Phenotypic consequences of 1,000 generations of selection at elevated CO₂ in a green alga

Abstract

Estimates of the effect of increasing atmospheric CO₂ concentrations on future global plant production rely on the physiological response of individual plants or plant communities when exposed to high CO₂ (refs 1-6). Plant populations may adapt to the changing atmosphere, however, such that the evolved plant communities of the next century are likely to be genetically different from contemporary communities^{7,8,9,10,11,12}. The properties of these future communities are unknown, introducing a bias of unknown sign and magnitude into projections of global carbon pool dynamics. Here we report a long-term selection experiment to investigate the phenotypic consequences of selection for growth at elevated CO₂ concentrations. After about 1,000 generations, selection lines of the unicellular green alga Chlamydomonas failed to evolve specific adaptation to a CO₂ concentration of 1,050 parts per million. Some lines, however, evolved a syndrome involving high rates of photosynthesis and respiration, combined with higher chlorophyll content and reduced cell size. These lines also grew poorly at ambient concentrations of CO₂. We tentatively attribute this outcome to the accumulation of conditionally neutral mutations in genes affecting the carbon concentration mechanism.