Light-limited growth rate modulates nitrate inhibition of dinitrogen fixation in the marine unicellular cyanobacterium Crocosphaera watsonii

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Abstract

© 2014 Garcia, Hutchins. Biological N 2 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 (NO 3 - ). Anthropogenic fixed N inputs to the ocean are increasing, but their effect on marine N 2 fixation is uncertain. Thus, global estimates of new oceanic N depend on a fundamental understanding of factors that modulate N source preferences by N 2 -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 N 2 fixation. When growth (μ) was limited by low light (μ=0.23 d -1 ), short-term experiments indicated that 0.4 μM NH 4 + reduced N 2 -fixation by ∼90% relative to controls without added NH 4 + . In fast-growing, highlight-acclimated cultures (μ=0.68 d -1 ), 2.0 μM NH 4 + was needed to achieve the same effect. In long-term exposures to NO 3 - , inhibition of N 2 fixation also varied with growth rate. In high-light-acclimated, fast-growing cultures, NO 3 - did not inhibit N 2 -fixation rates in comparison with cultures growing on N 2 alone. Instead NO 3 - supported even faster growth, indicating that the cellular assimilation rate of N 2 alone (i.e. dinitrogen reduction) could not support the light-specific maximum growth rate of Crocosphaera. When growth was severely light-limited, NO 3 - did not support faster growth rates but instead inhibited N 2 -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 N 2 fixation by fixed N sources based on the growth rate as controlled by light.

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Garcia, N. S., & Hutchins, D. A. (2014). Light-limited growth rate modulates nitrate inhibition of dinitrogen fixation in the marine unicellular cyanobacterium Crocosphaera watsonii. PLoS ONE, 9(12). https://doi.org/10.1371/journal.pone.0114465

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