Spatio-temporal distribution of phototrophic sulfur bacteria in the chemocline of meromictic Lake Cadagno (Switzerland)

In situ hybridization was used to study the spatio-temporal distribution of phototrophic sulfur bacteria in the permanent chemocline of meromictic Lake Cadagno, Switzerland. At all four sampling times during the year the numerically most important phototrophic sulfur bacteria in the chemocline were small-celled purple sulfur bacteria of two yet uncultured populations designated D and F. Other small-celled purple sulfur bacteria (Amoebobacter purpureus and Lamprocystis roseopersicina) were found in numbers about one order of magnitude lower. These numbers were similar to those of large-celled purple sulfur bacteria (Chromatium okenii) and green sulfur bacteria that almost entirely consisted of Chlorobium phaeobacteroides. In March and June when low light intensities reached the chemocline, cell densities of all populations, with the exception of L. roseopersicina, were about one order of magnitude lower than in August and October when light intensities were much higher. Most populations were evenly distributed throughout the whole chemocline during March and June, while in August and October a microstratification of populations was detected suggesting specific eco-physiological adaptations of different populations of phototrophic sulfur bacteria to the steep physico-chemical gradients in the chemocline of Lake Cadagn

Changes in freshwater bacterial community composition during measurements of microbial and community respiration

The respiration rates of a pelagic community and of its microbial fraction (< 1.2 μm) were measured at two depths in the oxic layer of a meromictic alpine lake (Cadagno, Switzerland) using the oxygen technique. The duration of the incubations were 12, 24 and 55 h. Bacterioplankton abundance (DAPI counts) and composition (whole cell hybridization using 11 group-specific rRNA-targeted oligonucleotide probes) were measured during the incubations. Respiration generally increased with time, especially in the microbial fraction, or remained similar. This result was not always consistent with changes in bacterial abundance and cell volume. The composition of the community also changed during the incubations. The abundance of β-Proteobacteria increased during the course of all the experiments. These results extend the previous conclusions drawn in marine environments to fresh waters and demonstrate that, in addition to changes in bacterial abundance, cell volume and biomass, changes in the taxonomic composition of the bacterial community can occur during discrete incubations of freshwater planktonic communitie

Phototropic sulfur and sulfate-reducing bacteria in the chemocline of meromictic Lake Cadagno, Switzerland

Lake Cadagno, a crenogenic meromictic lake located in the catchment area of a dolomite vein rich in gypsum in the Piora Valley in the southern Alps of Switzerland, is characterized by a compact chemocline with high concentrations of sulfate, steep gradients of oxygen, sulfide and light and a turbidity maximum that correlates to large numbers of bacteria (up to 107 cells ml-1). The most abundant taxa in the chemocline are large- and small-celled purple sulfur bacteria, which account for up to 35% of all bacteria, and sulfate- reducing bacteria that represent up to 23% of all bacteria. Depending on the season, as much as 45% of all bacteria in the chemocline are associated in aggregates consisting of different populations of small-celled purple sulfur bacteria of the genus Lamprocystis (up to 35% of all bacteria) and sulfate-reducing bacteria of the family Desulfobulbaceae (up to 12% of all bacteria) that are almost completely represented by bacteria closely related to Desulfocapsa thiozymogenes. Their association in aggregates is restricted to small-celled purple sulfur bacteria of the genus Lamprocystis, but not obligate since non-associated cells of bacteria related to D. thiozymogenes are frequently found, especially under limited light conditions in winter and early summer. Aggregate formation and concomitant growth enhancement of isolates of both partners of this association suggests synergistic interactions that might resemble a sulfide-based source-sink relationship between the sulfate-reducing bacterium that is able to sustain growth by a disproportionation of inorganic sulfur compounds (sulfur, thiosulfate, sulfite), with the purple sulfur bacteria acting as a biotic scavenger. The availability of these isolates opens up the door for future studies considering other facets of potential interactions in aggregates since both types of organisms are metabolically highly versatile and interactions may not be limited to sulfur compounds only

CO2 assimilation in the chemocline of Lake Cadagno is dominated by a few types of phototrophic purple sulfur bacteria

Lake Cadagno is characterized by a compact chemocline that harbors high concentrations of various phototrophic sulfur bacteria. Four strains representing the numerically most abundant populations in the chemocline were tested in dialysis bags in situ for their ability to fix CO2. The purple sulfur bacterium Candidatus ‘Thiodictyon syntrophicum' strain Cad16T had the highest CO2 assimilation rate in the light of the four strains tested and had a high CO2 assimilation rate even in the dark. The CO2 assimilation of the population represented by strain Cad16T was estimated to be up to 25% of the total primary production in the chemocline. Pure cultures of strain Cad16T exposed to cycles of 12 h of light and 12 h of darkness exhibited the highest CO2 assimilation during the first 4 h of light. The draft genome sequence of Cad16T showed the presence of cbbL and cbbM genes, which encode form I and form II of RuBisCO, respectively. Transcription analyses confirmed that, whereas cbbM remained poorly expressed throughout light and dark exposure, cbbL expression varied during the light-dark cycle and was affected by the available carbon sources. Interestingly, the peaks in cbbL expression did not correlate with the peaks in CO2 assimilatio

Dominance of a clonal green sulfur bacterial population in a stratified lake

For many years, the chemocline of the meromictic Lake Cadagno, Switzerland, was dominated by purple sulfur bacteria. However, following a major community shift in recent years, green sulfur bacteria (GSB) have come to dominate. We investigated this community by performing microbial diversity surveys using FISH cell counting and population multilocus sequence typing [clone library sequence analysis of the small subunit (SSU) rRNA locus and two loci involved in photosynthesis in GSB: fmoA and csmCA]. All bacterial populations clearly stratified according to water column chemistry. The GSB population peaked in the chemocline (c. 8 × 106 GSB cells mL−1) and constituted about 50% of all cells in the anoxic zones of the water column. At least 99.5% of these GSB cells had SSU rRNA, fmoA, and csmCA sequences essentially identical to that of the previously isolated and genome-sequenced GSB Chlorobium clathratiforme strain BU-1 (DSM 5477). This ribotype was not detected in Lake Cadagno before the bloom of GSB. These observations suggest that the C. clathratiforme population that has stabilized in Lake Cadagno is clonal. We speculate that such a clonal bloom could be caused by environmental disturbance, mutational adaptation, or invasio

Assessment of microplastic contamination on lakeshores of Lake Lugano

Due to the lack of knowledge on microplastic distribution on lakeshores and their ecological consequences in these ecosystems, the International Commission for the Protection of the Italian-Swiss Waters initiated a study to compare microplastic concentrations on shores of Lake Lugano with those of six Swiss lakes. Therefore, samples were collected on nine shores of Lake Lugano to quantify and identify microplastic particles (0.3-5 mm). After fractionation of the samples, the microplastic particles were manually extracted, quantified and characterized using a Fourier-transform infrared spectrometer (FTIR). Our data from Lake Lugano suggest that the microplastic distribution is heterogeneous among the shores; with the Agno bay being the most contaminated zone, probably due to its proximity to an urban-industrial area and the Vedeggio tributary. The observed microplastic particles were composed up to 89% of polyethylene, polypropylene and polystyrene. The data indicates that Lake Lugano shores have a higher concentration (3’063 ± 2’566 particles/m2) compared to shores of the six other Swiss lakes, albeit the order of magnitude is the same. Regarding spatial variations in the same lake, the pelagic zones of seven Swiss lakes (including Lake Lugano) are approximately four orders of magnitude less contaminated than their shores. Understanding and managing this environmental issue requires a more detailed knowledge of the microplastic distribution on lakeshores and its possible effects on biota.</p

Changes in freshwater bacterial community composition during measurements of microbial and community respiration

The respiration rates of a pelagic community and of its microbial fraction (< 1.2 μm) were measured at two depths in the oxic layer of a meromictic alpine lake (Cadagno, Switzerland) using the oxygen technique. The duration of the incubations were 12, 24 and 55 h. Bacterioplankton abundance (DAPI counts) and composition (whole cell hybridization using 11 group-specific rRNA-targeted oligonucleotide probes) were measured during the incubations. Respiration generally increased with time, especially in the microbial fraction, or remained similar. This result was not always consistent with changes in bacterial abundance and cell volume. The composition of the community also changed during the incubations. The abundance of β-Proteobacteria increased during the course of all the experiments. These results extend the previous conclusions drawn in marine environments to fresh waters and demonstrate that, in addition to changes in bacterial abundance, cell volume and biomass, changes in the taxonomic composition of the bacterial community can occur during discrete incubations of freshwater planktonic communities

Hyperspectral imaging sediment core scanning tracks high-resolution Holocene variations in (an)oxygenic phototrophic communities at Lake Cadagno, Swiss Alps

Pigments produced by anoxygenic phototrophic bacteria are valuable proxies of past anoxia in lacustrine and marine environments. Pigment measurement typically requires time-consuming and costly chemical extractions and chromatographic analyses, which limits the temporal resolution of paleoenvironmental reconstructions based on sedimentary pigments. Here, we evaluate the potential of in situ hyperspectral imaging (HSI) core scanning as a rapid, non-destructive method to document high-resolution changes in oxygenic and anoxygenic phototrophic communities at meromictic Lake Cadagno, Switzerland. Three distinct groups of pigments can be detected with the HSI method in the sediments of Lake Cadagno; each pigment group represents a different phototrophic community. Oxygenic phototrophs are indicated by total chloropigments (TChl; chlorophyll a, b, and derivatives). Two types of anoxygenic phototrophs were distinguished - purple sulfur bacteria (PSB), represented by bacteriochlorophyll a, and green sulfur bacteria (GSB), represented by bacteriochlorophyll c, d, and e. HSI pigment indices were validated by pigment measurements performed on extracted samples using spectrophotometer and high-performance liquid chromatography (HPLC). Bacteriochlorophylls were present throughout the past 10 kyr, confirming geochemical evidence of nearly continuous stratification and sulfidic conditions at Lake Cadagno. Major shifts in the anoxygenic phototropic communities are recorded at decadal to millennial scales. GSB and PSB communities coexisted from 10.2-8.8 kyr BP. Dominance of PSB over GSB from 8.8-3.4 kyr BP indicates strongly stratified conditions in the lake and strong light radiation at the chemocline. From 3.4-1.3 kyr BP, PSB were mostly absent, and GSB became dominant, implying lower light intensity at the chemocline due to a combination of factors including deforestation in the lake surroundings, increased flood frequency, cooler climatic conditions, and changes in groundwater solute concentrations. The high-resolution HSI data show that frequent flood events and mass movements disturbed the chemocline and the anoxygenic bacterial communities and that the PSB were particularly sensitive and slow to recover following these disturbance events. This study demonstrates for the first time that HSI can detect GSB-related pigments, making the method uniquely valuable as a rapid tool to study samples containing pigments of both oxygenic and anoxygenic phototrophs.ISSN:1726-4170ISSN:1726-417

Fine scale analysis of shifts in bacterial community structure in the chemocline of meromictic Lake Cadagno, Switzerland

Phototrophic sulfur bacteria in the chemocline of Lake Cadagno, Switzerland, were dominated by purple sulfur bacteria before 1998, but their composition shifted after a presumed disturbance to consist of mainly green sulfur bacteria. This study focused on comparative analyses of the distribution of the green sulfur compared to the purple sulfur bacteria by analyzing specific populations along fine scale depth profiles of the chemocline of Lake Cadagno. Water samples were collected from the chemocline on October 14th, 1998, and on September 28th, 2004. A detailed analysis of chemocline depth profiles revealed that total biomass of phototrophic sulfur bacteria was three times higher in 2004 than in 1998. The three-fold increase of biomass was entirely due to increments in abundance of one population of green sulfur bacteria, identified as Chlorobium clathratiforme. Abundance of purple sulfur bacteria remained unchanged with respect to overall numbers in the chemocline, but also with respect to distribution of different populations in depth in the chemocline. Aggregates of small-celled purple sulfur bacteria decreased in size about four-fold, but remained associated with sulfate-reducing bacteria of the genus Desulfocapsa. Compared to 1998, these had increased in numbers about three times in 2004, resulting in about ten times higher numbers per aggregate. These results demonstrate long-term effects of a presumed disturbance in autumn 1999 and 2000, on environmental conditions and on green sulfur and sulfate-reducing bacteria in the chemocline, however, without changes in the abundance and in the distribution of purple sulfur bacteria