Deep Cyanobacteria Layers: An Overlooked Aspect of Managing Risks of Cyanobacteria

Area of Science:

• Environmental microbiology
• Aquatic toxicology
• Public health

Background:

• Cyanotoxin exposure risk is linked to cyanobacteria location in water bodies.
• Deep water cyanobacteria layers are often unreported, raising concerns for drinking water safety.
• Sunfish Lake, Ontario, Canada, was studied due to its known deep cyanobacteria layer.

Purpose of the Study:

• To investigate the toxigenicity of a deep cyanobacteria layer in Sunfish Lake.
• To identify cyanobacteria species present in the deep layer and their toxin production.
• To assess the implications for drinking water safety and public health.

Main Methods:

• Screening for cyanotoxins and bioactive metabolites in the deep water layer.
• Analyzing the composition of the deep cyanobacteria layer, identifying dominant species.
• Quantifying microcystins and anabaenopeptins produced by the cyanobacteria.

Main Results:

• The deep cyanobacteria layer was found to be toxigenic, producing microcystins (0.03 μg L^-1 max) and anabaenopeptins (2.5 μg L^-1 max).
• The layer was primarily composed of *Planktothrix isothrix* with lower toxin quotas, and *Planktothrix rubescens* with higher toxin quotas at background levels.
• Co-occurrence of multiple toxigenic *Planktothrix* species was observed.

Conclusions:

• Routine surveillance of the entire water column is essential for early identification of toxigenic cyanobacteria.
• Environmental changes could favor *P. rubescens*, increasing cyanotoxin production and posing a future risk to drinking water.
• Enhanced monitoring is needed to predict and mitigate risks associated with deep cyanobacteria layers.