Phenotypic evolution in marine phytoplankton: when data and models meet
Friday seminar by Jorijntje Henderiks
The PhytoSCALE project aims to better understand the adaptive response of marine algae to climatic change. Our model organisms, the coccolithophores (class Prymnesiophyceae, division Haptophyta), are single-celled, golden-brown algae that produce calcite scales, coccoliths. Coccolith size relates to cell size and thus provides information on physiological constraints (such as intracellular pH regulation and resource uptake rates) in both extant and fossil species – offering a great opportunity to test hypotheses of adaptive evolution across temporal and spatial scales. At CEES, we form a cross-disciplinary team combining (a) fossil time series data, (b) controlled culture experiments and (c) evolutionary models of phenotypic evolution.
In this seminar, I will focus on results from the fossil record and our modeling efforts. The size variability within the Coccolithus lineage was determined in 205 deep-sea sediment samples from six different sites in the Atlantic, Indian and Pacific oceans, altogether spanning ~57 million years. In order to test whether the sampled populations have been fluctuating around a fixed or changing fitness optimum, as well as to identify any underlying processes driving the observed size variations, Reitan et al. (in review) have developed a model framework that builds upon a special class of stochastic equations (Ornstein-Uhlenbeck processes). The models that predict the coccolith size data best are very similar in structure and all suggest that the phenotype is influenced by both global and local processes. The largest contrasts in model parameters are found between the North Atlantic and tropical Pacific sites. Although the model parameters cannot be taken at face value, this outcome suggests that the same underlying driving process (environmental influence) operates at different rates in different regions, or that local factors, including ‘bottom-up’ selective pressures of resource availability and region-specific physical factors of the surface ocean, may override an underlying global process. However, this model outcome may also be to compensate for differences in the number of (cryptic) species at each site, in a way that the actual driving processes could be more similar and more connected (‘global’). The next generation of models (under development) will accommodate these scenarios. The culture experiments, currently being planned at CEES in close collaboration with the Marine Biology group, will inform our hypotheses by testing the phenotypic plasticity of individual clones.
Royal Swedish Academy Research Fellow/Associate Professor (Docent), Department of Earth Sciences, Palaeobiology, Uppsala; CEES, Department of Biology, University of Oslo