Nutrient limitation of algal growth: physiological assays and chemical analyses

"March 1997".

Project Number: Nutrient limitation of algal growth: physiological assays and chemical analyses - M/03/5088, SB/1/31; LWRRDC Project MDR8.

MDFRC item.

Associated publications with project: Wood, M.D. and Oliver, R.L. (1995) Fluorescence transients in response to nutrient enrichment of nitrogen- and phosphorus-limited Microcystis aeruginosa cultures and natural phytoplankton populations: a measure of nutrient limitation. Australian Journal of Plant Physiology 22(2): 331-340.

A rapid physiological assay (NIFT) was devised to identify the occurrence of nitrogen or phosphorus limitation in phytoplankton. The assay is based on the occurrence of transient fluctuations in the chlorophyll-a fluorescence signal that result from addition of the limiting nutrient to a sample containing either nitrogen or phosphorus limited phytoplankton. Nitrogen limited cultures showed a perturbation in the fluorescence signal on addition of nitrate or ammonium but did not respond to phosphate. Conversely, phosphorus limited phytoplankton showed a fluorescence perturbation in response to additions of ammonium or phosphate, but did not respond to additions of nitrate. Nutrient replete cells did not respond to any nutrient addition. These perturbations were observed in cultures of the cyanobacteria Microcystis aeruginosa and Anabaena circinalis, the green alga Selenastrum capricornutum, and the diatom Aulacoseira granulata. Algal growth bioassays confirmed the results of NIFT assays when both were used to test natural water samples. Samples collected from three field sites in the Murray-Darling Basin showed that nitrogen and phosphorus are often not limiting to phytoplankton growth, and that nitrogen limitation occurs just as frequently as phosphorus limitation. This provides an explanation for the occurrence of nitrogen fixing cyanobacteria, and at one field site the NIFT assay provided a direct demonstration of nitrogen limitation being associated with the development of a nitrogen fixer; as has been frequently postulated. Chemical assessment of nutrient limitation using elemental ratios corresponded poorly with the results of the physiological assay indicating that concentration data are difficult to interpret.