The acid–base relations of plant (including algal) environments are complex, comprising geological processes as modified by biology including, especially over the last 200 years, man. Some habitats (e.g. high intertidal rockpools and some freshwater bodies) have pH variations of up to three units over a diel cycle as a result of photosynthesis and respiration. Other habitats, e.g. nutrient-poor open ocean habitats, have diel variations that are more than an order of magnitude smaller. Anthropogenic influences on acid–base relations of different habitats include the input to the atmosphere of gases that dissolve to produce acidic solutions. The quantitatively predominant gas is CO2, but SO2, NOx and NHy (via nitrification) can also be significant. The influence of the acidic gases in aquatic habitats (including the upper layers of peat bogs) and on terrestrial photosynthetic organisms alters the inorganic carbon speciation and pH around the photosynthetic cells. The calcified coralline marine red macroalgae, with benthic and unattached (maerl) life forms, have extracellular calcification; their calcification rate will decline in the future, with a more CO2-rich ocean and decreasing CO3 concentrations. The marine planktonic coccolithophores have intracellular calcification, though the coccoliths themselves occur externally. While many coccolithophores show decreased calcification with increasing external CO2 and the attendant decrease in external CO3, this is not universal. For both coralline red algae and coccolithophores the external CaCO3 will dissolve when seawater becomes undersaturated with respect to the relevant crystal form of CaCO3. Overall, the effects of increased CO2 alone are negligible or result in increased growth of non-calcified algae, while there is most generally a decreased growth of calcified algae.

Raven J. A., 2011. Effects on marine algae of changed seawater chemistry with increasing CO2. Biology and Environment: Proceedings of the Royal Irish Academy 111B:1-17. Article.