Microalgal adaptation to changes in carbon dioxide

It is generally accepted that global levels of CO2 will roughly double over the next century. Because of their large population sizes and fast generation times, microalgae may adapt to global change through novel mutations fixed by natural selection, such that future populations may be genetically different from contemporary ones. The prediction that microalgae may respond evolutionarily to rising CO2 was tested using populations of Chlamydomonas reinhardtii grown for 1000 generations at increasing CO2. Laboratory populations grown at high CO2 did not show a direct response to selection at elevated CO2, instead evolving a range of non-adaptive syndromes. In addition, populations selected at elevated CO2 often grew poorly at ambient CO2. The same evolutionary responses were seen in natural populations isolated from CO2 springs. CO2 uptake was measured in a subset of the laboratory selection lines, which were found to have cells that either leaked CO2, had lost the ability to induce high-affinity CO 2 uptake, or both. These phenotypes were tentatively attributed to the accumulation of conditionally neutral mutations in genes involved in the carbon concentrating mechanism (CCM). The high-CO2-selected phenotypes were found to be reversible in terms of fitness when populations were backselected in air, though wild-type regulation of the CCM was not regained. It has been suggested that phytoplankton adaptation to changes in CO2 levels is constrained by selective history. This was tested by culturing genetically distinct populations of Chlamydomonas at decreasing levels of CO2. In this case, divergence between lines was attributable to chance rather than selective history.