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JOURNAL OF PLANKTON RESEARCH | VOLUME 16 | NUMBER 8 | PAGES 911-925 | 1994
© Oxford University Press

research-article

Fluorescence characteristics of a natural assemblage of freshwater picocyanobacteria

J.G.McMurter Heather1, R.Pick Frances and Dan E. Kelley

Ottawa-Carleton Institute of Biology, Department of Biology, University of Ottawa Ottawa, ON KIN 6N5, Canada

Received on June 16, 1993; accepted on March 4, 1994

The fluorescence characteristics of a freshwater assemblage of picocyanobacteria weredetermined in a mesotrophic lake using microspectrofluorometry. In Jack's Lake. Ontario. 72–98% of the assemblage was comprised of cells with a single excitation peak for chlorophyll (Chl) a (emission at 680 nm) at 565 ±3) nm. theexcitation spectra for Chl a resembling the spectral composition of downwelling irradiance. The assemblage was, therefore, dominated by a single phycobiliproteinpigment type, similar to type 2 phycoerythrin (PE) marine Synechococcus strains. The shape of excitation spectra did not change significantly with depth down to 0.6% of incident irradiance or between sampling dates, although the relative intensity of the PE excitation peak was generally greater for populations below the thermocline compared to surface populations during summer stratification. Two separate populations of PE-containing picocyanobacteria, distinguished based on their morphology and plane of division, could be further separated based on their emission spectra (using blue excitation): a Synechocystis type cell (PE-Sys) consistently had a more pronounced peak at 665 nm from allophycocyanin compared to a Synechococcus (PE-Syn) type cell. In addition, the ratio of the PE to Chl a peak emissions was higher in PE-Sys and increased significantly with depth below the thermocline. While nitrogen was limiting in the lake in summer, experimental additions of nitrogen did not significantly affect this ratio in surface water populations, but increased the ratio in PE-Syn populations at the base of the photic zone. For surface assemblages of picocyanobacteria, high irradiance may be more-important in regulating fluorescence characteristics than nitrogen stress.

1Deceased September 7, 1993


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