Evolutionary responses to ocean acidification in free-living protists.

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While many gross consequences that impact upon survival in an increasingly acidified ocean have attracted much attention, particularly, for example, the physiological effects upon calcifying organisms, we have yet to appreciate the long-term evolutionary response to this selective pressure and its concomitant effect on intraspecific biodiversity. This is a critical gap in our knowledge as biodiversity plays a key role in population persistence and thus affects ecosystem function. To address this issue, we propose an experimental approach on a model marine protist since this taxon is fast-growing, ecologically important, widespread, specious and occupies a range of environments. Specifically, we have selected the heterotrophic flagellate Oxyrrhis marina as it is widespread and easy to culture, and its growth rate is affected by pH conditions, although the specific physiological response varies among strains. Using a series of experiments on a bank of diverse (genetically and ecophysiologically) O.marina cultures sourced from habitats across the World's oceans, we aim to determine (1) the future consequences of ocean-acidification on patterns of intraspecific diversity and population structure and (2) identify whether there is a predictable outcome to future climate scenarios. Specifically we ask: 1. What is the extent of ecophysiological variation among O.marina-strains in response to acidification? 2. Is the outcome of evolution across an acidity gradient predictable from the ecophysiological profiles of strains? 3. Does system complexity affect the degree of determinism? 4. Does the rate of acidification affect the ability of O.marina to acclimate and thus impact on the outcome of evolution? We first characterise the spectrum of ecophysiological variation in growth rates in O.marina isolates to develop predictive models about their competitive ability under different ocean acidities. In addition, the level of genetic differences between strains will be quantified and used to determine whether response to pH has a distinct phylogeographic component. Next, we examine the scenario that selection is likely to be stronger, and the response to selection more deterministic/predictable, as acidity increases. Thus, the outcome of competition between strains with different growth responses to pH over 100-200 generations will be back-tracked to reveal the progress of 'evolution through time'. Data from these experiments will be compared to the predictive model of growth rates under different pHs, thus determining whether response to selection is predictable or stochastic. In addition, our current work reveals a dichotomy in biodiversity between the North Atlantic (low diversity) and the Mediterranean (high diversity) which may impact on the regions' overall response to environment change. To determine whether genetic diversity (population complexity) impacts upon the outcome of selection we will run another long-term (100-200 generation) selection experiment at a single pH with replicate populations founded with different levels of standing genetic variation. This experiment determines whether the outcome of competition between strains varies with population diversity, and whether the response to selection is stronger when there is greater standing genetic variation for select for selection to act upon. Finally we hypothesise that rapid changes will have greater impact than slow changes, which may allow strains to acclimate. Therefore, we will compare the responses of treatments of up to 10 divergent strains across a range of acclimation periods. Together these experiments will reveal not simply the immediate impact of ocean acidification but the potential consequences of this well accepted climate-change pressure on the evolution of life in the oceans, and thus the adaptability of our oceans to inevitable change.