Nitrogen isotope effects in the assimilation of inorganic nitrogenous compounds by marine diatoms

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Abstract
N isotope fractionation in the assimilation of inorganic nitrogenous compounds was studied using marine diatoms (Phaeodactylum tricornutum and Chaetoceros sp.). The isotopic composition (.delta.15N) of the diatoms ranged from 7 to -18.permill. relative to that of the N source, i.e., ammonium, nitrite or nitrate. When the growth was light-limited, the isotope fractionation in nitrate assimilation was inversely correlated with the growth rate. The highest fractionation factor of 1.016 was obtained when the growth rate was as low as 0.025/day. Fractionation was negligible when the growth rate was higher than 1/day. A steady-state kinetic model was applied to explain the isotope fractionation in nitrate assimilation. The N isotope fractionation primarily takes place at the step of N.sbd.O bond breaking in nitrate reduction to nitrite. The extent of the isotope fractionation associated with the nitrate uptake is very small and barely exceeds the limit of detection.