Multi-proxy approaches to investigate cyanobacteria invasion from a eutrophic lake into the circumjacent groundwater

To verify whether cyanobacteria can travel from eutrophic lakes into the surrounding groundwater, a large-scale field investigation, laboratorial incubations, and quartz column penetration tests were carried out in Lake Taihu (China). High-throughput sequencing of 16S rRNA gene amplicons indicated that cyanobacteria operational taxonomic units (OTUs) were present at fifteen out of forty total wells in four cardinal directions at varying distances from the shore of Lake Taihu, up to a maximum of forty-three kilometers. Six cyanobacteria genera were detected including Microcystis, Dolichospermum, Phormidium, Leptolyngbya, Pseudanabaena and Synechococcus. The proportions of Phormidium, Microcystis and Synechococcus OTUs in the total cyanobacterial community were 45.2%, 32.2% and 19.4%, respectively. The qRT-PCR results showed that cyanobacterial abundance decreased with increasing distance from the shore of Lake Taihu. Based on the microscopic analysis of cultures inoculated with groundwater, we found Microcystis, Dolichospermum and Phormidium. Five cyanobacterial genera were able to penetrate columns filled with quartz particles ranging from 100∼200 μm. Finer layers of quartz sands were found to be impenetrable. The rating of infiltration capabilities was Microcystis > Synechococcus > Nostoc > Phormidium > Cylindrospermopsis. Deficient concentrations of microcystins were found (< 1 µg L^-1) in the groundwater samples. Based on the consideration of different factors (cyanobacterial composition in Lake Taihu, peripheral groundwater, and algal soil crusts), it was deduced that Microcystis likely originated from the lake. Still, Phormidium was probably originated from the soil infiltration. These results suggest that cyanobacteria and their toxins could travel in the groundwater, but this is a size-dependent mechanism.