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

Abstract

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.

Introduction

Eutrophication of lakes and reservoirs may promote the growth of cyanobacteria, causing harmful cyanobacterial blooms. Cyanobacterial blooms can pose a threat to the biodiversity by outcompeting other beneficial phytoplankton species and depleting the dissolved oxygen, leading to the water quality deterioration and a variety of toxic secondary metabolites production (Chorus et al., 2000; Huisman et al., 2018; Ibelings et al., 2014; Paerl and Otten, 2013). Moreover, cyanobacteria blooms can cause severe economic (Merel et al., 2013), ecological (Huisman et al., 2005), and health problems (Carmichael and Boyer, 2016; Drobac et al., 2013).

During the exchange of water between the surface to the ground, it is still not clear whether the cyanobacteria can infiltrate from eutrophic lakes to the surrounding groundwater. Schinner et al. (2010) conducted a laboratory-scale experiment using columns packed with clean quartz and found that Microcystis aeruginosa and Anabaena flos-aquae can infiltrate through a column packed with quartz particles with average diameter of 763 μm. M. aeruginosa typically exist as colonial morphology, however, many single cells of M. aeruginosa have been found in eutrophic lakes (Zhu et al., 2018). This indicates that M. aeruginosa are capable of transporting through aeration zones in many cases. In addition, Scholl and Harvey (1992) showed that bacteria can transport through sandy aquifers, while Gkelis and Vlamis (2017) detected cyanobacteria traces in groundwater in Northern Greece using 16S rDNA amplification. Wu et al. (2008) reported that the cyanobacterium Microcystis had nearly no change in morphology, chlorophyll a, and photosynthetic activity being subjected to the darkness conditions for 30 days. (Tang et al., 2008) found that the cyanobacteria Phaeocystis antarctica decreased its photosynthetic capacity by only 60% after 150-days of dark condition. These findings strongly imply that cyanobacteria might be able to infiltrate from the eutrophic lakes into the surrounding groundwater systems and stay alive. Other genera of cyanobacteria could dominate eutrophic lakes, including Dolichospermum, Phormidium, and Oscillatoria (Dokulil and Teubner, 2000). However, the mechanisms and dominant factors that control cyanobacteria infiltration are still not well understood. Due to the difficulties in monitoring and tracing cyanobacterial cells in groundwater using conventional optical microscopy, the high-throughput sequencing method can be used to monitor the taxonomy and quantity of cyanobacteria. This method can also be used to detect dead bacteria due to the slow degradation rate of DNA and give an insight into the mechanisms which control the transport and survival of cyanobacteria in groundwater wells. This will help us to identify the risks related to groundwater utilization, such as drinking water production (Schmidt et al., 2019) and crop irrigation (Aw-Hassan et al., 2014; Massuel et al., 2017).

In recent years, microcystins were detected in groundwater wells in Saudi Arabia, however the microcystins were believed to have originated from the cyanobacteria living in the uncovered groundwater wells instead of infiltrating from the eutrophic surface water (Mohamed and Shehri, 2009). Yang et al. (2016) later reported that groundwater near Lake Chaohu, China was contaminated by the microcystins because of the toxic cyanobacteria blooms in that lake. This phenomenon was probably due to the exchange of dissolved microcystins between the groundwater and lake water. However, the evidence regarding the availability and quantity of cyanotoxins in groundwater is still scarce. Its quantification may become relevant if we consider water's uses (like crops, irrigation, or human or animal consumption (Aw-Hassan et al., 2014; Massuel et al., 2017; Schmidt et al., 2019).

The aims of this study were to a) verify if the cyanobacteria can infiltrate from the eutrophic lakes to the surrounding groundwater; if yes, b) to determine if this infiltration is genus dependent; and c) to measure the concentrations of detected microcystins isomers. Attending to all of this, in this study, we attempt through a multi-proxy approach including wells water environmental DNA (eDNA) examination, laboratory incubations, cyanotoxins detection, and quartz column penetration tests to a) verify if the photosynthetically active cyanobacteria could be found in the surrounding groundwater from a eutrophic lake, b) explore some mechanisms associated with their penetration in the soil, and c) determine if cyanotoxins could be detected in concentrations that could represent an environmental issue.