LIGHT-ABSORPTION AND PIGMENT CONTENT IN NATURAL-POPULATIONS AND CULTURES OF A NON-GAS VACUOLATE CYANOBACTERIUM OSCILLATORIA-BOURRELLYI (= TYCHOMEMA-BOURRELLYI)

Vertical profiles of temperature, the flux density and spectral composition of irradiance and the vertical distribution of Oscillatoria bourrellyi were measured in the North and South Basins of Windermere in the English Lake District. At the population maximum (8-14 m) the photon flux was 0.3-14-mu-mol m-2 s-1 with the waveband 512-580 nm contributing 54-60% of the photosynthetically active irradiance. Samples of O.bourrellyi taken concurrently from 0 and 12 m were analysed to determine the absorption properties of the populations and the phycoerythrin-related fluorescence of individual filaments. The 12 m populations were distinguished from the surface populations by higher beam absorption coefficients at all wavelengths throughout the visible spectrum. These differences were accentuated when the absorption characteristics were calculated using in situ irradiance profiles. The deeper populations consistently absorbed a greater fraction of the available irradiance than the shallow populations. This was due to an overall increase in the total pigment per cell rather than the differential synthesis of phycobiliprotein pigments. These observations were confirmed by both laboratory and field experiments using cultures of O.bourrellyi. In these experiments low white light was sufficient to induce the synthesis of both phycoerythrin and phycocyanin as well as chlorophyll. Mean individual filament fluorescence also distinguished populations from different depths. These measurements further demonstrated that filaments within a population located at a discrete depth have a wide range of fluorescence. This variation increased with decreasing light intensity and suggests that phycoerythrin could be used as a cellular marker to determine the provenance of individual filaments. The benefits of photo-adaptation in O.bourrellyi are analysed in relation to the variable underwater light climate of Windermere, UK. Their ecological significance for this alga during periods of intense but intermittent stratification in a nutrient-limited environment are discussed. The ecological implications of both intra- and inter-specific variations in the absorption properties of phytoplankton are discussed in relation to the use of simple community or populations means as the data entry points for models of photosynthetic production.