PEECE II mesocosm experiment: Dynamics of extracellular enzyme activities in seawater under changed atmospheric pCO2, 2011

As part of the PeECE II mesocosm project, we investigated the effects of pCO2 levels on the initial step of heterotrophic carbon cycling in the surface ocean. The activities of microbial extracellular enzymes hydrolyzing 4 polysaccharides were measured during the development of a natural phytoplankton bloom under pCO2 conditions representing glacial (190 µatm) and future (750 µatm) atmospheric pCO2. We observed that (1) chondroitin hydrolysis was variable throughout the pre-, early- and late-bloom phases, (2) fucoidanase activity was measurable only in the glacial mesocosm as the bloom developed, (3) laminarinase activity was low and constant, and (4) xylanase activity declined as the bloom progressed. Concurrent measurements of microbial community composition, using denaturing-gradient gel electrophoresis (DGGE), showed that the 2 mesocosms diverged temporally, and from one another, especially in the late-bloom phase. Enzyme activities correlated with bloom phase and pCO2, suggesting functional as well as compositional changes in microbial communities in the different pCO2 environments. These changes, however, may be a response to temporal changes in the development of phytoplankton communities that differed with the pCO2 environment. We hypothesize that the phytoplankton communities produced dissolved organic carbon (DOC) differing in composition, a hypothesis supported by changing amino acid composition of the DOC, and that enzyme activities responded to changes in substrates. Enzyme activities observed under different pCO2 conditions likely reflect both genetic and population-level responses to changes occurring among multiple components of the microbial loop.作为PeECE II中型实验生态系统（mesocosm）项目的一部分，本研究探讨了二氧化碳分压（pCO2）水平对表层海洋异养碳循环初始步骤的影响。本研究在代表冰期（190微大气压）和未来（750微大气压）大气pCO2的条件下，于自然浮游植物水华（phytoplankton bloom）的发生过程中，测定了水解4种多糖的微生物胞外酶活性。本研究观测到：（1）在水华前期、早期与后期阶段，软骨素水解活性均存在波动；（2）随着水华发展，岩藻聚糖酶（fucoidanase）活性仅在冰期中型实验生态系统中可被检出；（3）昆布多糖酶（laminarase）活性始终处于较低水平且保持稳定；（4）木聚糖酶（xylanase）活性随水华进程逐渐降低。采用变性梯度凝胶电泳（DGGE）同步测定微生物群落组成的结果显示，两个中型实验生态系统的群落结构随时间发生分化，且二者间的差异在水华后期尤为显著。酶活性与水华阶段及pCO2水平显著相关，表明不同pCO2环境下的微生物群落不仅发生了组成结构的改变，同时也存在功能层面的变化。但上述变化可能是对浮游植物群落发育过程中时间动态变化的响应，而这类动态变化本身又因pCO2环境的不同而存在差异。本研究提出假说：不同pCO2环境下的浮游植物群落所产生的溶解有机碳（DOC）在组成上存在差异，这一假说得到了DOC氨基酸组成发生变化的实验结果支持；同时，酶活性会对底物组成的变化产生响应。在不同pCO2条件下观测到的酶活性，可能同时反映了微生物环（microbial loop）中多种组分发生变化时，微生物群落所产生的遗传层面及种群层面的响应。