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Abstract
Increased seawater pCO2 has the potential to alter phytoplankton biochemistry, which in turn may negatively affect the nutritional quality of phytoplankton as food for grazers. Our aim was to identify how Antarctic phytoplankton, Pyramimonas gelidicola, Phaeocystis antarctica, and Gymnodinium sp., respond to increased pCO2. Cultures were maintained in a continuous culture setup to ensure stable CO2 concentrations. Cells were subjected to a range of pCO2 from ambient to 993 µatm. We measured phytoplankton response in terms of cell size, cellular carbohydrate content, and elemental, pigment and fatty acid composition and content. We observed few changes in phytoplankton biochemistry with increasing CO2 concentration which were species-specific and predominantly included differences in the fatty acid composition. The C:N ratio was unaffected by CO2 concentration in the three species, while carbohydrate content decreased in Pyramimonas gelidicola, but increased in Phaeocystis antarctica. We found a significant reduction in the content of nutritionally important polyunsaturated fatty acids in Pyramimonas gelidicola cultures under high CO2 treatment, while cellular levels of the polyunsaturated fatty acid 20:5ω3, EPA, in Gymnodinium sp. increased. These changes in fatty acid profile could affect the nutritional quality of phytoplankton as food for grazers, however, further research is needed to identify the mechanisms for the observed species-specific changes and to improve our ability to extrapolate laboratory-based experiments on individual species to natural communities.
Highlights
Human activities have led to an increase in atmospheric CO2 concentration of which an estimated30% have been absorbed by the oceans, causing global average surface seawater pH to drop by0.1 units [1,2,3], a process termed ocean acidification [4,5]
Growth rates were not affected by CO2 concentrations in any of the three species (Figure 2)
Antarctic phytoplankton species examined here, we found subtle but highly variable responses under elevated CO2 concentrations, with opposite changes observed in cellular carbohydrate contents as well as in fatty acid profiles
Summary
Human activities have led to an increase in atmospheric CO2 concentration of which an estimated30% have been absorbed by the oceans, causing global average surface seawater pH to drop by0.1 units [1,2,3], a process termed ocean acidification [4,5]. Human activities have led to an increase in atmospheric CO2 concentration of which an estimated. High latitudes will be vulnerable due to their capacity to store more CO2, and upwelling and subsequent entrainment of CO2-rich deep waters during winter [6,7,8,9,10], rendering its inhabitants among the first to be affected by ocean acidification. Little is known presently about the susceptibility of polar organisms to increased pCO2, and this is true for Antarctic phytoplankton [11,12]. Ocean acidification has the potential to alter phytoplankton biochemistry. Elevated CO2 concentration has been shown to influence the ratio of carbon to nutrient uptake rates in phytoplankton [13,14,15,16] and an increase in C:N:P ratio [17,18,19]. A reduction in the percentage of polyunsaturated fatty acids (PUFA) in the diatom Thalassiosira pseudonana has been reported [20]
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