MEPS 359:25-36 (2008)  -  DOI: https://doi.org/10.3354/meps07241

ABSTRACT: The diazotrophic cyanobacterium Trichodesmium is a significant contributor to marine nitrogen and carbon cycles and has been incorporated in biogeochemical ocean circulation models. To date, parameterization of light as a controlling factor for nitrogen fixation has been based on field observations, where factors other than light also affect Trichodesmium physiology. Here we present data on light-dependent (15 to 1100 µmol quanta m^–2 s^–1) diazotrophic growth from controlled laboratory experiments and their implications for modeling approaches. We supply a simple empirical model to describe nitrogen fixation by Trichodesmium in batch cultures. Diazotrophic growth of axenic Trichodesmium IMS-101 was light saturated at 180 µmol quanta m^–2 s^–1 and did not vary significantly at higher photon irradiances up to 1100 µmol quanta m^–2 s^–1 (μ_{carbon based} ≈ 0.26 d^–1). Chlorophyll a (chl a) normalized N₂ fixation rates were significantly affected by light intensity during mid-exponential growth (0.74 to 4.45 mol N fixed mol chl a^–1 h^–1) over the range of photon irradiances tested. In contrast, nitrogen fixation rates normalized to the cellular carbon content were relatively unaffected by light intensity (0.42 to 0.59, averaging 0.5 mmol N mol particulate organic carbon [POC]^–1 h^–1). Trichodesmium carbon biomass can be used to estimate the nitrogen input by this diazotroph into the ocean; the maximum input rate is 350 nmol N fixed l^–1 h^–1.

KEY WORDS: Trichodesmium · Light · Nitrogen fixation · Marine nitrogen cycle · Marine carbon cycle · Marine cyanobacteria · Diazotrophic growth