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（OA）情境下，海水中溶解无机碳（DIC）可用性的提升或可增强海草生产力。然而，海草利用额外DIC的能力可能受光照可用性调控——而光照往往会因地表径流而减弱。为验证该假说，研究将两种热带海草物种羽叶二药藻（Cymodocea serrulata）与单脉二药藻（Halodule uninervis），暴露于两种DIC浓度（447 μatm与1077 μatm的pCO₂分压）及三种光照处理（35、100、380 μmol·m⁻²·s⁻¹）中，持续培养两周。研究通过测量净光合速率，单独探究了两种海草的DIC摄取机制：具体操作为对C. serrulata与H. uninervis施加光照变化，并添加碳酸氢根（HCO₃⁻）利用抑制剂——碳酸酐酶抑制剂乙酰唑胺，以及三羟甲基氨基甲烷（TRIS）缓冲液（pH 8.0）。结果显示，在所有光照水平下，C. serrulata均高度依赖能量驱动的H⁺-HCO₃⁻协同转运过程（TRIS可破坏H⁺外排过程），这表明其在低光照条件下对CO₂的依赖性更强。该结论在为期两周的暴露培养实验后得到验证：相较于高光处理组的C. serrulata（光合速率提升4%），DIC富集对低光照（35 μmol·m⁻²·s⁻¹）下培养的该物种的最大光合速率（Pmax）与光合效率（α）的促进作用更为显著，增幅可达36%至60%。不过，C. serrulata的生长随DIC富集与光照水平提升均呈增加趋势。而H. uninervis的生长、净初级生产力（NPP）以及光合响应均随光照处理强度升高而增强，且与DIC可用性无关。此外，研究发现H. uninervis在HCO₃⁻摄取通路上具有更高的灵活性。本研究表明，光照可用性可通过碳固定与碳获取两种途径，影响海草生产力对DIC富集的响应，这凸显了水质在未来海洋酸化响应中的关键作用。