Saturating light and not increased carbon dioxide under ocean acidification drives photosynthesis and growth in Ulva rigida (Chlorophyta)

Carbon physiology of a genetically identified Ulva rigida was investigated under different CO(2(aq)) and light levels. The study was designed to answer whether (1) light or exogenous inorganic carbon (Ci) pool is driving growth; and (2) elevated CO(2(aq)) concentration under ocean acidification (OA) will downregulate CA(ext)-mediated [Image: see text] dehydration and alter the stable carbon isotope (δ(13)C) signatures toward more CO(2) use to support higher growth rate. At pH(T) 9.0 where CO(2(aq)) is <1 μmol L(−1), inhibition of the known [Image: see text] use mechanisms, that is, direct [Image: see text] uptake through the AE port and CA(ext)-mediated [Image: see text] dehydration decreased net photosynthesis (NPS) by only 56–83%, leaving the carbon uptake mechanism for the remaining 17–44% of the NPS unaccounted. An in silico search for carbon-concentrating mechanism elements in expressed sequence tag libraries of Ulva found putative light-dependent [Image: see text] transporters to which the remaining NPS can be attributed. The shift in δ(13)C signatures from –22‰ toward –10‰ under saturating light but not under elevated CO(2(aq)) suggest preference and substantial [Image: see text] use to support photosynthesis and growth. U. rigida is Ci saturated, and growth was primarily controlled by light. Therefore, increased levels of CO(2(aq)) predicted for the future will not, in isolation, stimulate Ulva blooms.