![]() hauckii strain was isolated directly from the field and grown on a nitrate-based medium that would not support DDA growth. Symbiont-free host cell cultures could not be established however, a symbiont-free H. hauckii symbiosis did not use nitrate when added, although ammonium was consumed by the H. In contrast to published results on the Rhizosolenia-Richelia symbiosis, the H. Maximum laboratory N 2 fixation rates were ~6 to 8-fold higher than literature-derived field rates of the H. Maximum N 2 fixation rates (84 fmol N 2 heterocyst −1h −1) in low light adapted cultures (50 µmol m −2s −1) were approximately 40–50% of rates (144–154 fmol N 2 heterocyst −1h −1) in high light (150 and 200 µmol m −2s −1) adapted cultures. Nitrogen fixation also exhibited light saturation kinetics. At the onset of the scotophase, nitrogen-fixation rates declined over several hours to near-zero values. Nitrogen fixation rates by the symbiont while within the host followed a diel pattern where rates increased from near-zero in the scotophase to a maximum 4–6 h into the photophase. hauckii symbioses with a growth compensation light intensity (E C) of 7–16 µmol m −2s −1and saturation light level (E K) of 84–110 µmol m −2s −1. Growth rates followed light saturation kinetics in H. hauckii symbioses in N-free medium with N 2 as the sole N source were 0.74–0.93 div d −1. Maximum growth rates of multiple strains of H. Symbiont diazotrophy was sufficient to support both the host diatom and cyanobacteria symbionts, and the entire symbiosis replicated and grew without added nitrogen. ![]() The symbioses rates reported here are for the joint diatom-cyanobacteria unit. We document the influence of light and nutrients on nitrogen fixation and growth rates of the host diatom Hemiaulus hauckii Grunow together with its diazotrophic endosymbiont Richelia intracellularis Schmidt, as well as less complete results on the Hemiaulus membranaceus- R. This study reports the successful isolation and growth in monocultures of multiple strains of a diatom-cyanobacteria symbiosis from the Gulf of Mexico using a modified artificial seawater medium. Diazotrophic symbioses between cyanobacteria and diatoms can dominate nitrogen-fixation regionally, particularly in major river plumes and in open ocean mesoscale blooms. Nitrogen fixers (diazotrophs) are often an important nitrogen source to phytoplankton nutrient budgets in N-limited marine environments.
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