Microbial responses to ocean alkalinity enhancement in seasonally hypoxic coastal sediments

Enhanced benthic weathering (EBW) is a promising marine carbon dioxide removal (mCDR) strategy, where alkaline minerals are added to the seafloor. Upon weathering, these artificially deposited minerals release alkalinity to the water column, which ultimately leads to CO₂ uptake from the atmosphere. However, feedbacks of the benthic bacterial community to this mCDR approach remain unknown.

To investigate the effects of EBW, sediment core incubations with added alkalizing minerals were carried out and shifts in bacterial communities identified and linked to sedimentary properties. Surface sediments with supernatant water from a seasonally hypoxic basin in the Baltic Sea off the eastern coast of northern Germany were collected and either calcite (limestone) or silicate (dunite) added and kept under oxic (avg. 263 ± 34 μM O₂) or oxygen-limited (0 to maximum 162 μM O₂) conditions.

Bacterial surface sediment communities responded to the different oxygen availabilities in the overlying bottom waters independent of the mineral addition as reflected in cluster formation via Principal Coordinates Analysis (PCoA). Although, by contrast, mineral addition induced less pronounced changes, the relative abundances of several rare taxa changed significantly, such as chemolithoautotrophs associated with the sulfur and nitrogen cycles. Candidatus Electrothrix was selectively promoted by calcite addition under oxygen-limited conditions in two of three cores, whereas Nitrosomonas exhibited increased growth tendencies following dunite addition under both oxic and oxygen depleted bottom water conditions. Sulfur oxidizing bacteria (SOB) of Beggiatoaceae declined under oxygenated water conditions, independent of the type of mineral added. The dunite amendment was associated with an increase in other SOB while calcite amendment led to the enrichment of organotrophic taxa.

Our findings provide important insights into the response of microbial communities to EBW, but also highlight the necessity of field-based studies to accurately assess the effects of EBW on coastal ecosystems.