The effect of light quality on the selection of natural populations of Green Sulfur Bacteria (Chlorobiaceae) is considered to be a classic factor in the determination of their ecological niches. From the comparison among phototrophic bacterial populations of lakes, it is shown that brown and green pigmented groups of Chlorobiaceae have a differential distribution depending on depth. Statistical analyses prove that green species, which dominate at shallow oxic/anoxic boundaries, are correlated to light spectra enriched in long wavelengths, while brown ones are found when light spectra are enriched in the central region of the spectrum, as in deeper lake layers. Physiological experiments have been made withChlorobium limicola andC. phaeobacteroides cultures placed under different light quality conditions, in order to verify these hypotheses made on a field data basis. Results show that red and white light has more positive effects on the green bacterium than on the brown. Blue and green light illuminations have opposite consequences. Therefore, the effect of shallow depths and Chromatiaceae shading-which also increases the proportion of long wavelengths in light spectra-benefits the bacteriochlorophyll-based strategies of green species. On the other hand, the carotenoid-based strategies of brown ones are favored by the light climates usually dominant at greater depths. Thus, brown species are considered to be singular adaptations of Chlorobiaceae to depth, where bacteriochlorophyll light-harvesting is strongly limited by light quality.

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A laser in situ scattering and transmissometry (Lisst-100) probe has been used for estimating the particle-size distribution of phytopankton, purple photosynthetic sulphur bacteria (Chromatiaceae), and suspended inorganic sediments in different lakes. Results from Lisst-100 have been compared to laboratory measurements, such as those obtained by using a Galai laser size analyzer (GL), an optical microscope (OM), and a flow cytometer (FC). Although all of these instruments were shown to provide reliable values of the particle number concentration for the given populations, the Lisst-100 was the fastest and most reliable instrument because it did not require manipulation of the samples-which is not the case of GL, OM and FC instruments -and avoided the tedious procedure of microscopic counts. The total particle volume concentration results obtained with Lisst-100 differed from those obtained with GL for populations with large and porous aggregates, such as phytoplankton cells. The difference was attributed to the breakage of fragile algal aggregates resulting from the measuring procedure used by GL. Although for suspended sediment particles both instruments gave the same results for the total particle volume concentration, the particle-size distribution obtained with GL was found always shifted to smaller diameters than with Lisst-100, probably because inorganic sediment particles present compact aggregates. When these aggregates break, they split into a high number of small particles that contribute the same to the total volume concentration as the previous aggregates. Finally, results of the total particle volume concentration with Lisst-100 were in accordance with those obtained with GL for the Chromatiaceae population, because cells remained in a dispersed phase. A good correlation was found between the total particle volume concentration of Chromatiaceae measured with Lisst-100 and the concentration of bacteriochlorophyll a (BChl a), which is the parameter habitually used to estimate the concentration of Chromatiaceae. Therefore, Lisst-100 was found to be a reliable instrument to estimate the Chromatiaceae concentration in aquatic ecosystems.

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The composition and the structure of the metalimnetic communities of phototrophic microorganisms were studied in 24 lakes of Wisconsin and Michigan (USA), during the period of summer stratification, and related to environmental parameters. The presence of phototrophic bacteria was reported for the first time in some lakes. Different types of phototrophic microorganisms were separated in three different clusters, by decreasing values of photosynthetic available radiation (PAR) and redox potential (Eh): (1) cyanobacteria and eukaryotic phytoplankton, (2) Chromatiaceae and multicellular filamentous green bacteria, (3) green sulfur bacteria. Each cluster conformed one different layer of the vertical distribution of the community. At each layer, usually one type of microorganisms outcompeted the others, but at times they were found together in competition. The three layers were mixed at the bottom of the lake at the beginning of the stratification period, before widening and colonising the water column. At the end of summer, the contraction of the metalimnion resulted in overlapping or suppressing some layers. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.

The composition and the structure of the metalimnetic communities of phototrophic microorganisms were studied in 24 lakes of Wisconsin and Michigan (USA), during the period of summer stratification, and related to environmental parameters. The presence of phototrophic bacteria was reported for the first time in some lakes. Different types of phototrophic microorganisms were separated in three different clusters, by decreasing values of photosynthetic available radiation (PAR) and redox potential (Eh): (1) cyanobacteria and eukaryotic phytoplankton, (2) Chromatiaceae and multicellular filamentous green bacteria, (3) green sulfur bacteria. Each cluster conformed one different layer of the vertical distribution of the community. At each layer, usually one type of microorganisms outcompeted the others, but at times they were found together in competition. The three layers were mixed at the bottom of the lake at the beginning of the stratification period, before widening and colonising the water column. At the end of summer, the contraction of the metalimnion resulted in overlapping or suppressing some layers.

The relative content of the four main bacteriochlorophyll (BChl) e homologs of several populations of brown‐colored photosynthetic sulfur bacteria grown in different waterbodies have been measured by high performance liquid chromatography and statistically compared by principal component analysis. All the studied populations possessed representative pigment patterns enriched in highly alkylated bacteriochlorophyll e homologs, with average contents of 0.02±0.01%, 24.92±1.01%, 35.2±0.70%, and 39.9±0.71% for bacteriochlorophyll e1, e2, e3, and e4, respectively. These values clearly differ from those obtained for the same species growing under optimal conditions in laboratory batch cultures (4.99±1.11%, 50.34±1.73%, 28.99±0.63%, and 15.6±1.10% for bacteriochlorophyll e1, e2, e3, and e4, respectively). Multivariate statistical analyses grouped samples into two main clusters, both related to the developmental state of the population. Within these clusters, samples were arranged in several groups according to the physiological pigment response of bacterial populations to light limitation. Although bacteriochlorophyll homolog distribution cannot be considered a real taxonomic character, the data presented demonstrate that it can be useful in field studies since it reflects both the physiological status of the cells and the light regime under which the population has been growing.

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