D. tertiolecta and P. micans grew significantly faster in higher CO₂ concentrations, but D. brightwellii stayed the same across CO₂ treatments. Only D. tertiolecta showed a significant change in cell area across CO₂ treatments; P. micans and D. brightwellii stayed the same.

    D. tertiolecta and P. micans used significantly more carbon as the concentrations of CO₂ increased. Both organisms increased in growth rates and D. tertiolecta increased its cell area, which could mean that these species used this extra carbon to grow and reproduce. D. brightwellii also used more carbon as it was provided, but it did not change in morphology or physiology, meaning that the extra carbon could have been stored or released as exudates.

    None of these organisms showed a significant increase in their nutrient uptake. For D. tertiolecta and P. micans we would have expected an increase in nutrient uptake because these species grew faster and D. tertiolecta increased in cell size. Therefore, both organisms might decrease in food quality to predators. D. brightwellii did not increase its nutrient uptake, but this was expected because D. brightwellii did not increase in cell area or growth rate.

    The responses of the phytoplankton to elevated CO₂ appear to be species-specific (Rost et al. 2008). Phytoplankton have different affinities to CO₂ and will use it in different ways. Our results showed that some species, like D. tertiolecta and P. micans, may be able to deal with the changes and use more CO₂ from the atmosphere by increasing growth rates and cell size (only D. tertiolecta). These changes in morphology and physiology could create cascading effects on the marine biota, like changes in grazing and predation, which would need future research. Organisms like D. brightwellii could deal with higher CO₂ concentrations, but may throw away the excess as exudates. Also, species like P. micans and D. tertiolecta could be dominant because they are able to grow faster and increase in cell size. But, those organisms like D. tertiolecta and P. micans, which do not increase their nutrient uptake, could lead to bottom-up effects in the marine trophic web. We generally conclude that phytoplankton might be able to mitigate some effects of increased CO₂ in ocean waters, but this could have consequences for the entire marine biota.

Rost, B., Zondervan, I., Wolf-Gladrow, D., 2008. Sensitivity of phytoplankton to future changes in ocean carbonate chemistry: Current knowledge, contradictions and research directions. Marine Ecology Progress Series 373, 227-337.