Background
The authors write that "marine phytoplankton are the foundation of ocean ecosystems," noting that (1) "these small but mighty microbes are responsible for roughly half of global carbon fixation," and that (2) they "form a fundamental part of the biological carbon pump that exports fixed carbon to the deep ocean." And in light of the tremendous significance of these minute but "mighty microbes," they further note the great concern that holds sway in many quarters of the scientific (and political) world with respect to the potential negative effects the projected future increase in the atmosphere's CO2 concentration may have on them, as well as on the several trickle-down effects that would follow in their wake. But they also indicate that "empirical studies so far predict changes in phytoplankton communities using single or a few genotypes to represent functional groups," whereas the real-world variation in responses within functional groups "has not been quantified." Thus, they proceed to describe what they did to initiate the acquisition of that essential knowledge.

What was done
In the words of the four researchers, they used "16 ecotypes of Ostreococcus tauri from nine habitat types" - which "were obtained from the Roscoff Culture Collection and the Plymouth Marine Laboratory, grown in Keller medium and made clonal by dilution, so that each culture originated from single cells" that were "acclimated for 5-7 asexual generations to 380 ppm CO2 or 1,000 ppm CO2 in a closed-system and grown in semi-continuous batch cultures at low densities" - in order to quantify variations in plastic responses to elevated CO2 for ecologically relevant traits such as photosynthesis," while also characterizing "changes in traits affecting food quality for five of these ecotypes" and noting as an aside that "O. tauri is [1] the smallest known free-living eukaryote, is [2] globally distributed, has [3] distinct ecotypes and is [4] an important primary producer, making it [5] ideal for eco-evolutionary studies."

What was learned
Schaum et al. say they were able to "link plasticity in photosynthesis rates to changes in the relative fitness of ecotypes during asexual growth," and that they were further able to "use this link to predict which ecotypes are likely to rise in frequency in a high-CO2 environment." More specifically, they report that the 2.63-fold increase in the air's CO2 content of their experiment led to increases in photosynthetic rates among the 16 ecotypes they studied that ranged from 1.02- to 2.18-fold greater than the current mean, while CO2-induced size differences among ecotypes were found to range from 1.3- to 1.9-fold greater than the current mean. Likewise, differences in plastic responses for C/N ratios, which partly determine the food quality of phytoplankton, were found to range from 1.06- to 1.56-fold greater than the current mean.

What it means
The four scientists conclude by stating that "as CO2 levels increase, O. tauri will grow and photosynthesize faster, and have larger cells with a higher C/N ratio than contemporary cells," with the result that "Ostreococcus, along with other green algae and cyanobacteria, are likely to increase in abundance in high-CO2 conditions" with concomitant benefits for the biosphere.