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

Carbon physiology of a genetically identified Ulva rigida was investigated under different CO2(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 CO2(aq) concentration under ocean acidification (OA) will downregulate CAext-mediated inline image dehydration and alter the stable carbon isotope (delta13C) signatures toward more CO2 use to support higher growth rate. At pHT 9.0 where CO2(aq) is <1 ?mol/L, inhibition of the known inline image use mechanisms, that is, direct inline image uptake through the AE port and CAext-mediated inline image 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 inline image transporters to which the remaining NPS can be attributed. The shift in delta13C signatures from -22 per mil toward -10 per mil under saturating light but not under elevated CO2(aq) suggest preference and substantial inline image use to support photosynthesis and growth. U. rigida is Ci saturated, and growth was primarily controlled by light. Therefore, increased levels of CO2(aq) predicted for the future will not, in isolation, stimulate Ulva blooms.本研究针对经基因鉴定的硬石莼(Ulva rigida)，在不同水溶液二氧化碳(CO2(aq))浓度与光照强度条件下开展碳生理特性探究。本实验设计旨在解答两个核心科学问题：其一，光照还是外源无机碳(inorganic carbon, Ci)库主导藻类生长；其二，海洋酸化(Ocean Acidification, OA)背景下升高的水溶液二氧化碳浓度，是否会下调胞外碳酸酐酶(CAext)介导的碳酸氢根(HCO3^-)脱水过程，并改变稳定碳同位素δ¹³C(stable carbon isotope δ¹³C)特征，通过更多利用二氧化碳的方式支撑更高生长速率。当实验体系酸碱度为pHT 9.0且水溶液二氧化碳浓度低于1 μmol/L时，已知的碳酸氢根利用机制——即通过阴离子交换(AE, Anion Exchanger)端口直接摄取碳酸氢根以及胞外碳酸酐酶介导的碳酸氢根脱水——受到抑制后，净光合速率(net photosynthesis, NPS)仅下降56%~83%，仍有17%~44%的净光合速率对应的碳摄取机制尚未阐明。通过对石莼属表达序列标签(expressed sequence tag, EST)文库开展碳浓缩机制(carbon-concentrating mechanism, CCM)元件的计算机模拟(in silico)检索，发现了潜在的光依赖性碳酸氢根转运蛋白，该类转运蛋白可解释剩余的净光合速率来源。饱和光照条件下，稳定碳同位素δ¹³C特征从-22‰向-10‰偏移，但在升高的水溶液二氧化碳浓度条件下未出现该偏移，这表明硬石莼偏好并大量利用碳酸氢根以支撑光合与生长过程。硬石莼的无机碳供应已达饱和状态，其生长主要受光照调控。因此，未来预测的水溶液二氧化碳浓度升高，单独作用下并不会诱发石莼属藻华(bloom)。