of mutation, changes with temperature in intrinsic growth rate, and use the experimental setup and results to place constraints on the way mutations occur. In particular, all numerical experiments with mutation result in a lag time ~30-140 generations before a significant increase in realized growth rate occurs. This lag after a favorable mutation results from the number of generations required for a single favorable mutant cell to reach a significant fraction of the ~105 cells in the culture. A numerical experiment that includes a simple plastic response formulation shows that plasticity could remove this lag and yield results more in agreement with those observed in the laboratory study.
Highlights
The climate has been changing and is expected to continue to change—and possibly at an increasing rate (Collins et al, 2013; Rhein et al, 2013) The oceans are intimately involved in both regulating and responding to that change, and marine ecosystems are and will continue to change in response to changes associated with a changing climate (Hoegh-Guldberg et al, 2014; Pörtner et al, 2014; Wong et al, 2014)
The objective of this paper is to develop a model at the trait level of genetic mutation by the coccolithophorid Emiliania huxleyi based on observations taken over 4 years of laboratory culture experiments (Schlüter et al, 2014)
Modeling even the adaptive response to abrupt change in a single environmental variable in an asexual phytoplankton population of a single trait led to unexpected results
Summary
The climate has been changing and is expected to continue to change—and possibly at an increasing rate (Collins et al, 2013; Rhein et al, 2013) The oceans are intimately involved in both regulating and responding to that change, and marine ecosystems are and will continue to change in response to changes associated with a changing climate (Hoegh-Guldberg et al, 2014; Pörtner et al, 2014; Wong et al, 2014). Coupled climate-ecosystem models that predict future changes in marine ecosystems, for the most part use fixed compartment model structures for ecosystems with minimally-adaptive parameters: mainly variable C:N ratios and a temperature dependence of some intrinsic rates such as phytoplankton growth rate (e.g., Chust et al, 2014). While we use these models to predict the future structure and function of marine ecosystems, considerable skepticism remains (e.g., Planque, 2015). Other documented changes in response to warming are in phenology: for example, open ocean and coastal zooplankton reaching their biomass maximum ∼1 month earlier over 40 years, correlated with the total number of “degree-days” above 6◦C over the spring months of March-April-May (Mackas et al, 2007)
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