Particulate dimethylsulfoniopropionate at DYNAPROC station

In the framework of the projects DYFAMED and PICASSO, diel variations of particulate dimethylsulfoniopropionate (DMSPp) and of its size fraction higher than 10 µm (DMSPp>10 µm) were studied in surface waters of the central Ligurian Sea in May 1990 and May 1995, and in the harbour of the city of Barcelona (Spain) in July 1998. Time series performed in stratified and nitrate depleted surface waters of the Ligurian Sea revealed that DMSPp>10 µm was undergoing diurnal variations. DMSPp-containing particles in the size range higher than 10 µm also markedly affected the DMSPp-to-chlorophyll (chl) a ratio of surface waters on a daily basis. The ratios were 35% to 72% lower at dawn than at dusk. The fact that the diadinoxanthin (DD)-to-chl a ratio of surface phytoplanktonic populations did not exhibit diurnal cycles suggests that physiological adaptation of cellular DMSP and chl a to the light conditions was not a likely process to account for the diurnal changes of the DMSPp-to-chl a ratio. It is suggested that such diurnal variability resulted from changes in plankton composition due to vertical migrations of DMSP-containing organisms larger than 10 µm. We have demonstrated from samples collected in the harbour of the city of Barcelona that DMSP-containing dinoflagellates are active diel migrants. However, the results obtained in the open sea in May 1990 suggest that dinoflagellates and also ciliates contribute to the pool of DMSPp in the size range larger than 10 µm. The results of May 1995 are ambiguous as to the role of dinoflagellates because, in the absence of specific cell counts, DMSPp>10 µm and the pigment peridinin, which is usually present in dinoflagellates (but peridinin-free dinoflagellates exist) showed very different vertical and temporal patterns.

在DYFAMED与PICASSO项目框架下，本研究针对颗粒态二甲基巯基丙酸内盐（particulate dimethylsulfoniopropionate，DMSPp）及其粒径大于10μm的组分（DMSPp>10μm）的日变化特征展开分析：分别于1990年5月、1995年5月在利古里亚海中部表层水域，以及1998年7月在西班牙巴塞罗那港开展观测。在利古里亚海层化且硝酸盐耗竭的表层水域进行的时间序列研究显示，DMSPp>10μm存在显著昼夜变化。粒径大于10μm的含DMSPp颗粒，亦会在日尺度上显著影响表层水体的DMSPp与叶绿素a（chl a）比值，该比值在黎明时较黄昏时低35%至72%。表层浮游植物种群的硅甲藻黄素（diadinoxanthin，DD）与chl a比值无昼夜周期，这一事实表明，细胞内DMSP与chl a对光照条件的生理适应，并非解释DMSPp/chl a比值昼夜变化的合理机制。研究认为，此类昼夜变异性源自粒径大于10μm的含DMSP生物的垂直迁移所引发的浮游群落组成改变。基于巴塞罗那港采集的样本，我们证实含DMSP的甲藻属于活跃的昼夜垂直迁移类群。但1990年5月公海观测结果显示，甲藻与纤毛虫均参与构成了粒径大于10μm的DMSPp库。1995年5月的研究结果则对甲藻的作用界定模糊：由于缺乏特异性细胞计数数据，DMSPp>10μm与通常存在于甲藻中的色素多甲藻黄素（peridinin，尽管存在不含多甲藻黄素的甲藻）呈现出截然不同的垂直与时间分布模式。