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

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.

Controlling factors on the global distribution of a representative marine heterotrophic diazotroph phylotype (Gamma A)

“…However, the moderate explanatory power of our prediction model indicates that there must be other unknown factors and mechanisms also impacting heterotrophic diazotrophs. For instance, heterotrophic diazotrophs found in the guts of copepods (Scavotto et al, 2015) imply that they are subject to top-down controls, which was also suggested for marine autotrophic diazotrophs (Landolfi et al, 2021;Wang et al, 2019;Wang and Luo, in press). The uneven spatial samplings of Gamma A may also introduce biases into our analyses.…”

Nitrogen fixation in mesoscale eddies of the North Pacific Subtropical Gyre: patterns and mechanisms

“…Little is known about the identity of diazotrophic grazers, or the extent to which grazing controls diazotroph abundances and NFRs in the NPSG (Landolfi et al, 2021). Early reports identified mesoplanktonic copepods grazing on Trichodesmium (Azimuddin et al, 2016; O’Neil et al, 1996; O’Neil, 1998), and later on UCYN-C (Hunt et al, 2016), UCYN-A (Scavotto et al, 2015), UCYN-B, Het-1, and Het-2 (Conroy et al, 2017).…”

Riverine nitrogen supply to the global ocean and its limited impact on global marine primary production: a feedback study using an Earth system model

“…Because there can be strong seasonal variations in nutrient fluxes and fluvial nutrient imports can have different effects on the biogeochemistry of a coastal ecosystem depending on the timing of the fluxes (Eisele and Kerimoglu, 2015;Holmes et al, 2012;Townsend-Small et al, 2011), we used the seasonally cycling climatology of freshwater runoff from UVic to estimate seasonal variations in N supply. Although freshwater discharge and riverine nutrient export are not always correlated, the discharge has an important impact on the nutrient loads of rivers (e.g., Lu et al, 2011Lu et al, , 2009Sigleo and Frick, 2007;DeMaster and Pope, 1996). Here, we assumed a constant seasonal cycle in runoff and that nitrogen concentrations in the discharged river water are constant throughout the seasonal cycle.…”

“…Note that part of our knowledge of N 2 fixation and most modern model concepts are still based on the original limited assumptions based on a few species, especially Trichodesmium. Also, emphasis has traditionally been put on bottom-up controls, despite accumulating evidence that top-down controls such as selective grazing on dominant species may have substantial impacts on the distribution of diazotrophs and nitrogen fixation (Landolfi et al, 2021).…”