Ocean Acidification Negatively Impacts the Productivity of Photosynthetic Coral Symbionts

Many corals have a symbiotic relationship with symbiotic dinoflagellates. These photosynthetic symbionts are vital to the survival and health of their coral hosts. Ocean acidification has been shown to inhibit the calcification coral structures, but few studies have focused on the impact on the symbiotic dinoflagellates. Crawley et al. (2010) tested the impact of ocean acidification on several key aspects of dinoflagellates of the genus Symbiodinium associated with the coral Acropora formosa. The authors found that a decrease in ocean pH resulted in an increase of the pigment chlorophyll a per cell, an increase in xanthophyll de-epoxidation, and a decrease in photosynthetic capacity per chlorophyll. Phosphoglycolate phosphatase (PGPase), an enzyme that enables carbon fixation by the symbionts, was also impacted by an increase in ocean acidity. These findings suggest symbiont productivity might increase under conservative increases in ocean acidity, but will likely decrease according to the current trajectory.––Emily Putnam
Crawley, A., Kline, D.I., Dunn, S., Anthony, K., Dove, S., 2010. The effect of ocean acidification on symbiont photorespiration and productivity in Acropora formosa. Global Change Biology 16, 851–863.

Crawley and colleagues at the University of Queensland performed a variety of tests to assess both physiological changes due to increased CO2 and the genetic expression of the enzyme PGPase. The physical tests executed were the respirometry assays, cell counting, and pigment analysis. Respirometry assays are designed to examine photosynthesis as a product of controlled amounts of light. This test determines respiration that is produced without light, as well as the photosynthetic efficiency and capacity of the symbionts. The symbiont cells were counted for each trial to see whether acidification impacted the number of dinoflagellates found on a coral specimen. The pigments produced by the symbionts were separated by chromatographic techniques. The authors specifically looked for the pigment xanthophyll and evidence of xanthophyll de-epoxidation. Xanthophyll de-epoxidation is an important indicator for this study because the process of de-epoxidation quenches extra energy from light and protects the symbiont from damage.
Genetics testing centered on expression of PGPase. The sequence for PGPase was obtained from a database and then compared with similar sequences from other photosynthetic organisms­­, including diatoms and terrestrial plants. RNA was extracted from A. formosa specimens. Quantitative real time reverse transcription polymerase chain reaction was performed to measure the expression of PGPase in the symbionts. A test was done using coral RNA with no symbionts to ensure that the primers used to isolate the sequence coding for PGPase did not amplify the coral’s RNA as well.
The chlorophyll a concentrations per cell increased along with increases in acidity. The density of cells was not affected by acidification. Xanthophyll de-epoxidation also increased as a result of acidification. PGPase expression was reduced by 50% in the severely acidified conditions. Crawley et al. claim that a coincidental decrease in photosynthetic productivity suggests a link between PGPase expression and photosynthetic output, but will require further testing to confirm this hypothesis. Productivity increases under conservative increases in acidity, but did not for the current trajectory of ocean acidification. The authors posit that sharp increases in acidity may overwhelm the symbionts capacity for dissipating energy and that the loss of these systems may be involved in the loss of productivity.

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