PHOTOSYNTHETIC PRODUCTION AND PHOTOADAPTATION OF PHOTOTROPHIC SULFUR BACTERIA IN LAKE CADAGNO (SWITZERLAND)

Lake Cadagno is a meromictic lake of 21-m depth in the central Swiss A lps at 1,923 m asl. Its mixolimnion reaches from the surface down to 1 0-m depth. A dense population of phototrophic bacteria dominated by Ch romatium okenii is located in the subsequent 2-m layer containing up t o 200 mg bacteriochlorophyll (Bchl) m(-3). Maximum cell concentration was found at 10.8-m depth during the summer season. The light intensit y at the upper edge of the bacterial layer averages 4.7% of the subsur face radiation (at 0.05 m depth) and at the density peak of the layer it averages 0.4%. Variations over the summer season in the photosynthe tic properties of the bacterial population at the depth of highest cel l density have been observed in in situ C-14-CO2 incorporation experim ents. These changes are random, and no photoadaptation effect was foun d. Thus, the population of phototrophic bacteria was physiologically u niform during the investigation period and it had the following photos ynthetic properties (medians for the 25th and 75th percentiles are in parentheses): P-max, 0.154 mg C (mg Bchl)(-1) h(-1) (0.048, 0.174); I- k, 0.016 mol quanta m(-2) h(-1) (0.012, 0.019); alpha, 7.7 mg C (mg Bc hl)(-1) h(-1) (mol quanta m(-2) h(-1))(-1) (4.8, 11.6). The specific d ark production rate was 0.016 mg C (mg Bchl)(-1) h(-1) (0.014, 0.023). The average quantum yield for CO2 assimilation at the depth of maximu m cell density, phi(z), was low at 0.012 (0.007, 0.020). The cells wer e sensitive to light intensities higher than the optimum found at simi lar to 0.036 mol quanta m(-2) h(-1). Maximum inhibition by excessive l ight was determined to be 80% at light intensities >0.108 mol quanta m (-2) h(-1) (=30 mu mol quanta m(-2) s(-1)) when cells were exposed for periods of 2.5 h or longer. The bacterial biomass increment assuming no losses, mu, was calculated to be 0.05 d(-1) (0.03, 0.08) and the l oss rate, lambda, to be 0.03 d(-1) (-0.10, 0.06).