Light Levels Affect Carbon Utilisation in Tropical Seagrass under Ocean Acidification

Under future ocean acidification (OA), increased availability of dissolved inorganic carbon (DIC) in seawater may enhance seagrass productivity. However, the ability to utilise additional DIC could be regulated by light availability, often reduced through land runoff. To test this, two tropical seagrass species, Cymodocea serrulata and Halodule uninervis were exposed to two DIC concentrations (447 μatm and 1077 μatm pCO2), and three light treatments (35, 100, 380 μmol m-2 s-1) for two weeks. DIC uptake mechanisms were separately examined by measuring net photosynthetic rates while subjecting C. serrulata and H. uninervis to changes in light and addition of bicarbonate (HCO3-) use inhibitors (carbonic anhydrase inhibitor, acetazolamide) and TRIS buffer (pH 8.0). We observed a strong dependence on energy driven H+-HCO3- co-transport (TRIS, which disrupts H+ extrusion) in C. serrulata under all light levels, indicating greater CO2 dependence in low light. This was confirmed when, after two weeks exposure, DIC enrichment stimulated maximum photosynthetic rates (Pmax) and efficiency (α) more in C. serrulata grown under lower light levels (36–60% increase) than for those in high light (4% increase). However, C. serrulata growth increased with both DIC enrichment and light levels. Growth, NPP and photosynthetic responses in H. uninervis increased with higher light treatments and were independent of DIC availability. Furthermore, H. uninervis was found to be more flexible in HCO3- uptake pathways. Here, light availability influenced productivity responses to DIC enrichment, via both carbon fixation and acquisition processes, highlighting the role of water quality in future responses to OA.

在未来海洋酸化（Ocean Acidification, OA）情境下，海水中溶解无机碳（Dissolved Inorganic Carbon, DIC）可利用性的提升可能会增强海草的生产力。然而，海草利用额外DIC的能力可能受光照可利用性调控——而光照往往因陆地径流而降低。为验证这一假说，研究将两种热带海草物种：圆叶丝粉藻（Cymodocea serrulata）与单脉二药藻（Halodule uninervis）分别置于两种DIC浓度（447 μatm与1077 μatm pCO₂）以及三种光照处理（35、100、380 μmol·m⁻²·s⁻¹）下培养两周。为单独探究DIC的摄取机制，本研究在调控光照条件的基础上，向两种海草施加碳酸氢根（HCO₃⁻）摄取抑制剂——碳酸酐酶抑制剂（carbonic anhydrase inhibitor）乙酰唑胺（acetazolamide）——与TRIS缓冲液（pH 8.0），并同步测量其净光合速率。研究观察到，在所有光照水平下，圆叶丝粉藻均强烈依赖能量驱动的H⁺-HCO₃⁻协同转运机制（TRIS可阻断H⁺外排过程），这表明其在低光照条件下对CO₂的依赖性更高。两周暴露实验结束后，这一结论得到验证：经DIC富集处理后，低光照（35 μmol·m⁻²·s⁻¹）组圆叶丝粉藻的最大光合速率（Pmax）与光合效率（α）提升幅度（36%~60%）显著高于高光组（仅提升4%）。不过，圆叶丝粉藻的生长随DIC富集与光照水平提升均有所增加。而单脉二药藻的生长、净初级生产力（Net Primary Productivity, NPP）与光合响应均随光照强度升高而增强，且不受DIC可利用性的影响。此外，研究发现单脉二药藻在HCO₃⁻摄取途径上具有更高的可塑性。本研究表明，光照可利用性通过碳固定与碳获取两个过程，影响海草对DIC富集的生产力响应，凸显了水质在海洋生物应对未来海洋酸化过程中的关键作用。