Seawater carbonate chemistry and Microalgal photophysiology and macronutrient distribution in summer sea ice in the Amundsen and Ross Seas, Antarctica@en

Our study addresses how environmental variables, such as macronutrients concentrations, snow cover, carbonate chemistry and salinity affect the photophysiology and biomass of Antarctic sea-ice algae. We have measured vertical profiles of inorganic macronutrients (phosphate, nitrite + nitrate and silicic acid) in summer sea ice and photophysiology of ice algal assemblages in the poorly studied Amundsen and Ross Seas sectors of the Southern Ocean. Brine-scaled bacterial abundance, chl a and macronutrient concentrations were often high in the ice and positively correlated with each other. Analysis of photosystem II rapid light curves showed that microalgal cells in samples with high phosphate and nitrite + nitrate concentrations had reduced maximum relative electron transport rate and photosynthetic efficiency. We also observed strong couplings of PSII parameters to snow depth, ice thickness and brine salinity, which highlights a wide range of photoacclimation in Antarctic pack-ice algae. It is likely that the pack ice was in a post-bloom situation during the late sea-ice season, with low photosynthetic efficiency and a high degree of nutrient accumulation occurring in the ice. In order to predict how key biogeochemical processes are affected by future changes in sea ice cover, such as in situ photosynthesis and nutrient cycling, we need to understand how physicochemical properties of sea ice affect the microbial community. Our results support existing hypothesis about sea-ice algal photophysiology, and provide additional observations on high nutrient concentrations in sea ice that could influence the planktonic communities as the ice is retreating.本研究旨在探明大量营养盐（macronutrients）浓度、积雪覆盖、碳酸盐化学特征与盐度等环境变量，如何对南极海冰藻类的光生理学（photophysiology）及生物量产生影响。我们于夏季海冰中测定了无机大量营养盐（磷酸盐、亚硝酸盐+硝酸盐及硅酸）的垂直剖面，并针对南大洋研究程度较低的阿蒙森海与罗斯海海域的冰藻群落检测了其光生理学特征。按盐水体积换算的细菌丰度、叶绿素a（Chl a）与大量营养盐浓度在海冰中普遍处于较高水平，且三者间呈显著正相关。对光系统II（photosystem II，PSII）快速光曲线的分析显示，在磷酸盐与亚硝酸盐+硝酸盐浓度较高的样品中，微藻细胞的最大相对电子传递速率与光合效率均出现降低。我们还观测到PSII参数与积雪深度、海冰厚度及盐水盐度之间存在紧密关联，这表明南极浮冰藻类存在广泛的光适应（photoacclimation）现象。海冰季晚期的浮冰大概率处于水华后状态，此时海冰内光合效率较低且养分积累程度较高。为了预测海冰覆盖的未来变化（如原位光合作用与养分循环）如何影响关键生物地球化学过程，我们需要阐明海冰的物理化学性质如何作用于微生物群落。本研究结果验证了现有关于海冰藻类光生理学的假说，并补充了海冰中高浓度营养盐的观测数据——这类高浓度营养盐在海冰消融时，可能会对浮游生物群落产生影响。