Co2 Alters Picophytoplankton Community Structure In Freshwater Ecosystems

Seasonal in situ microcosm experiments were conducted from August 2013 to April 2014 in East Lake Taihu to study the responses of picophytoplankton to CO2 enrichment in freshwater ecosystems. Three CO2 levels (270 ppm, 380 ppm and 750 ppm) were implemented to simulate pre-industrial, present and predicted CO2 concentrations in the year 2100, respectively. The results showed that picophytoplankton accounted for a major proportion of phytoplankton in East Lake Taihu, and their chlorophyll-a content contributed a maximum of 79 % to the total chlorophyll-a concentration in winter. Phycocyanin-rich picocyanobacteria (PC-cells) dominated the picophytoplankton community in terms of cell number in East Lake Taihu in all seasons except winter, when only photosynthetic picoeukaryotes (PPEs) were observed. PC-cells were not sensitive to changes in CO2 within the tested CO2 range in this study. In contrast, CO2 elevation significantly increased the abundances of PPEs in spring but decreased their abundances in winter. High-throughout sequencing of PPEs was performed for sorted PPE samples in spring and winter from the three CO2 treatments. The results showed that Chlamydomonas and Cryptophyceae were not favored by the high CO2 treatment, whereas Chrysophyceae benefitted from an increase in CO2 level.