Species-specific effects of two bioturbating polychaetes on sediment chemoautotrophic bacteria

Bioturbation has major impacts on sediment biogeochemistry, which can be linked tothe functional traits of the macrofauna involved. <i>Nereis (Hediste)</i> diversicolor and <i>Marenzelleriaviridis</i> are 2 functionally different bioturbating polychaetes that strongly affect the ecology andbiogeochemistry of coastal sediments. However, the different effects of these polychaetes on theactivity and composition of microbial communities and on chemoautotrophic bacteria have notbeen extensively studied. We performed experiments with sediment aquaria that contained eachspecies separately as well as a non-bioturbated control. Bacterial communities in different sedimentzones (surface, burrow, subsurface) were characterized by phospholipid-derived fatty acidanalysis combined with stable isotope labeling (¹³C bicarbonate) to quantify the dark CO₂ fixationby chemoautotrophic bacteria. Pore water chemistry (SH₂S and dissolved inorganic carbon) wasadditionally assessed in each treatment. The strong ventilation but low bioirrigation capacity inthe open-ended burrows of N. diversicolor resulted in enhanced aerobic chemoautotrophic activity,potentially by sulfur oxidizing and nitrifying bacteria along the burrow. In contrast, slowerventilation and higher irrigation by M. viridis induced an advective mode of pore water transport.This promotes anaerobic chemoautotrophy around the blind-ended burrow and within the subsurfacesediment. Sulfate-reducing bacteria were the dominant anaerobic chemoautotrophs thatprobably disproportionate sulfur. In conclusion, our analysis shows that bioturbating fauna influencethe microbial community and chemoautotrophic activity in sediments, but that the effectstrongly depends on the structure of the burrow and on species-specific ventilation behavior andirrigation capacity