Search

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.

Signature pigments of photosynthetic green sulfur bacteria (GSB) were found in ancient sediments collected from an abandoned clay quarry located in the Banyoles lacustrine area (Spain). The sediments belong to the Interglacial Waalian of the lower Pleistocene (0.7–1.5 millions years old) and were deposited after a marshy event occurring during that geologic period. Reverse-phase high performance liquid chromatography (RP-HPLC) analyses of acetone:methanol sediment extracts revealed that the main pigments found were carotenoids of both eukaryotic and prokaryotic photosynthetic organisms. In particular, isorenieratene (Isr) and β-isorenieratene (β-Isr) constituted the larger bacterial fraction (35–40% of the total carotenoid content), whereas okenone, a signature pigment of purple sulfur bacteria, accounted for less than 2%. Xanthophylls from oxygenic photosynthetic organisms accounted for the remaining carotenoids. Likewise, traces of degradation products of both bacteriochlorophyll (BChl) and chlorophyll (Chl) were also detected. The low concentration of Chl degradation products made proper identification of these compounds impossible. In contrast, degradation products of BChl-e such as bacteriochlorophyllide-e and bacteriopheophorbide-e were identified in the HPLC analyses, suggesting that chemical degradation of naturally occurring BChl aggregates of GSB was slower in the clay quarry sediments. The presence of signature pigments of brown-colored species of GSB (Isr, β-Isr and degradation products of BChl-e) in the sediments suggests an ancient aquatic environment where GSB were present and where episodes of sulfide-rich anoxia reaching the photic zone could be envisaged. Similarly, the large percentage of algal xanthophylls supports that algae could be an important part of the microbial photosynthetic community in this ancient ecosystem., EU. TMR Program, Contract No. FMRXCT96-0081.

The pigment composition of two species of green-colored BChl c-containing green sulfur bacteria (Chlorobium limicola and C. chlorovibrioides) and two species of brown-colored BChl e-containing ones (C. phaeobacteroides and C. phaeovibrioides) incubated at different light intensities have been studied. All species responded to the reduction of light intensity from 50 to 1 μEinstein(E) m(-2) s(-1) by an increase in the specific content of light harvesting pigments, bacteriochlorophylls and carotenoids. At critical light intensities (0.5 to 0.1 μE m(-2) s(-1)) only brown-colored chlorobia were able to grow, though at low specific rates (0.002 days(-1) mg prot(-1)). High variations in the relative content of farnesyl-bacteriochlorophyll homologues were found, in particular BChl e 1 and BChl e 4, which were tentatively identified as [M, E] and [I, E] BChlF e, respectively. The former was almost completely lost upon reduction of light intensity from 50 to 0.1 μE m(-2) s(-1), whereas the latter increased from 7.2 to 38.4% and from 13.6 to 42.0% in C. phaeobacteroides and C. phaeovibrioides, respectively. This increase in the content of highly alkylated pigment molecules inside the chlorosomes of brown species is interpreted as a physiological mechanism to improve the efficiency of energy transfer towards the reaction center. This study provides some clues for understanding the physiological basis of the adaptation of brown species to extremely low light intensities.

The effects of inhibition of carotenoid biosynthesis by 2-hydroxybiphenyl on the photosynthetic growth, pigment composition and chlorosome structure of Chlorobium phaeobacteroides strain CL1401 were examined. At a concentration of 20 micrograms 2-hydroxybiphenyl .ml-1, carotenoid synthesis was largely inhibited (85%), but the photosynthetic growth rate was almost unaffected (mu control = 0.00525 +/- 0.00007 h-1 and mu HBP-treated = 0.00505 +/- 0.0005 h-1). Cells grown in the presence of the inhibitor were 5 microns-70 microns long, while control cells were between 2-5 microns long. Moreover, 2-hydroxybiphenyl-treated cells contained fewer, unevenly distributed chlorosomes per micron of cytoplasmic membrane with an irregular arrangement (2.5 +/- 1.5 vs of 9.1 +/- 1.9). This was concomitant to the 83% decrease in the content of bacteriochlorophyll (BChl) e in 2-hydroxybiphenyl-treated cells. Electron microscopy revealed that the shape of carotenoid-depleted chlorosomes changed from ellipsoidal to spherical, although the mean volume was similar to that of control chlorosomes. SDS-PAGE analysis of the chlorosome polypeptide composition showed that the amount of CsmA protein decreased by 60% in carotenoid-depleted chlorosomes. This was paralleled by a decrease in the baseplate BChl a content. The data suggest that carotenoids are close to the chlorosomal baseplate, where they carry out both structural and photoprotective functions.

Among photosynthetic microorganisms, Green Sulfur Bacteria are unique due to their capacity to grow at very low light intensities. Their antenna complexes, the chlorosomes, contain the bacteriochlorophylls (BChl c, d or e) the carotenoids (Car), and BChl a in the baseplate [1]. The high packaging and arrangement of pigments within the chlorosomes convert them in highly efficient light-harvesting structures. Brown coloured species can grow under extremely low light conditions both by increasing their specific content of antenna pigments and by modifying their BChl e homolog composition [2]. In contrast, such adaptations were found to be less apparent in green coloured species.

Green species of Chlorobiaceae contain either BChl c or d as main antenna pigments, though in some cases may coexist [1-3]. Inside the chlorosomes, BChl molecules exist as a series of aggregated homologs arranged as rod elements [3]. Chlorosome bacteriochlorophylls capture light energy, which is sequentially transferred to the RC through distinct excitation carriers: the BChl a chlorosome baseplate, FMO proteins, and the membrane bound BChl a-protein complexes [4].

Green sulfur bacteria are unique within photosynthetic microorganisms in their capacity to grow at extremely low light intensities. This ability is mainly related with the structure of their photosynthetic antenna, the chlorosomes, oblong bodies attached to the cytoplasmic membrane and tightly packed with antenna pigments. All green bacteria contain bacteriochlorophyll (BChl) c, d or e, in a mixture of several homologs. Together with antenna BChls, chlorosomes contain variable amounts of carotenoids and a BChl a baseplate, which acts as an intermediate in energy transfer [1].

A reversed-phase High Performance Liquid Cromatography (HPLC) method has been developed to accurately separate bacteriochlorophyllsc, d ande homologues in a reasonably short run time of 60 minutes. By using this method, two well-defined groups of bacteriochlorophyll homologue peaks can be discriminated. The first one consists of 4 peaks (min 24 to 30), which corresponds to the four main farnesyl homologues. The second peak subset is formed by a cluster of up to 10 minor peaks (min 33 to 40). These peaks can be related with series of several alcohol esters of the different chlorosome chlorophylls. The number of homologues was, however, quite variable depending on both, the bacteriochlorophyll and the bacterial species. The method hereby described, also provides a good separation of other photosynthetic pigments, either bacterial (Bacteriochlorophylla, chlorobactene, isorenieratene and okenone) or algal ones (Chlorophylla, Pheophytina and β-carotene). A preliminary screening of the homologue composition of several green photosynthetic bacterial species and isolates, has revealed different relative quantitative patterns. These differences seem to be related to physiological aspects rather than to taxonomic ones. The application of the method to the study of natural populations avoids the typical drawbacks on the pigment identification of overlapping eukaryotic and prokaryotic phototrophic microorganisms, giving further information about their physiological status.

The annual dynamics of the phototrophic bacterial populations developing in the anoxic layers has been monitored in three basins of the northern area of Lake Banyoles (Spain). Although two of the studied basins are meromictic and one is holomictic, chemical properties of the water are almost identical. Therefore, differences in both the spatial and temporal distribution, as well as in the composition of phototrophic bacterial communities, dominated by Chlorobium phaeobacteroides and Chromatium minus, are discussed on the basis of the structural and morphometric characteristics of each basin. Both species showed the same physiological adaptations to light intensity changes by modifying the carotenoid/bacteriochlorophyll ratio. Light reaching the oxic-anoxic boundary appears to be the most important factor controlling the growth of phototrophic bacteria in Lake Banyoles. The oxic-anoxic boundary becomes shallower as summer advances, until enough light is available for bacterial photosynthesis.