Combined effects of ocean acidification and light or nitrogen availabilities on 13C fractionation in marine dinoflagellates@en

Along with increasing oceanic CO2 concentrations, enhanced stratification constrains phytoplankton to shallower upper mixed layers with altered light regimes and nutrient concentrations. Here, we investigate the effects of elevated pCO2 in combination with light or nitrogen-limitation on 13C fractionation (epsilon p) in four dinoflagellate species. We cultured Gonyaulax spinifera and Protoceratium reticulatum in dilute batches under low-light (LL) and high-light (HL) conditions, and grew Alexandrium fundyense and Scrippsiella trochoidea in nitrogen-limited continuous cultures (LN) and nitrogen-replete batches (HN). The observed CO2-dependency of epsilon p remained unaffected by the availability of light for both G. spinifera and P. reticulatum, though at HL epsilon p was consistently lower by about 2.7 per mil over the tested CO2 range for P. reticulatum. This may reflect increased uptake of (13C-enriched) bicarbonate fueled by increased ATP production under HL conditions. The observed CO2-dependency of epsilon p disappeared under LN conditions in both A. fundyense and S. trochoidea. The generally higher epsilon p under LN may be associated with lower organic carbon production rates and/or higher ATP:NADPH ratios. CO2-dependent epsilon p under non-limiting conditions has been observed in several dinoflagellate species, showing potential for a new CO2-proxy. Our results however demonstrate that light- and nitrogen-limitation also affect epsilon p, thereby illustrating the need to carefully consider prevailing environmental conditions.随着海洋二氧化碳浓度升高，增强的层化作用将浮游植物限制在更浅的上层混合层中，同时改变了光照条件与营养盐浓度。本研究针对四种甲藻，探究了升高的二氧化碳分压（pCO₂）分别与光照限制、氮限制共同作用对其碳同位素分馏（¹³C fractionation，εₚ）的影响。本研究采用稀释分批培养法，在低光照（LL）与高光照（HL）条件下培养了刺孢原多甲藻（Gonyaulax spinifera）和网状原角藻（Protoceratium reticulatum）；同时采用氮限制连续培养（LN）与氮充足分批培养（HN）体系，培养了塔玛亚历山大藻（Alexandrium fundyense）和锥状斯氏藻（Scrippsiella trochoidea）。对于刺孢原多甲藻和网状原角藻而言，εₚ的二氧化碳依赖性并未受光照有效性的影响；但在高光照条件下，网状原角藻的εₚ在测试的二氧化碳分压范围内始终低约2.7‰。这一现象或可归因于高光照条件下三磷酸腺苷（ATP）生成量增加，进而促进了对富集¹³C的碳酸氢盐的摄取。而在氮限制条件下，塔玛亚历山大藻与锥状斯氏藻的εₚ二氧化碳依赖性均消失。氮限制条件下εₚ普遍偏高，这可能与有机碳生成速率降低以及/或三磷酸腺苷（ATP）与烟酰胺腺嘌呤二核苷酸磷酸（NADPH）的比值升高有关。此前已有多项研究在多种甲藻的非限制培养条件下观测到了二氧化碳依赖性的εₚ，这表明其具备开发新型二氧化碳替代指标（CO₂-proxy）的潜力。但本研究结果表明，光照限制与氮限制同样会对εₚ产生影响，这提示在实际应用中需审慎考量当前的环境条件。