Seawater carbonate chemistry, biomass and metabolic rates (leucine incorporation, CO2 fixation and respiration) of Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217) in a laboratory experiment@en

Experimental results related to the effects of ocean acidification on planktonic marine microbes are still rather inconsistent and occasionally contradictory. Moreover, laboratory or field experiments that address the effects of changes in CO2 concentrations on heterotrophic microbes are very scarce, despite the major role of these organisms in the marine carbon cycle. We tested the direct effect of an elevated CO2 concentration (1000 ppmv) on the biomass and metabolic rates (leucine incorporation, CO2 fixation and respiration) of 2 isolates belonging to 2 relevant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217). Our results demonstrate that, contrary to some expectations, high pCO2 did not negatively affect bacterial growth but increased growth efficiency in the case of MED217. The elevated partial pressure of CO2 (pCO2) caused, in both cases, higher rates of CO2 fixation in the dissolved fraction and, in the case of MED217, lower respiration rates. Both responses would tend to increase the pH of seawater acting as a negative feedback between elevated atmospheric CO2 concentrations and ocean acidification.有关海洋酸化对海洋浮游微生物影响的实验结果迄今仍存在较大分歧，且偶尔会出现相互矛盾的结论。此外，尽管异养微生物在海洋碳循环中发挥着关键作用，但探讨二氧化碳浓度变化对异养微生物影响的实验室或现场实验仍十分匮乏。本研究测试了升高的二氧化碳浓度（1000 ppmv）对2株隶属于重要海洋细菌科的菌株的生物量及代谢速率（亮氨酸掺入量、二氧化碳固定速率与呼吸速率）的直接影响，受试菌株分别为红杆菌科（Rhodobacteraceae）的MED165菌株与黄杆菌科（Flavobacteriaceae）的MED217菌株。研究结果显示，与部分预期相悖的是，高二氧化碳分压（pCO2）并未对细菌生长产生抑制作用，反而提升了MED217菌株的生长效率。升高的二氧化碳分压（pCO2）使得两株菌株的溶解组分中二氧化碳固定速率均有所提升；同时针对MED217菌株，其呼吸速率出现下降。这两种响应均会倾向于提升海水pH值，从而在大气二氧化碳浓度升高与海洋酸化之间形成负反馈效应。