Microbial Responses to Ocean Alkalinity Enhancement at Bülk

How does increased alkalinity affect life on the seafloor?

Ocean Alkalinity Enhancement (OAE) is being explored as a way to remove carbon dioxide (CO₂) from the atmosphere and store it in the ocean. At the Bülk wastewater treatment plant in Kiel Bay, Germany, the start-up Planeteers is installing a reactor that uses limestone (calcite) to convert captured CO₂ into dissolved bicarbonate. The treated water is then released into the Baltic Sea, increasing the alkalinity of the surrounding coastal waters.

The outfall at Bülk already releases water with more than twice the alkalinity of the surrounding Baltic Sea. The additional increase resulting from reactor operation provides an opportunity to investigate how sustained alkalinity enhancement affects the marine environment under real-world conditions. 

Our focus: Microorganisms in marine sediments

Marine sediments are home to diverse microbial communities that drive essential processes in the carbon, sulfur, nitrogen and iron cycles. These microorganisms break down organic matter, transform nutrients and influence how chemical elements move between the seafloor and the overlying water.

Changes in seawater chemistry may affect these microbial communities and the processes they perform. Our research investigates whether and how increased alkalinity alters microbial community composition, metabolic activity and biogeochemical processes in the upper sediment layer. 

How do we investigate these changes?

Through independent scientific monitoring, we compare conditions before and after the reactor starts operating and examine locations exposed to the alkalinity increase alongside unaffected reference sites. Seasonal field sampling combines sediment geochemistry with molecular analyses and measurements of microbial activity, allowing us to investigate both which microorganisms are present and what they are doing.

By following these changes over time, we aim to distinguish potential effects of increased alkalinity from natural seasonal variability and other environmental influences. Our findings will contribute to a better understanding of how benthic microbial communities and sediment processes respond to sustained ocean alkalinity enhancement, providing important field-based evidence for assessing its potential environmental impacts. 

Bülk-Team

Sediment Geochemistry & Microbiology - Stefanie Böhnke-Brandt, Mirjam Perner, Andy Dale

Benthic Macrofauna - Lukas Krause, Florian Weinberger

Water Column Physics & Chemistry - Mario Esposito, Eric Achterberg, Bryce van Dam (HEREON), Achim Kopf (MARUM), Charly Moras (Universität Hamburg)

Coordination, Data Management and Stakeholder Communication - Andreas Oschlies, Michael Sswat, Ulrike Bernitt, Tim Boxhammer, Leila Kittu

 

Outreach

GEOMAR News (28.09.2026)

Kiel Magazin (07.10.2026)