Trichodesmium is a globally significant marine
diazotroph responsible for supplying new nitrogen to
the oligotrophic regions in which it is found. Though it
has been studied for decades, our understanding of the
ways in which environmental factors can affect its
nitrogen fixation rate remains limited. A continuous
culture of Trichodesmium was established in which
steady state growth and nitrogen fixation were
maintained at dilution rates ranging from 0.27 to 0.67
d^-1. Our results clearly show that, as growth rate
increased, biomass decreased linearly and nitrogen
fixation rate increased linearly. C:N:P ratios remained
constant over the range of growth rates studied,
suggesting a tight coupling between macronutrient
uptake and the maintenance of balanced growth at
steady state.

We used cultures at steady state to determine the
impact of nitrate exposure and uptake on nitrogen
fixation. Nitrate inhibits nitrogen fixation by up to 70%
in a concentration-dependent manner at initial nitrate
concentrations less than 10 micromolar. Nitrate uptake
accounted for as much as 86% of total N uptake and, at
initial nitrate concentrations greater than 2.5
micromolar, more than made up for the observed
inhibition of nitrogen fixation.

A field study of this diazotroph shows that nitrogen
fixation scales with light intensity from a maximum at
50% surface irradiance. Estimated areal nitrogen
fixation rates in the Gulf of Mexico, based on vertical
abundance profiles and the relationship between
nitrogen fixation and surface irradiance, are
comparable to measurements made in other
oligotrophic regions. Stable isotopic composition of the
particulate organic matter and the zooplankton confirms
that Trichodesmium nitrogen and carbon are moving
into the food chain and are important to higher trophic
levels. As much as 60% of the zooplankton carbon
was derived from Trichodesmium. Our work
established that this diazotroph is ecologically
important in the water column of the Gulf of Mexico, with
important implications for nitrogen and carbon cycling.
Findings from our field and culture studies can be
added to models used to quantify the importance of
Trichodesmium nitrogen fixation on an oceanic scale.