Effects of gilvin on the composition and dynamics of metalimnetic communities of phototrophic bacteria in freshwater North-American lakes

The spectral distribution of light reaching the populations of phototrophic bacteria in the metalimnion of stratified lakes is a selective factor dete rmining the community composition. At deep metalimnia, light spectra are en riched in photons of the central part of the spectrum (500-600 nm) and bene fit Chromatiaceae, brown-coloured Chlorobiaceae and phycoerythrine-containi ng cyanobacteria. Their carotenoids (okenone, spiriloxanthine, isorenierate ne) and phycoerythrines allow these phototrophic bacteria to use light from the narrow central spectral wavebands. Otherwise, shallow metalimnetic com munities receive light from a wide range (400-800 nm) and their composition is more diverse and usually enriched in green-coloured Chlorobiaceae, whic h are unable to take advantage of the central part of the spectrum. Gilvin compounds (humic substances dissolved in water), have strong effects on lig ht absorption, especially at shorter wavelengths. Therefore, light spect-ra in lakes with high gilvin contents are enriched in photons of long wavelen gths (> 600 nm). Several Wisconsin lakes with different gilvin contents wer e studied during the period of summer stratification in 1994. Spectral dist ribution of light reaching their metalimnia changed with increasing gilvin contents (measured as g(440)). In the latter, phototrophic metalimnetic bac terial communities were absolutely dominated by green-coloured Chlorobiacea e. Intermediate lakes could experiment changes on their community compositi on depending on variations in gilvin content, as happened in Little Long la ke. The dynamics of this lake was studied during summer 1995. The ratio of green-coloured species in respect to brown-coloured species increased after a sudden increase of gilvin due to strong rainfall. These results agree wi th the photosynthetic advantage of green-coloured Chlorobiaceae under led-l ight illumination, inferred from laboratory experiments, and suggest a bact eriochlorophyll-dependent, light-harvesting strategy of these phototrophic sulphur bacteria.