Enhanced benthic weathering as a CO2 removal strategy in seasonal hypoxic coastal waters of the Baltic Sea is influenced by cable bacteria activity after re-ventilation of bottom waters

Since the industrial revolution, atmospheric CO2 has increased dramatically resulting in warming of Earth’s climate. The oceans are fundamental for mitigating global climate change as they absorb heat and CO2. Consequently, the oceans have changed, affecting marine life and biogeochemical cycles. To slow down climate change, different CO2 removal (CDR) strategies are being tested such as enhanced benthic weathering (EBW). In EBW, alkaline minerals are added to the seafloor, accelerating weathering and releasing alkalinity to the water body. In the Baltic Sea, sediments exposed to seasonal hypoxic bottom waters appear ideal settings for EBW; yet, feedbacks of microbial metabolisms to EBW remain unknown. We addressed this knowledge gap by performing sediment core incubations adding alkaline minerals to Baltic Sea surface sediments, naturally exposed to seasonal hypoxia. Sediment cores were incubated under oxygen limited conditions and shifts in geochemistry and transcriptomes were monitored over time. Calcite amended incubations indicated the highest CDR potential. The combination of calcite dissolution related bicarbonate bioavailability and limited oxygen availability in bottom waters were likely the main drivers of Cand. Electrothrix (cable bacteria) enrichment. Cand. Electrothrix’s acidifying activity promotes carbonate- and FeS-mineral dissolution, as evidenced by bicarbonate, manganese, iron and sulfide accumulation in sediment porewaters. In line, transcriptomics suggested stimulation of overall microbial sulfur-, iron- and manganese oxidation. Moreso, transcript abundances posited shifts towards denitrification. Carbonic anhydrases (bicarbonate <-> CO2) were upregulated suggesting a calcite dissolution related bicarbonate source, coinciding with upregulated genes of autotrophic CO2-fixation. Cand. Electrothrix appeared to promote EBW. In turn, artificial calcite addition also increased buffering capacity of the sediments, affecting relative FeS-mineral dissolution. Relatively less available ferrous iron may restrict some formation of iron-oxides, known to act as barrier to sulfide emissions from sediments during phases of anoxia. Conclusively, according to shifts in transcript abundances, feedbacks may include stimulated EBW but also loss of dinitrogen from sediments as well as reduced retention of sulfide during anoxia. Metabolic rate measurements are urgently required to scrutinize the holistic effects of EBW on such coastal systems.