Electrophysiological monitoring of nutrient stress in Oscillatoria sp. cohorts: Toward an early-warning tool for harmful algal blooms.

Cyanobacterial harmful algal blooms (HABs) are a growing challenge for freshwater management, traditionally addressed through phosphorus (P) limitation strategies. However, increasing evidence highlights nitrogen (N) availability as a key driver of bloom initiation and persistence. Here, we report extracellular voltage recordings from Oscillatoria cohorts exposed to four days of N starvation followed by ammonium (NH₄⁺) repletion using 5 mg L-1. Under N deprivation, electrical signalling developed progressively with a median of 1-2 events min-1 and amplitudes between 3-4 µV on day 1 and 2, following a marked increase in activity by days 3 and 4, with median spike rates of 5 events min-1 and amplitudes up to 17 µV. Following NH4 + repletion, signalling activity declined within 24 h, indicating rapid restoration of N balance. These results suggest that electrophysiological monitoring can serve as a real-time, non-invasive indicator of nutrient stress in Oscillatoria and provides a potential early warning tool for HAB onset and associated taste and odour (T&O) outbreaks in freshwater systems. Supplementary Information: The online version contains supplementary material available at 10.1557/s43580-025-01486-3.