FLUORESCENCE TRANSIENTS IN RESPONSE TO NUTRIENT ENRICHMENT OF NITROGEN-LIMITED AND PHOSPHORUS-LIMITED MICROCYSTIS-AERUGINOSA CULTURES AND NATURAL PHYTOPLANKTON POPULATIONS - A MEASURE OF NUTRIENT LIMITATION

This study presents the results of an investigation of the application of chlorophyll fluorescence perturbations as a bioassay for nutrient limitation in microalgae. The variable chlorophyll fluorescence (Delta F) response of Microcystis aeruginosa, chemostatically cultured under different nitrogen (N) and phosphorus (P) nutrient regimes, to spiking with NO3-, NH4+ and PO43- was monitored. Differences were observed in the short-term (<10 min) fluorescence response (F) which related to the original nutrient status of the test organism. Cells growing exponentially in full nutrient media showed no transient changes in Delta F. The addition of NH4+ or NO3- to N-limited cells resulted in transient changes to Delta F, while no response was observed following the addition of PO43-. Spiking of P-limited cells with either PO43- or NH4+ resulted in transient changes to Delta F, while no response was observed for NO3-. Thus a simple, rapid, nutrient-specific assay for nutrient limitation was available for cultured cells of M. aeruginosa. When the assay was tested on field samples from the Maude Weir Pool (34 degrees 28'S,144 degrees 18'E), Murrumbidgee River, NSW, Australia, apparent nutrient limitation, either N or P, was detected. Use of microalgal fluorescence responses to nutrient spiking may allow the rapid and specific detection of nutrient limitation in both cultures and field populations of phytoplankton.