Revealing hydrogen peroxide as an external stressor in macrophyte-dominated coastal ecosystems

Benthic primary producers in coastal ecosystems provide important habitat for marine organisms through the provision of complex 3D habitat. Primary producers produce organic matter, while simultaneously producing reactive oxygen species, including hydrogen peroxide (H₂O₂), a driver of oxidative stress. Through their high biomass, productivity and effect on local hydrodynamics, benthic primary producers can potentially increase H₂O₂ concentrations surrounding the biogenic structures they form. The aim of this study was to identify the potential role of H₂O₂ produced by benthic primary producers as an external stressor in coastal ecosystems. This was achieved by measuring H₂O₂ concentrations within sea lettuce blooms (Ulva sp.), giant kelp forests (Macrocystis pyrifera), and seagrass meadows (Zostera muelleri); quantifying H₂O₂ production rates of these species; and testing heterotrophic bacterial response to relevant H₂O₂ concentrations. Ulva sp. produced five times more H₂O₂ than other species. At in situ concentrations, H₂O₂ inhibited bacterial production and carbon flow through the microbial loop by 75%. This study reveals H₂O₂ as an additional stressor in bloom-forming Ulva sp. with higher H₂O₂ production compared to the ecosystem engineers M. pyrifera and Z. muelleri. H₂O₂ production by benthic primary producers can affect carbon flow through the microbial loop, with the potential to propagate a stress signal up the food web.