Meiofauna improve oxygenation and accelerate sulfide removal in the seasonally hypoxic seabed

Oxygen depleted areas are widespread in the marine realm. Unlike macrofauna, meiofauna are abundant in hypoxic sediments. We studied to what extent meiofauna affect oxygen availability, sulfide removal and microbial communities. Meiofauna were extracted alive and added to intact sediments simulating abundance gradients previously reported in the area. A total of 324 porewater microprofiles were recorded over a 3-week incubation period and microbial community structure and cable bacteria densities were determined at the end of the experiment. At high abundances meiofauna activity deepened oxygen penetration by 85%, 59%, and 62% after 5, 14, and 22 days, respectively, compared to control sediment with scarce meiofauna. After 6 days, meiofauna increased the volume of oxidized, sulfide-free sediment by 68% and reduced sulfide fluxes from 8.8 to 0.4 mmol m^-2 d^-1. After 15 days, the difference with the control attenuated due to the presence of a cable bacteria population, which facilitated sulfides oxidation in all treatments. 16S rRNA gene analysis revealed that meiofauna affected microbial community structure (beta diversity). Thus, meiofauna bioturbation plays an important role in deepening oxygen penetration, counteracting euxinia and in structuring microbial diversity of hypoxic sediments. Co-existence with cable bacteria demonstrates neutralism interaction between these two ecosystem engineers.