PONE-D-14-40234R2 The Physiological Response of Two Green Calcifying Algae From the Great Barrier Reef Towards High Dissolved Inorganic and Organic Carbon (DIC and DOC) Availability

Increasing dissolved inorganic carbon (DIC) concentrations associated with ocean acidification can affect marine calcifiers, but local factors, such as high dissolved organic carbon (DOC) concentrations through sewage and algal blooms, may interact with this global factor. For calcifying green algae of the genus Halimeda, a key tropical carbonate producer that often occurs in coral reefs, no studies on these interactions have been reported. These data are however urgently needed to understand future carbonate production. Thus, we investigated the independent and combined effects of DIC (pCO2 402 µatm/ pHtot 8.0 and 996 µatm/ pHtot 7.7) and DOC (added as glucose in 0 and 294 µmol L-1) on growth, calcification and photosynthesis of H. macroloba and H. opuntia from the Great Barrier Reef in an incubation experiment over 16 days. High DIC concentrations significantly reduced dark calcification of H. opuntia by 130 % and led to net dissolution, but did not affect H. macroloba. High DOC concentrations significantly reduced daily oxygen production of H. opuntia and H. macroloba by 78 % and 43 %, respectively, and significantly reduced dark calcification of H. opuntia by 70%. Combined high DIC and DOC did not show any interactive effects for both algae, but revealed additive effects for H. opuntia where the combination of both factors reduced dark calcification by 162 % compared to controls. Such species-specific differences in treatment responses indicate H. opuntia is more susceptible to a combination of high DIC and DOC than H. macroloba. From an ecological perspective, results further suggest a reduction of primary production for Halimeda-dominated benthic reef communities under high DOC concentrations and additional decreases of carbonate accretion under elevated DIC concentrations, where H. opuntia dominates the benthic community. This may reduce biogenic carbonate sedimentation rates and hence the buffering capacity against further ocean acidification.

与海洋酸化相关的溶解无机碳（dissolved inorganic carbon, DIC）浓度升高，可对海洋钙化生物造成影响；而诸如污水与藻华引发的高溶解有机碳（dissolved organic carbon, DOC）浓度这类局地因子，可能会与这一全球尺度因子产生交互作用。对于广泛分布于珊瑚礁中的关键热带碳酸盐生产者——仙掌藻属（Halimeda）钙化绿藻而言，目前尚无针对这类交互作用的相关研究报道。然而，要理解未来海洋碳酸盐生产格局，亟需这类研究数据。因此，本研究开展了为期16天的室内培养实验，探究了DIC（设置两组浓度梯度：分压二氧化碳pCO₂为402 µatm、总pH（pH_tot）8.0，以及996 µatm、总pH7.7）与DOC（以葡萄糖为添加源，设置0与294 µmol·L⁻¹两个浓度）对采自大堡礁的大叶仙掌藻（H. macroloba）与团扇仙掌藻（H. opuntia）的生长、钙化作用与光合作用的独立及联合效应。
高DIC浓度可使团扇仙掌藻的黑暗钙化量较对照组显著降低130%，并引发净溶解现象，但未对大叶仙掌藻造成显著影响。高DOC浓度则分别使团扇仙掌藻与大叶仙掌藻的每日产氧量显著降低78%与43%，同时使团扇仙掌藻的黑暗钙化量显著降低70%。高DIC与高DOC的联合处理并未对两种藻类表现出任何交互效应，但对团扇仙掌藻呈现出加性效应：相较于对照组，两种因子的联合作用使其黑暗钙化量降低162%。
这类处理响应的物种特异性差异表明，相较于大叶仙掌藻，团扇仙掌藻对高DIC与高DOC的联合胁迫更为敏感。从生态学视角来看，本研究结果进一步表明，当底栖礁群落以仙掌藻属物种为主时，高DOC浓度会降低其初级生产力；而在以团扇仙掌藻为优势种的底栖礁群落中，高DIC浓度会进一步减少碳酸盐沉积量。这可能会降低生物碳酸盐沉积速率，进而削弱其对进一步海洋酸化的缓冲能力。