http://www.co2science.org/subject/o/summaries/acidificationdiatoms.php

Ocean Acidification (Effects on Marine Plants: Phytoplankton, Diatoms) -- Summary
In conclusion, and has been found to be the case for essentially all types of marine phytoplankton, the real-world data that have been obtained to date suggest that earth's diatoms will manage just fine as the air's CO2 content continues to climb to ever-greater heights. And as diatoms serve as primary producers in numerous marine food chains, the several trophic levels above them should also be similarly benefited by the dreaded phenomenon of "ocean acidification."

http://sites.duke.edu/writing20_12_f2011/2011/09/05/ocean-acidification-and-diatoms/

Ocean Acidification and Diatoms

Another experimented conducted entailed the creation of an equilibrium of atmospheric carbon dioxide with bubbled aqueous carbon dioxide. When the carbon dioxide was made to be twice that of normal conditions, consumption increased by 27%. When the carbon dioxide was tripled, the diatoms’ consumption was 39% higher. Estimates say that such carbon dioxide consumption as that described here may in have kept atmospheric levels to 90% of what they would be otherwise since start of the industrial revolution. In yet another study, it was found that certain species of diatoms grow 20% faster when exposed to increased carbon dioxide.

This potentially positive consequence of the increase in atmospheric carbon dioxide is not nearly enough to outweigh the negative results of anthropogenic carbon dioxide. Some algae do not, in fact, benefit from increased levels of carbon dioxide. Zooxanthellae, for example, exist symbiotically with coral reefs. If the zooxanthellae colonies grow too large, then they will be doing so at the expense of their coral homes. Some species of phytoplankton may react poorly to the increased acidity. Then we must factor in things such as coral bleaching, coastal erosion, decalcification, and the loss of biodiversity. Indeed, for every possible upside that comes from ocean acidification, it seems that there are two potentially devastating ramifications.