Effects of high dissolved inorganic and organic carbon availability on the physiology of the hard coral Acropora millepora from the Great Barrier Reef

Coral reefs are facing major global and local threats due to climate change-induced increases in dissolved inorganic carbon (DIC) and because of land-derived increases in organic and inorganic nutrients. Recent research revealed that high availability of labile dissolved organic carbon (DOC) negatively affects scleractinian corals. Studies on the interplay of these factors, however, are lacking, but urgently needed to understand coral reef functioning under present and near future conditions. This experimental study investigated the individual and combined effects of ambient and high DIC (<i>p</i>CO₂ 403 μatm/ pH_Total 8.2 and 996 μatm/pH_Total 7.8) and DOC (added as Glucose 0 and 294 μmol L^-1, background DOC concentration of 83 μmol L^-1) availability on the physiology (net and gross photosynthesis, respiration, dark and light calcification, and growth) of the scleractinian coral <i>Acropora millepora</i> (Ehrenberg, 1834) from the Great Barrier Reef over a 16 day interval. High DIC availability did not affect photosynthesis, respiration and light calcification, but significantly reduced dark calcification and growth by 50 and 23%, respectively. High DOC availability reduced net and gross photosynthesis by 51% and 39%, respectively, but did not affect respiration. DOC addition did not influence calcification, but significantly increased growth by 42%. Combination of high DIC and high DOC availability did not affect photosynthesis, light calcification, respiration or growth, but significantly decreased dark calcification when compared to both controls and DIC treatments. On the ecosystem level, high DIC concentrations may lead to reduced accretion and growth of reefs dominated by <i>Acropora</i> that under elevated DOC concentrations will likely exhibit reduced primary production rates, ultimately leading to loss of hard substrate and reef erosion. It is therefore important to consider the potential impacts of elevated DOC and DIC simultaneously to assess real world scenarios, as multiple rather than single factors influence key physiological processes in coral reefs.

珊瑚礁正面临全球性与地方性的多重严重威胁：一方面是气候变化导致的溶解无机碳（dissolved inorganic carbon, DIC）升高，另一方面是陆源输入的有机与无机营养盐增加。近期研究表明，易变溶解有机碳（labile dissolved organic carbon, DOC）的高可用性会对石珊瑚产生负面影响。然而，目前针对这些因子交互作用的研究仍有缺失，但为理解当前及近未来情境下的珊瑚礁生态功能，此类研究亟需开展。本实验研究探究了环境背景与高浓度DIC（二氧化碳分压pCO₂分别为403 μatm/总pH pHTotal 8.2，以及996 μatm/pHTotal 7.8）、DOC（以葡萄糖形式添加，浓度分别为0与294 μmol L⁻¹，背景DOC浓度为83 μmol L⁻¹）的单一及联合作用，对采集自大堡礁的石珊瑚米氏轴孔珊瑚（Acropora millepora, Ehrenberg, 1834）的生理指标——包括净光合与总光合速率、呼吸作用、暗钙化与光钙化作用以及生长速率——的影响，实验周期为16天。
高DIC浓度对光合作用、呼吸作用及光钙化无显著影响，但可使暗钙化速率与生长速率分别显著降低50%与23%。高DOC浓度可使净光合与总光合速率分别降低51%与39%，但对呼吸作用无显著影响。添加DOC未改变钙化作用，但可使生长速率显著提升42%。高DIC与高DOC的联合处理未对光合作用、光钙化、呼吸作用及生长产生影响，但相较于对照组与单一DIC处理组，可显著降低暗钙化速率。
从生态系统层面来看，高DIC浓度可能导致轴孔珊瑚（Acropora）占优势的礁体堆积与生长受到抑制；而当DOC浓度升高时，此类礁体的初级生产力大概率会下降，最终引发硬质底质流失与礁体侵蚀。因此，鉴于珊瑚礁的关键生理过程受多重而非单一因子调控，在评估现实世界情境时，需同时考虑DOC与DIC浓度升高的潜在影响，这一点至关重要。