Can Top-Down Controls Expand the Ecological Niche of Marine N2 Fixers?

Angela Landolfi; A E Friederike Prowe; Markus Pahlow; Christopher J Somes; Chia-Te Chien; Markus Schartau; Wolfgang Koeve; Andreas Oschlies

doi:10.3389/fmicb.2021.690200

Can Top-Down Controls Expand the Ecological Niche of Marine N₂ Fixers?

Front Microbiol. 2021 Aug 18:12:690200. doi: 10.3389/fmicb.2021.690200. eCollection 2021.

Authors

Angela Landolfi^{1

2}, A E Friederike Prowe², Markus Pahlow², Christopher J Somes², Chia-Te Chien², Markus Schartau², Wolfgang Koeve², Andreas Oschlies²

Affiliations

¹ Institute of Marine Sciences, National Research Council, Rome, Italy.
² Marine Biogeochemistry, GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany.

Abstract

The ability of marine diazotrophs to fix dinitrogen gas (N₂) is one of the most influential yet enigmatic processes in the ocean. With their activity diazotrophs support biological production by fixing about 100-200 Tg N/year and turning otherwise unavailable dinitrogen into bioavailable nitrogen (N), an essential limiting nutrient. Despite their important role, the factors that control the distribution of diazotrophs and their ability to fix N₂ are not fully elucidated. We discuss insights that can be gained from the emerging picture of a wide geographical distribution of marine diazotrophs and provide a critical assessment of environmental (bottom-up) versus trophic (top-down) controls. We expand a simplified theoretical framework to understand how top-down control affects competition for resources that determine ecological niches. Selective mortality, mediated by grazing or viral-lysis, on non-fixing phytoplankton is identified as a critical process that can broaden the ability of diazotrophs to compete for resources in top-down controlled systems and explain an expanded ecological niche for diazotrophs. Our simplified analysis predicts a larger importance of top-down control on competition patterns as resource levels increase. As grazing controls the faster growing phytoplankton, coexistence of the slower growing diazotrophs can be established. However, these predictions require corroboration by experimental and field data, together with the identification of specific traits of organisms and associated trade-offs related to selective top-down control. Elucidation of these factors could greatly improve our predictive capability for patterns and rates of marine N₂ fixation. The susceptibility of this key biogeochemical process to future changes may not only be determined by changes in environmental conditions but also via changes in the ecological interactions.

Keywords: N2 fixation; bottom-up control; ecological niche; environmental controls; marine diazotrophs; selective grazing; top-down control.