First findings on the decomposition of organic material in the deep sea

Many processes in the deep sea are not yet well understood, and the role of microbial communities in particular is often a big unknown. This includes, for example, how organic material that sinks from the water surface to the ocean floor is metabolized - an important building block to better understand the global carbon cycle. The team of authors from MARUM – Center for Marine Environmental Sciences and the departments of geosciences and biology/chemistry at the University of Bremen, led by first author Qing-Zeng Zhu, has now published the study results in the journal Science Advances.

As soon as organisms get food, metabolism begins. This process is well known, but rarely studied for marine ecosystems - in contrast to ecosystems on land.  This also applies to the specific microbial communities involved in this process.

For their study, the team fed organic material with labeled carbon isotope substrates (13C) - either an algae-lipid mix or crude proteins - and observed it in the laboratory for 400 days. The microbial communities originate from a sediment core obtained off Helgoland. The initial question was: What happens to fresh biomass and which organisms are involved in its processing? This was the first time that the formation of new biomass and its turnover was quantified more precisely.

The team found that microbial communities can be stimulated by the addition of lipids and proteins and not only break down more easily digestible fresh biomass, but also old carbon. This new finding is relevant because the input of fresh organic matter is increased by man-made, climate-induced environmental changes, such as expanding low oxygen zones in the ocean, melting sea ice or glacier retreat.

“Our study is the first to link degradation of labile organic matter with growth of microbes and the consequences for the composition of organic matter that eventually is buried in marine sediment. To our surprise we discovered that fresh organic matter additions had a disproportionally large and long-lasting impact on the degradation of old, supposed to be refractory organic matter”, says Prof Jack Middelburg from Utrecht University, Co-Author of the study and Excellence Chair at the Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface” based at MARUM. Within the frame of this cluster’s research this study provided pivotal data to link organic matter deposition events with long-term carbon burial and as such links the research units Receiver, Reactor and Recorder.

Pulses of fresh organic matter delivery to the seafloor will increase due to climate-induced ecosystem changes. These will directly impact not only the organisms living at the seafloor but also the organic carbon sequestration and as such feedback to the climate system.

Publication

Qing-Zeng Zhu, Xiuran Yin, Heidi Taubner, Jenny Wendt, Michael W. Friedrich, Marcus Elvert, Kai-Uwe Hinrichs, Jack J. Middelburg: Secondary production and priming reshape the organic matter composition in marine sediments. Science Advances 2024. DOI: 10.1126/sciadv.adm8096