Identifying and quantifying nitrogen pools is essential for understanding the nitrogen cycle in aquatic ecosystems. The ubiquitous diatoms represent an overlooked nitrate pool as they can accumulate nitrate intracellularly and utilize it for nitrogen assimilation, dissipation of excess photosynthetic energy, and Dissimilatory Nitrate Reduction to Ammonium (DNRA). Here, we document the global co-occurrence of diatoms and intracellular nitrate in phototrophic microbial communities in freshwater (n = 69), coastal (n = 44), and open marine (n = 4) habitats. Diatom abundance and total intracellular nitrate contents in water columns, sediments, microbial mats, and epilithic biofilms were highly significantly correlated. In contrast, diatom community composition had only a marginal influence on total intracellular nitrate contents. Nitrate concentrations inside diatom cells exceeded ambient nitrate concentrations ∼100–4000-fold. The collective intracellular nitrate pool of the diatom community accounted for <1% of total nitrate in pelagic habitats and 65–95% in benthic habitats. Accordingly, nitrate-storing diatoms are emerging as significant contributors to benthic nitrogen cycling, in particular through Dissimilatory Nitrate Reduction to Ammonium activity under anoxic conditions.
Bibliografisk noteFunding Information:
This study was supported by grants from the German Science Foundation awarded to P.S. (STI 202/6) and A.K. (KA 3187/2-1), by the Max Planck Society (Germany), by the European Union’s Seventh Framework Program for Research and Technological Development including Demonstration Activities awarded to Aarhus Institute of Advanced Studies (Grant agreement No. 609033), and by the European Union’s Horizon 2020 Research and Innovation Program (Grant agreement No. 669947) and the Danish National Research Foundation through the Danish Center for Hadal Research (DNRF145) and the Danish Center for Electromicrobiology (DNRF136). Sample collection was supported by HYDRA Marine Sciences GmbH (Bühl, Germany), PULZ im Rieselfeld (Freiburg, Germany), and MACRORE (European Union’s Horizon 2020 Research and Innovation Program (Grant agreement No. 654182; ENVRIplus)). We would like to thank the following colleagues and friends for their kind and active engagement in collecting samples: Kathrin Graffe, Nicole Ehlers, Tillman Keller, Jutta Drees, Jasper Dammann, Fabian Seredszus, Stefanie Tenten, Georg Supp, Herdís G. R. Steinsdóttir, Mon O. Yee, Hans F. Hansen, Michael W. Hansen, Lorenzo Rovelli, Irina-Elena Antonescu, Irene Olivé, Kelsey I. Miller, Marco Bonaglia, Artur Fink, Miriam Weber, Frank Wenzhöfer, Marit van Erk, and Elisa Merz. Microalgal isolates were kindly provided by Olga Kourtchenko (Gothenburg University Marine Algae Culture Collection, GUMACC), Tina Trautmann, Jan-Hendrik Hehemann, Daniel Doherty, and Hans F. Hansen. The technical support by Erik Laursen, Susanne Nielsen, Anja Niclas, and Gaby Eickert-Groetzschel is acknowledged.
© 2022, The Author(s).