Acidification increases bacterioplankton dependency and strengthens fungal community resilience

Microorganisms form the basis of ocean ecosystems yet the effects of perturbations such as decreasing pH on community structure, interactions and functionality remains less understood than compared with marine multicellular organisms. Using an experimental manipulation of seawater from the Southern Ocean we subjected bacterioplankton and mycoplankton to artificial pH decreases. Acidification led to a substantial increase in the diversity of bacterioplankton compared with mycoplankton. Bacterioplankton community structure and network robustness were also altered at lower pH, suggesting instability due to shift in community composition. In contrast, the mycoplankton responses to pH change were limited suggesting a higher stability compared to bacterioplankton. Additionally, the activities of extracellular enzymes linked to nitrogen acquisition decreased under acidified conditions, potentially reflecting shifts in organic matter cycling. Our findings suggest that bacterioplankton and mycoplankton may exhibit contrasting responses to future ocean acidification, with potentially unpredictable impacts on the community structure and biogeochemical cycling in marine ecosystems.