How does rising atmospheric CO2 affect marine organisms?

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The Photosynthetic Response of a Marine Diatom to Atmospheric CO2 Enrichment
Chen, X. and Gao, K.  2004.  Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2Functional Plant Biology 31: 399-404.

According to Chen and Gao, "marine diatoms, a group of unicellular photosynthetic eukaryotes, are responsible for approximately 40% of marine primary productivity," and "Skeletonema costatum is a unicellular marine diatom that is widely distributed in coastal waters all over the world and constitutes a major component of natural assemblages of most marine phytoplankton."

What was done
The two scientists grew S. costatum (Grenville) Cleve. (strain 2042) in filtered nutrient-enriched seawater maintained at 20°C under a light/dark cycle of 12/12 hours at a light intensity of 200 ėmol m-2 s-1, while continuously aerating both cultures with air of either 350 or 1000 ppm CO2.  Simultaneously, they measured a number of physiological parameters related to the diatom's photosynthetic activity.

What was learned
Chen and Gao report that cell numbers of the alga "increased steadily throughout the light period and they were 1.6 and 2.1 times higher after the 12 h light period for the alga grown at 350 and 1000 ppm CO2, respectively."  They also say that chlorophyll a concentrations in the bulk of the two CO2 cultures "increased 4.4- and 5.4-fold during the middle 8 h of the light period for the alga grown at 350 and 1000 ppm CO2, respectively," and that "the contents of cellular chlorophyll a were higher for the alga grown at 1000 ppm CO2 than that at 350 ppm CO2."  In addition, they report that the initial slope of the light saturation curve of photosynthesis and the photochemical efficiency of photosystem II "increased with increasing CO2, indicating that the efficiency of light-harvesting and energy conversion in photosynthesis were increased."

What it means
The authors say their experiments "showed that the alga mainly use free CO2 from the medium," and that "S. costatum benefited from CO2 enrichment," noting once again that their data "showed that the light-saturated photosynthesis rate based on cell number, the chlorophyll a content, the photosynthetic chemistry of photosysten II and the efficiency of the light reaction all increased to various degrees with elevated CO2."

Reviewed 11 August 2004