Effect of temperature on the unimodal size scaling of phytoplankton growth

contrary to predictions of fundamental laws in biology, there are evidences that phytoplankton growth rates peak at intermediate cell sizes. however, it is still unknown if this pattern may result from the effect of experimental temperature. here we test whether temperature affects the unimodal size scaling pattern of phytoplankton growth by 1) growing synechococcus sp., ostreococcus tauri, micromonas commoda and pavlova lutheri at 18℃ and 25℃, and 2) using thermal response curves available in the literature to estimate the growth rate at 25ºc as well as the maximum growth rate at optimal temperature for 22 species assayed previously at 18ºc. we also assess the sensitivity of growth rate estimates to the metric employed for measuring standing stocks, by calculating growth rates based on in vivo fluorescence, chlorophyll a concentration, cell abundance and biomass (particulate organic carbon and nitrogen content). our results show that the unimodal size scaling pattern of phytoplankton growth, with a peak at intermediate cell sizes, is observed at 18℃, 25℃ and at the optimal temperature for growth, and that it prevails irrespective of the standing-stock metric used. the unimodal size scaling pattern of phytoplankton growth is supported by two independent field observations reported in the literature: i) a positive relationship between cell size and metabolic rate in the picophytoplankton size range and ii) the dominance of intermediate-size cells in nutrient-rich waters during blooms.

尽管与生物学基本定律的预测相悖，但有证据表明浮游植物的生长速率在中等细胞尺寸时达到峰值。然而，尚不清楚这一模式是否源于实验温度的影响。在本研究中，我们通过以下两种方式检验温度是否会影响浮游植物生长的单峰尺寸比例模式：一是将蓝藻（Synechococcus sp.）、海洋球藻（Ostreococcus tauri）、微小拟球藻（Micromonas commoda）和帕夫洛夫菌（Pavlova lutheri）在18℃和25℃下培养；二是利用文献中可用的热响应曲线，估计25℃下的生长速率以及先前在18℃下测定的22个物种在最佳温度下的最大生长速率。此外，我们还通过计算基于体内荧光、叶绿素a浓度、细胞数量和生物量（颗粒有机碳和氮含量）的生长速率，评估了生长速率估计对测量现存生物量所采用的计量方法的敏感性。我们的结果表明，浮游植物生长的单峰尺寸比例模式，即峰值出现在中等细胞尺寸，在18℃、25℃以及生长的最佳温度下均有观察到，并且这一模式在使用不同的现存生物量计量方法时均普遍存在。浮游植物生长的单峰尺寸比例模式得到了文献中两篇独立现场观察的证实：一是微浮游植物尺寸范围内细胞尺寸与代谢率之间的正相关关系；二是富营养化水体中在中等尺寸细胞的优势地位。