近岸水域，與外海或大洋相比，生物生產力高，生物同化與異化作用導致明顯的pH晝夜變化。這種碳酸鹽化學的晝夜變化，與全球海洋酸化相比，可能對生物產生更明顯的影響。為此，研究海水碳酸鹽系統振蕩及其條件下的海洋酸化生理生態效應，是認識近海生物響應酸化機制的重要環節。

高坤山教授課題組以近岸（威氏海鏈藻，Thalassiosira weissflogii）及大洋硅藻（大洋海鏈藻，Thalassiosira oceanica）為研究對象，探討了海水碳酸鹽系統晝夜變化條件下CO₂濃度升高（酸化）對其生長及光合生理的影響。結果表明，酸化及海水碳酸鹽系統晝夜變化對兩種硅藻表現出截然不同的影響。酸化提高威氏海鏈藻的呼吸作用，促進胞內有機物的生產速率，對其生長沒有影響。而大洋海鏈藻的生長速率明顯受到酸化處理的抑制。海水碳酸鹽系統晝夜變化對威氏海鏈藻沒有明顯影響，卻顯著降低了大洋海鏈藻的生長速率及胞內有機物的生產速率。顯然，兩種不同生態位的硅藻對酸化具有不同的響應，其適應策略也會不同。另外，該結果暗示，浮游植物對碳酸鹽系統晝夜變化的適應能力，可能是影響其地理分布的一個重要因素。

該研究成果近日以“Physiological responses of coastal and oceanic diatoms to diurnal fluctuations in seawater carbonate chemistry under two CO₂ concentrations”為題，發表在Biogeosciences期刊上。第一作者為2014級博士生李富田，通訊作者為高坤山教授。 

Figure 1. Measured pH values under different treatments over a diel cycle. LCs (blue closed triangles): steady regime under ambient CO₂ level; LCf (blue open triangles): fluctuating regime under ambient CO₂ level; HCs (red closed circles): steady regime under elevated CO₂ level; HCf (red open circles): fluctuating regime under elevated CO₂ level.

Figure 2 Specific growth rate, respiration rate, POC and PON production rates of the oceanic diatom Thalassiosira oceanica in steady (columns without hatching) and fluctuating  (columns with hatching) regimes under ambient (LC, blue columns) and elevated (HC, red columns) CO₂levels.

Abstract: Diel and seasonal fluctuations in seawater carbonate chemistry are common in coastal waters, while in the open-ocean carbonate chemistry is much less variable. In both of these environments, ongoing ocean acidification is being superimposed on the natural dynamics of the carbonate buffer system to influence the physiology of phytoplankton. Here, we show that a coastal Thalassiosira weissflogii isolate and an oceanic diatom, Thalassiosira oceanica, respond differentially to diurnal fluctuating carbonate chemistry in current and ocean acidification (OA) scenarios. A fluctuating carbonate chemistry regime showed positive or negligible effects on physiological performance of the coastal species. In contrast, the oceanic species was significantly negatively affected. The fluctuating regime reduced photosynthetic oxygen evolution rates and enhanced dark respiration rates of T. oceanica under ambient CO₂ concentration, while in the OA scenario the fluctuating regime depressed its growth rate, chlorophyll a content, and elemental production rates. These contrasting physiological performances of coastal and oceanic diatoms indicate that they differ in the ability to cope with dynamic pCO₂. We propose that, in addition to the ability to cope with light, nutrient, and predation pressure, the ability to acclimate to dynamic carbonate chemistry may act as one determinant of the spatial distribution of diatom species. Habitat-relevant diurnal changes in seawater carbonate chemistry can interact with OA to differentially affect diatoms in coastal and pelagic waters.

Reference:  Li, F., Wu, Y., Hutchins, D. A., Fu, F., and Gao, K.: Physiological responses of coastal and oceanic diatoms to diurnal fluctuations in seawater carbonate chemistry under two CO₂ concentrations, Biogeosciences, 13, 6247-6259, doi:10.5194/bg-13-6247-2016, 2016.

Link to full text: http://www.biogeosciences.net/13/6247/2016/bg-13-6247-2016.pdf