Assessment of microbial activity in water based on hydrogen peroxide decomposition rates

This study proposes a new and simple assay that allows rapid assessment of microbial activity in water samples. The assay consists of standardized hydrogen peroxide (H₂O₂) addition to a water sample and subsequent spectrophotometric determination of H₂O₂ reduction over time. The H₂O₂ decomposition rate constant reflects the level of enzymatic activity from planktonic and particle-associated bacteria as well as algae and protozoans. The proof of concept was verified on water samples from recirculating aquaculture systems (RAS), showing that the vast majority of H₂O₂ decomposition was related to microbial activity. Only 3% of the total H₂O₂ decomposition was related to abiotic processes when 0.20 μm sterile filtered RAS water was compared with unfiltered RAS water. Planktonic bacteria (size range 0.20–1.6 μm) accounted for 16% of H₂O₂ decomposition, while bacterial aggregates, particle-associated bacteria and microbiota above 1.6 μm were responsible for the remaining 81%. H₂O₂ decomposition rate constants were positively correlated to BOD₅ (r = 0.893; p < 0.001; n = 18) and to the number of 1–30 μm micro particles (r = 0.909; p < 0.001; n = 72) in RAS water, substantiating the biologically mediated decomposition processes in the water phase. The H₂O₂ decomposition assay thus represents a new alternative to existing methods that allows rapid (1–2 h) and simple quantification of microbial activity in fresh- and saltwater samples from aquaculture systems. Potential applications of the assay are discussed.