Calcite addition as a CO2 removal strategy in seasonal hypoxic coastal waters promotes cable bacteria initiating changes in local biogeochemical cycles

Enhanced benthic weathering (EBW) is a promising carbon dioxide (CO2) removal (CDR) technique. By adding alkaline minerals to the seafloor, weathering is accelerated and alkalinity is released to the water column fostering CDR. In the Baltic Sea, seasonally hypoxic bottom waters are considered ideal settings for EBW, but feedbacks of microbial metabolisms remain unknown. Here, Baltic Sea sediments exposed to seasonal hypoxia were used for sediment core incubations and alkaline minerals supplemented to sediment surfaces. Calcite addition gave the highest CDR potential and cable bacteria became enriched. Based on transcript abundances their activity stimulated microbial sulfur-, iron- and manganese oxidation, conversion of arsenic species and dinitrogen release from the sediments, indicating considerable shifts in biogeochemical cycling. Carbonic anhydrases (converting bicarbonate into CO2) were massively upregulated indicative of a bicarbonate source in the sediments likely related to calcite dissolution. This appeared to trigger autotrophic CO2-fixation. The fate of the synthesized biomass remains unknown and may pose a CO2 sink or an unaccounted CO2 source. The increased buffering capacity of the sediments after artificial calcite addition may essentially affect Fe-oxide crust formation. These Fe-oxides are hypothesized to hinder sulfide release from the sediments during seasonal hypoxia, thus restricted mineral precipitation could promote the expansion of anoxia. Several feedback scenarios are posited following calcite addition that need to be better constrained. Albeit, they need to be taken with caution as they are based on transcript abundances alone and actual rate measurements are urgently required to scrutinize the impacts of EBW for such systems.