Like land plants, phytoplankton use sunlight to convert carbon dioxide (CO2) and water into organic carbon and oxygen. CO2 is therefore an essential nutrient for phytoplankton and higher concentrations of CO2 in the surface ocean have the potential to support higher rates of phytoplankton photosynthesis and growth. However, since the pH of water drops (reflecting an increase in acidity) as CO2 rises, the positive effects of higher CO2 may be offset by negative effects of low pH.
In laboratory experiments with pure cultures of individual species of phytoplankton, little or no change in growth is typically observed in cultures grown with CO2 levels expected in the year 2100. However, other more subtle effects have been noted including interactions of elevated CO2 with demand for other nutrients such as nitrogen, phosphorus, and iron, which suggest that higher CO2 may be most beneficial under nutrient rich conditions, and lower growth in cultures exposed to elevated CO2 in combination with light or temperature stress. In a group of phytoplankton known as coccolithophores that produce calcium carbonate plates on their cell surface, growth, calcification, and organic carbon production can be altered as a result of the decrease in pH associated with elevated CO2. Lower pH may also negatively affect the ability of an important group of photosynthetic bacteria known as Trichodesmium to provide nitrogen to the ocean through the process of nitrogen fixation, particularly in the vast areas of the ocean that are limited by the availability of the essential trace element, iron. To add to this potential stress, the lower pH associated with acidification likely lowers the bioavailability of essential trace metals in marine phytoplankton.
As multiple factors influence photosynthesis and growth of phytoplankton in the ocean, it is difficult to predict the effects of higher CO2 on the productivity of natural phytoplankton communities. For example, concurrent increases in both CO2 and temperature could have a positive effect on certain species of phytoplankton, but not others. Similarly, increases in CO2 expected over the next 100 years may favor growth of larger diatoms, which are most susceptible to CO2 diffusion limitation and generally have higher nutrient requirements. Both effects could lead to changes in phytoplankton community composition with potential secondary effects on biogeochemical cycles and food web structures in the ocean.
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