title: Light-limited growth rate modulates nitrate inhibition of dinitrogen fixation in the marine unicellular cyanobacterium Crocosphaera watsonii.
abstract: Biological N2 fixation is the dominant supply of new nitrogen (N) to the oceans, but is often inhibited in the presence of fixed N sources such as nitrate (NO3-). Anthropogenic fixed N inputs to the ocean are increasing, but their effect on marine N2 fixation is uncertain. Thus, global estimates of new oceanic N depend on a fundamental understanding of factors that modulate N source preferences by N2-fixing cyanobacteria. We examined the unicellular diazotroph Crocosphaera watsonii (strain WH0003) to determine how the light-limited growth rate influences the inhibitory effects of fixed N on N2 fixation. When growth (µ) was limited by low light (µ = 0.23 d-1), short-term experiments indicated that 0.4 µM NH4+ reduced N2-fixation by ∼90% relative to controls without added NH4+. In fast-growing, high-light-acclimated cultures (µ = 0.68 d-1), 2.0 µM NH4+ was needed to achieve the same effect. In long-term exposures to NO3-, inhibition of N2 fixation also varied with growth rate. In high-light-acclimated, fast-growing cultures, NO3- did not inhibit N2-fixation rates in comparison with cultures growing on N2 alone. Instead NO3- supported even faster growth, indicating that the cellular assimilation rate of N2 alone (i.e. dinitrogen reduction) could not support the light-specific maximum growth rate of Crocosphaera. When growth was severely light-limited, NO3- did not support faster growth rates but instead inhibited N2-fixation rates by 55% relative to controls. These data rest on the basic tenet that light energy is the driver of photoautotrophic growth while various nutrient substrates serve as supports. Our findings provide a novel conceptual framework to examine interactions between N source preferences and predict degrees of inhibition of N2 fixation by fixed N sources based on the growth rate as controlled by light.
authors.affiliation: Marine and Environmental Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, United States of America.
authors.forename: Nathan S
authors.initials: NS
authors.lastname: Garcia
authors.affiliation: Marine and Environmental Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, United States of America.
authors.forename: David A
authors.initials: DA
authors.lastname: Hutchins
mesh_terms: D000458:Cyanobacteria / Q000187:drug effects* / Q000254:growth & development* / Q000378:metabolism / Q000528:radiation effects; D008027:Light; D009566:Nitrates / Q000494:pharmacology*; D009586:Nitrogen Fixation / Q000187:drug effects*; D013997:Time Factors
chemical_list: D009566:Nitrates