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Stoichiometry of iron, manganese and cobalt in the marine diazotroph Crocosphaera subtropica ATCC51142 in iron and phosphorous limited continuous cultures

Alexandra Marki, Robert Fischer, Thomas J. Browning, Evangelia Louropoulou, Robert Ptacnik, Martha Gledhill*

*Corresponding author:

ABSTRACT: We investigated trace element stoichiometries of the nitrogen fixing marine cyanobacteria Crocosphaera subtropica ATCC51142 under steady state growth conditions. We utilised Exponentially Fed Batch (EFB) cultures and varied iron (Fe) concentrations to establish nutrient limitation in C. subtropica growing at one constant growth rate (0.11 day-1). No statistical difference in cell density, chlorophyll a, particulate organic carbon (C), nitrogen (N) and phosphorous (P) were observed between consecutive days post day 14, and cultures were assumed to be at steady state with respect to growth for the remaining 11 days of the experiment. Cultures were limited by P in the highest Fe treatment (41 nmol L-1 Fe) and by Fe in the two lower Fe treatments (1 and 5 nmol L-1 Fe). Cell size and in-vivo fluorescence changed throughout the experiment in the 1 nmol L-1 Fe treatment suggesting ongoing acclimation of C. subtropica to our lowest Fe supply. Nevertheless Fe:C ratios were not significantly different between the Fe treatments and we calculated an average Fe:C ratio of 32 ± 14 µmol mol-1 for growth at 0.11 day-1. Steady state P limited cells had lower P quotas whilst Fe limited cells had higher Mn and Co quotas. We attribute the increase in Mn and Co quotas at low Fe to a competitive effect resulting from changes in the supply ratio of trace elements. Such an effect has implications for variability in elemental stoichiometry in marine phytoplankton, and potential consequences for trace metal uptake and cycling in marine systems.