ABSTRACT: Bioturbation has major impacts on sediment biogeochemistry, which can be linked to the functional traits of the macrofauna involved. Nereis (Hediste) diversicolor and Marenzelleria viridis are 2 functionally different bioturbating polychaetes that strongly affect the ecology and biogeochemistry of coastal sediments. However, the different effects of these polychaetes on the activity and composition of microbial communities and on chemoautotrophic bacteria have not been extensively studied. We performed experiments with sediment aquaria that contained each species separately as well as a non-bioturbated control. Bacterial communities in different sediment zones (surface, burrow, subsurface) were characterized by phospholipid-derived fatty acid analysis combined with stable isotope labeling (13C bicarbonate) to quantify the dark CO2 fixation by chemoautotrophic bacteria. Pore water chemistry (ΣH2S and dissolved inorganic carbon) was additionally assessed in each treatment. The strong ventilation but low bioirrigation capacity in the open-ended burrows of N. diversicolor resulted in enhanced aerobic chemoautotrophic activity, potentially by sulfur oxidizing and nitrifying bacteria along the burrow. In contrast, slower ventilation 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 subsurface sediment. Sulfate-reducing bacteria were the dominant anaerobic chemoautotrophs that probably disproportionate sulfur. In conclusion, our analysis shows that bioturbating fauna influence the microbial community and chemoautotrophic activity in sediments, but that the effect strongly depends on the structure of the burrow and on species-specific ventilation behavior and irrigation capacity.
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