Coupled Phosphorus and Manganese Controls on Ocean Ecosystem Fertility

• Applicant: Viktoria Steck, Ph.D.
• Subject Area: Oceanography ; Metabolism, Biochemistry and Genetics of Microorganisms
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545769867

Marine phytoplankton are integral for our planetary life support system by contributing nearly half of the global net primary production. However, the scarcity of essential nutrients such as phosphorus (P) can constrain their physiology, community structure, primary productivity, and ultimately influence carbon cycling and climate. To cope with P scarcity, marine microorganisms express metal-containing hydrolytic enzymes to access the alternative pool of dissolved organic phosphorus (DOP), a process dependent on trace metal availability in the marine environment. Among these enzymes, ecologically prevalent alkaline phosphatases with zinc- or iron cofactors are believed to shape P acquisition in large parts of the global ocean. On the contrary, I established an unprecedented role for manganese (Mn) in microbial DOP utilization. In a computational survey of 25,000 proteins involved in P metabolism, Mn emerged as the second most abundant metal overall, rivaling zinc and exceeding iron more than 2-fold in alkaline phosphatases. Despite the crucial function of Mn in photosynthesis, a biological relevance for P metabolism has not been described to date. Building on this survey, I used purified enzymes from marine diatoms to demonstrate, for the first time, that marine hydrolases can employ Mn as enzymatic cofactor for the degradation of diverse DOP compounds. In pioneering incubation experiments, I established that ambient Mn availability regulates environmental DOP hydrolysis within natural mixed microbial communities. This new evidence indicates a paradigm-shifting link between P nutrition and Mn availability and a hitherto unknown potential for P-Mn interactions controlling ocean ecosystem fertility. My research will address these open questions on the molecular, species, and community level. First, I will assess the potential of marine phosphatases to utilize Mn for compound-specific DOP degradation and supporting nutritional requirements of marine microbes using pure overexpressed enzymes and axenic cultures of ecologically representative model phytoplankton. Second, I will quantify the prevalence of manganese-dependent DOP utilization across marine plankton assemblages and its impacts on nutrient budgets in the present and future ocean through field work along a gradient between the P-replete and anthropogenically influenced Baltic Sea, and the chronically P-deplete but metal rich Mediterranean Sea. Third, the influence of other linkages between P and Mn on phytoplankton physiology will be evaluated, such as the sorption and hydrolysis of DOP on Mn (oxyhydr)oxide minerals, and the storage of Mn on polyphosphate in intracellular vacuoles. Given that incorporating preferential DOP remineralization increased model estimates of primary productivity by ten percent, the novel data generated by this research will be instrumental for further refining climate predictions while transforming our view of integrated biochemical cycles in a changing ocean.

• DFG Programme: Emmy Noether Independent Junior Research Groups
• International Connection: USA
• Major Instrumentation: Flow Cytometer
• Instrumentation Group: 3500 Zellzähl- und Klassiergeräte (außer Blutanalyse), Koloniezähler
• Cooperation Partners: Kiefer Forsch, Ph.D.; Makoto Saito, Ph.D.; Ichiko Sugiyama, Ph.D.