The anaerobic zones of Lakes Cis6 and Vilar (Banyoles karstic area, NE Spain) had mass developments of purple sulfur bacteria during summer 1982. In Lake Vilar, Chromatium spp. was dominant (up to 92% of the microbial biovolume). In Lake Ciso, the predominant microorganisms were Chromatium spp. (up to 7 1%) and another purple sulfur bacterium forming aggregates (20%).The bacterial layer could be divided according to the physiological state of the cells into a top part of maximal specific activity, a peak of maximal abundance, and a bottom part of inactive cells. The bacteria in the peak were predominantly limited by light; sulfide, phosphate, and acetate were not limiting in the middle of the day. The light limitation started at the depth having the maximal concentration of cells; the top of the layer appeared to be sulfide-limited. Specific contents of photopigments, elemental sulfur, and reserve polymers decreased from the top to the bottom of the bacterial layer. These phenomena point to the crucial role of light in the development of layers of phototrophic bacteria in stratified lakes.

Two kinds of predatory bacteria have been observed and characterized by light and electron microscopy in samples from freshwater sulfurous lakes in northeastern Spain. The first bacterium, named Vampirococcus, is Gram-negative and ovoidal (0.6 jam wide). An anaerobic epibiont, it adheres to the surface of phototrophic bacteria (Chromatium spp.) by specific attachment structures and, as it grows and divides by fission, destroys its prey. An important in situ predatory role can be inferred for Vampirococcus from direct counts in natural samples. The second bacterium, named Daptobacter, is a Gram-negative, facultatively anaerobic straight rod (0.5 x 1.5 ,um) with a single polar flagellum, which collides, penetrates, and grows inside the cytoplasm of its prey (several genera of Chromatiaceae). Considering also the well-known case of Bdellovibrio, a Gram-negative, aerobic curved rod that penetrates and divides in the periplasmic space of many chemotrophic Gram-negative bacteria, there are three types of predatory prokaryotes presently known (epibiotic, cytoplasmic, and periplasmic). Thus, we conclude that antagonistic relationships such as primary consumption, predation, and scavenging had already evolved in microbial ecosystems prior to the appearance of eukaryotes. Furthermore, because they represent methods by which prokaryotes can penetrate other prokaryotes in the absence of phagocytosis, these associations can be considered preadaptations for the origin of intracellular organelles.Although symbiotic bacteria have been extensively studied and their evolutionary importance in the origin of eukaryotic cells has been recognized (1, 2), predatory behavior in bacteria is known only for Bdellovibrio (3, 4) and Vampirovibrio (5,6). Antagonistic relationships among large organisms are considered to be properties of ecosystems and integrated into ecological theory (7); however, such behavior (e.g., primary consumption, predation, and scavenging) attributed only to animals and plants (8) MATERIALS AND METHODS Studies were conducted in Lake Estanya (420 02' N, 0O 32' E) and Lake Cis6 (420 08' N, 20 45' E) in northeastern Spain.Both lakes are sinkholes formed in karstic areas, rich in calcium sulfate as gypsum and anhydrite. They receive most of their water inputs through seepage. The water conductivity, about 1800 gS cm-l for Lake Estanya and 1300 ;LS-cm-'for Lake Cis6, is high, primarily as a consequence of dissolved salts as sulfates (siemens are reciprocal ohms; S = 1/fl). From 7 to 10 mM sulfate is present in solution in the hypolimnia of both lakes. Lake Estanya, figure-eight shaped, has two basins 12 and 20 m deep, respectively. They are separated by a 2-m-deep sill (10). Lake Cis6, an almost semispherical basin, is 9 m deep and 25 m in average diameter at the surface. Because of high production of hydrogen sulfide in the sediments, it is completely anoxic during mixing (11). Details of lake ecology and methods of study have been published (12)(13)(14). In both lakes light penetrates down to the thermocline,...

A photosynthetic microbial mat was investigated in a large pond of a Mediterranean saltern (Salins-de-Giraud, Camargue, France) having water salinity from 70 per thousand to 150 per thousand (w/v). Analysis of characteristic biomarkers (e.g., major microbial fatty acids, hydrocarbons, alcohols and alkenones) revealed that cyanobacteria were the major component of the pond, in addition to diatoms and other algae. Functional bacterial groups involved in the sulfur cycle could be correlated to these biomarkers, i.e. sulfate-reducing, sulfur-oxidizing and anoxygenic phototrophic bacteria. In the first 0.5 mm of the mat, a high rate of photosynthesis showed the activity of oxygenic phototrophs in the surface layer. Ten different cyanobacterial populations were detected with confocal laser scanning microscopy: six filamentous species, with Microcoleus chthonoplastes and Halomicronema excentricum as dominant (73% of total counts); and four unicellular types affiliated to Microcystis, Chroococcus, Gloeocapsa, and Synechocystis (27% of total counts). Denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments confirmed the presence of Microcoleus, Oscillatoria, and Leptolyngbya strains (Halomicronema was not detected here) and revealed additional presence of Phormidium, Pleurocapsa and Calotrix types. Spectral scalar irradiance measurements did not reveal a particular zonation of cyanobacteria, purple or green bacteria in the first millimeter of the mat. Terminal-restriction fragment length polymorphism analysis of PCR-amplified 16S rRNA gene fragments of bacteria depicted the community composition and a fine-scale depth-distribution of at least five different populations of anoxygenic phototrophs and at least three types of sulfate-reducing bacteria along the microgradients of oxygen and light inside the microbial mat.

Accurate studies of the pigment composition and isolation in pure cultures of Chlorobiaceae from samples of eight Spanish lakes show that there are two main coexisting groups of green and brown Chlorobium spp. represented respectively by Chlorobium limicola and Chlorobium phaeobacteroides. Laboratory experiments with pure and mixed cultures of the isolated strains show that light quality plays a selective role on the species composition among Chlorobiaceae. This selection depends on the pigment composition which determines the in vivo absorption spectrum of the cells as well as on their ability to adjust the intracellular concentration of light-harvesting pigments to the spectral distribution and energy of light. Correlation analysis performed with field data resulted in significant, but low, correlation coefficients. Nevertheless, they were consistent with laboratory data showing that brown Chlorobiaceae were dominant in deep layers in meromictic lakes, whereas green Chlorobiaceae dominated in layers nearer the surface or underneath plates of Chromatiaceae. The combination of laboratory and field observations stress the role of biological light filtering in determining the species composition among Chlorobiaceae in lakes.

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