Increased CO2 modifies the carbon balance and the photosynthetic yield of two common Arctic brown seaweeds: Desmarestia aculeata and Alaria esculenta

Ocean acidification affects with special intensity Arctic ecosystems, being marine photosynthetic organisms a primary target, although the consequences of this process in the carbon fluxes of Arctic algae are still unknown. The alteration of the cellular carbon balance due to physiological acclimation to an increased CO2 concentration (1300 ppm) in the common Arctic brown seaweeds Desmarestia aculeata and Alaria esculenta from Kongsfjorden (Svalbard) was analysed. Growth rate of D. aculeata was negatively affected by CO2 enrichment, while A. esculenta was positively affected, as a result of a different reorganization of the cellular carbon budget in both species. Desmarestia aculeata showed increased respiration, enhanced accumulation of storage biomolecules and elevated release of dissolved organic carbon, whereas A. esculenta showed decreased respiration and lower accumulation of storage biomolecules. Gross photosynthesis (measured both as O2 evolution and 14C fixation) was not affected in any of them, suggesting that photosynthesis was already saturated at normal CO2 conditions and did not participate in the acclimation response. However, electron transport rate changed in both species in opposite directions, indicating different energy requirements between treatments and species specificity. High CO2 levels also affected the N-metabolism, and 13C isotopic discrimination values from algal tissue pointed to a deactivation of carbon concentrating mechanisms. Since increased CO2 has the potential to modify physiological mechanisms in different ways in the species studied, it is expected that this may lead to changes in the Arctic seaweed community, which may propagate to the rest of the food web.

海洋酸化对北极生态系统的影响尤为显著，海洋光合生物是其主要作用靶点，但目前学界对该过程对北极藻类碳通量的影响仍不明晰。本研究针对采自斯瓦尔巴（Svalbard）群岛康斯峡湾（Kongsfjorden）的两种常见北极褐藻——针状网地藻（Desmarestia aculeata）与美味翅藻（Alaria esculenta）展开分析，探究其在二氧化碳浓度升高（1300 ppm）条件下通过生理驯化引发的细胞碳平衡变化。研究结果显示，二氧化碳富集对针状网地藻的生长速率产生负面影响，却对美味翅藻的生长速率具有促进作用，这源于两种藻类的细胞碳预算重组模式存在差异。针状网地藻表现出呼吸作用增强、储存生物分子积累增加以及溶解性有机碳（dissolved organic carbon, DOC）释放量提升的特征；而美味翅藻则呈现呼吸作用减弱、储存生物分子积累降低的变化。总光合速率（分别通过氧气释放量与¹⁴C固定量测定）在两种藻类中均未受影响，这表明在正常二氧化碳浓度条件下，光合作用已达到饱和状态，未参与本次驯化响应。然而，电子传递速率在两种藻类中呈现相反的变化趋势，这说明不同处理组间的能量需求存在差异，且该响应具有物种特异性。高二氧化碳浓度同样影响了氮代谢（N-metabolism），藻类组织的¹³C同位素分馏值表明其碳浓缩机制（carbon concentrating mechanisms, CCM）出现失活。鉴于二氧化碳浓度升高可通过不同途径改变所研究物种的生理机制，预计这将引发北极褐藻群落结构的变化，并进一步传递至整个食物网。