Surviving the depths: metazoan resilience in sulphidic aquaria environments

Hydrogen sulphide (H2S) is widely acknowledged as a potent respiratory toxin for eukaryotic cells. However, macrofauna have been observed thriving in environments with elevated H2S concentrations. Here we report on a saltwater aquarium hosting a community of invertebrates and inhabited by an epibenthic microbial mat. The aquarium was left undisturbed for the duration of the COVID-19 pandemic stay-at-home order, leading to the development of high concentrations of H2S. Remarkably, the invertebrate community did not collapse. This success offers valuable insights into how invertebrates respond to physiochemical stressors at both individual and community levels. We also observed persistent disequilibrium between H2S and oxygen (O2), exhibiting out-of-phase periodic cycles driven by a simulated solar cycle. During daylight, photosynthetic O2 production increased, resulting in more active behaviour from the metazoan community. Conversely, H2S production peaked during the dark cycle, causing a moribund animal community. Additionally, over time, overall community diversity in the tank decreased, while macrofaunal abundance appeared largely unaffected. Polychaete worms and cnidarians demonstrated resilience to the high-sulphide conditions for the entire duration of the experiment, whereas others experienced gradual declines in abundance until they perished. These findings challenge conventional expectations of eukaryotic tolerance to H2S and underscore the significance of behavioural adaptations in withstanding high-sulphide environments. Our findings provide insights into how primitive metazoans may have survived in sulphidic to euxinic Ediacaran seas.