Utilization of limiting concentrations of ortho-phosphate and production of extracellular organic phosphates in cultures of marine diatoms

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Abstract
The utilization of ortho-phosphate by two coastal marine diatom species, Nitzschia closterium and Cyclotella cryptica, was studied in batch cultures. The hypothesis was tested that threshold concentrations in the phosphate uptake determine the lower limit of environmental phosphate, permitting the existence of species. The turn-over time of residual medium phosphate in cultures is ˜10 min, indicating a rapid equilibration of concentration dependent on uptake with leakage of ortho-phosphate. Increasing phosphate starvation in cultures diminished the residual ortho-phosphate in the range of ˜60–−1, as measured radiochemically after elution on Sephadex® G-10 gel. These concentrations encompass the range of limiting phosphate concentration in continuous cultures of the few microalgae, for which these concentrations are actually measured. The diatoms excreted ˜20–100 nmol I−1 of organic phosphate. One dominating compound, probably an unusual nucleotide, was incompletely or not resorbed under phosphate starvation. In contrast, Nitzschia closterium excreted under ample phosphate supply a series of three related compounds, probably phospholipids, that were resorbed under depletion. The association of the organic phosphates with macromolecular exudates is interpreted, along with the other observations, as an indication for a hardly explored periplasmatic phosphate metabolism in these algae.

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