Drying harvested blue-green algae sludge halves methane emissions in experiments

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In some lakes around the world, including some in China, cyanobacterial blooms are increasingly managed through large-scale mechanical harvesting to improve water quality and ensure a continued supply of drinking water. However, as a recent study shows, this does not prevent greenhouse gas emissions.

Just like the affected lakes, the harvested cyanobacterial sludge also releases methane, particularly when it still has a high moisture content. The results of the collaboration between two Chinese research institutions and the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) were published in the Journal of Cleaner Production. Cyanobacteria, also known as blue-green algae, often lead to water quality issues in freshwater ecosystems during the summer months.

This is because, following a bloom, large numbers of cyanobacteria die and decompose, consuming oxygen in the process. Furthermore, cyanobacteria can produce toxic substances. Lakes contaminated with cyanobacteria also release increasing amounts of greenhouse gases.

According to estimates, they account for around 19% of global methane emissions. For this reason, large-scale algal blooms are increasingly being harvested mechanically by skimming the algal mats off the surface. In China, this method is routinely implemented as a rapid-response measure during cyanobacterial blooms.

Until now, however, little was known about the quantities of greenhouse gases that can be released when harvesting blue-green algae sludge. Lake Dianchi, a hypereutrophic lake in southwestern China, is often affected by large cyanobacterial blooms. A Chinese research team, together with IGB researcher Hans-Peter Grossart, conducted two experiments to investigate how much methane is produced in the lake and in the harvested blue-green algae sludge.

In one of the experiments, they analyzed methane production at different levels of sludge moisture. They found that the concentrations of greenhouse gases were positively correlated with chlorophyll a and turbidity, but negatively correlated with dissolved oxygen and pH. This suggests that higher algal biomass and the associated physico-chemical changes promote the emission of greenhouse gases.