Proposal for an Abridged Nitrogen Turnover Cycle in Certain Marine Planktonic Systems Involving Hypoxanthine-Guanine Excretion by Ciliates and their Reutilization by Phytoplankton

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Abstract
At the concentration of 0.125 mM, the purine hypoxanthine is a good and energy efficient N source for phototrophic growth of at least 60% of 26 tested species of marine microalgae from 10 taxonomic classes. At 10-fold higher concentration, this purine generally augmented growth of the species showing fair-to-good growth on the low concentration and this growth increase was dramatic for most flagellates. The higher concentration level caused weak or no growth improvement of the species showing poor purine utilization at the lower level. There was no indication of hypoxanthine concentration toxicity and the maximal growth yields achieved by the best purine utilizers indicated assimilation of all 4 N atoms/molecule of hypoxanthine. These results, combined with reported excretion of hypoxanthine-guanine by marine ciliates, suggests a short-circuited N-turonver, cycle in certain planktonic associations whereby the excreted purines may be directly reutilized by a large porportion of marine phytoplankton.