Characterization of functional bacterial groups in a hypersaline microbial mat community (Salins-de-Giraud, Camargue, France)

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.     

Challenges for simultaneous nitrification, denitrification, and phosphorus removal in microbial aggregates: mass transfer limitation and nitrous oxide production

The microbial community composition and activity was investigated in aggregates from a lab-scale bioreactor, in which nitrification, denitrification and phosphorus removal occurred simultaneously. The biomass was highly enriched for polyphosphate accumulating organisms facilitating complete removal of phosphorus from the bulk liquid; however, some inorganic nitrogen still remained at the end of the reactor cycle. This was ascribed to incomplete coupling of nitrification and denitrification causing NO(3)(-) accumulation. After 2 h of aeration, denitrification was dependent on the activity of nitrifying bacteria facilitating the formation of anoxic zones in the aggregates; hence, denitrification could not occur without simultaneous nitrification towards the end of the reactor cycle. Nitrous oxide was identified as a product of denitrification, when based on stored PHA as carbon source. This observation is of critical importance to the outlook of applying PHA-driven denitrification in activated sludge processes.     

Microsensor study of photosynthesis and calcification in the scleractinian coral, Galaxea fascicularis: active internal carbon cycle

Sources of inorganic carbon (Ci) for photosynthesis and calcification and the mechanisms involved in their uptake in scleractinian corals were investigated in microcolonies of Galaxea fascicularis. Direct measurements of Ca²⁺, pH and O₂ on the surface and inside the polyp's coelenteron were made with microsensors. Gross photosynthesis (Pg) and net photosynthesis (Pn) were measured on the surface. Light respiration (LR) was calculated from Pg and Pn. The effect of light/dark and dark/light switches on Ca²⁺ and pH dynamics on the surface and inside the coelenteron were followed. To evaluate the different sources of Ci for photosynthesis and calcification, Ci-free seawater and 6-Ethoxyzolamide and Acetazolamide, inhibitors for carbonic anhydrase (CA) were used.In normal seawater, Pg was about seven times higher than Pn, the LR was ca. 80-90% of the Pg. Thus, most of the O₂ produced in Pg are immediately consumed in respiration, indicating the presence of a highly active internal C-cycle. As the internal C-cycle is highly active, a large part of the Ci for calcification will have passed through the metabolism of the symbiont. The high LR provides ATP for energy requiring processes in light.Ci for photosynthesis and calcification can come from seawater in the form of free Ci, respiration of photosynthates (internal C-cycle) or respiration of the ingested plankton. These sources form a common carbon pool (C-pool) that is used for the different processes.In Ci-free seawater, Pg decreased by about 12.5%, indicating that most of the photosynthetically fixed Ci can temporarily be supplied from internal sources. The initial decalcification, observed directly upon the switch to Ci-free seawater, showed that the Ca-pools in the coral are exchangeable. Part of the Pg in Ci-free seawater may depend on this decalcification for its Ci supply.Three localities of CA were defined. One on the surface facing seawater and one on endodermal cells facing the coelenteron, while the third is intracellular. The inhibition of CA decreased Pg by about 30%, while it increased the concentration of Ca²⁺ as a result of a decrease in its precipitation. The reduction of photosynthesis and calcification by CA inhibition demonstrated that both processes need the enzyme for the supply of Ci. The pH on the surface and inside the coelenteron decreased upon 6-Ethoxyzolamide addition indicating a role of CA in pH control.     

Diversity and vertical distribution of magnetotactic bacteria along chemical gradients in freshwater microcosms

The vertical distribution of magnetotactic bacteria along various physico-chemical gradients in freshwater microcosms was analyzed by a combined approach of viable cell counts, 16S rRNA gene analysis, microsensor profiling and biogeochemical methods. The occurrence of magnetotactic bacteria was restricted to a narrow sediment layer overlapping or closely below the maximum oxygen and nitrate penetration depth. Different species showed different preferences within vertical gradients, but the largest proportion (63-98%) of magnetotactic bacteria was detected within the suboxic zone. In one microcosm the community of magnetotactic bacteria was dominated by one species of a coccoid "Alphaproteobacterium", as detected by denaturing gradient gel electrophoresis in sediment horizons from 1 to 10 mm depth. Maximum numbers of magnetotactic bacteria were up to 1.5 x 10(7) cells/cm3, which corresponded to 1% of the total cell number in the upper sediment layer. The occurrence of magnetotactic bacteria coincided with the availability of significant amounts (6-60 microM) of soluble Fe(II), and in one sample with hydrogen sulfide (up to 40 microM). Although various trends were clearly observed, a strict correlation between the distribution of magnetotactic bacteria and individual geochemical parameters was absent. This is discussed in terms of metabolic adaptation of various strains of magnetotactic bacteria to stratified sediments and diversity of the magnetotactic bacterial communities.     

Spatial distribution of calcification and photosynthesis in the scleractinian coral Galaxea fascicularis

The spatial heterogeneity of photosynthesis and calcification of single polyps of the coral Galaxea fascicularis was investigated. Photosynthesis was investigated with oxygen microsensors. The highest rates of gross photosynthesis (Pg) were found on the tissue covering the septa, the tentacles, and the tissues surrounding the mouth opening of the polyp. Lower rates were found on the tissues of the wall and the coenosarc. Calcification was investigated by radioactive tracers. The incorporation pattern of ⁴⁵Ca and ¹⁴C in the corallites was imaged with use of a Micro-Imager. The -images obtained showed that the incorporation of the radioactive tracers coincided with the Pg distribution pattern with the highest incorporation rates found in the corallite septa. Thus, the high growth rate of the septa is supported by the high rates of Pg by the symbiont in the adjacent tissues. The total incorporation rates were higher in light than in dark, however, the distribution pattern of the radioisotope incorporation was not affected by illumination. This further emphasizes the close relation between calcification and photosynthesis.     

Nitrogen transformations in microenvironments of river beds and riparian zones

The soil of flooded riparian zones, the rhizosphere of riparian plants, biofilms at solid surfaces in the river, and the surface layer of sediments all constitute important environments for the oxidative or reductive transformations of inorganic nitrogen compounds. The exact microzonation and coupling of the processes have recently been studied intensively with ¹⁵N enrichment methods and microsensors for NH₄⁺, NO₂⁻, NO₃⁻, and N₂O. Microsensor analyses of gradients in sediments and biofilms have shown that nitrate production takes place in an aerobic surface zone that has a maximum thickness of a few millimeters in most shallow-water sediments and may be as thin as 100 μm in biofilms from very eutrophic environments. In the anoxic zone, denitrification is also concentrated in a zone of maximum a few millimeters, and typically half of the nitrate produced by nitrification is denitrified while the other half escapes to the water. The supply of nitrate from above is primarily controlled by the oxic layer acting as a diffusion barrier, and therefore denitrification is generally a linear function of the nitrate concentration in the water. The overlying water is thus a much more important source of nitrate for denitrification if the concentration is high. The rate and location of denitrification are also affected by bioturbating animals, benthic microphytes, plants, and bacteria performing dissimilatory nitrate reduction to ammonium (DNRA).     

Microphytobenthic primary production within the flocculent layer, its fractions and aggregates, studied in two shallow Baltic estuaries of different eutrophic status

Investigations concerning the primary production of the microphytobenthos were carried out in two shallow estuarine areas different in their trophic status at the southern Baltic Sea. On top of the sediment surface, a flocculent layer was present at both sites, mainly consisting of bioseston, detritus and mineral particles. Separation of this fluffy layer from the sediment below showed a clear dominance of photoautotrophic versus heterotrophic processes within the layer. In the bare sediment, formerly buried algae were exposed to light again, and immediately, significant photosynthetic activity could be measured. Both findings are of important ecological meaning in the shallow Bodden, where even low wind velocities will lead to a resuspension of the fluffy layer and lateral transport might occur. During deposition, the particles of the flocculent layer will be fractionated due to their different sinking velocity, and therefore might contribute differently to pelagic production. Experimentally gained fractions of the flocculent layer all showed positive oxygen net production already under low light intensities (20-50 μE m⁻² s⁻¹), but the finest fraction contained the highest pigment concentrations and metabolic activities. The photoautotrophic character was also confirmed for the single aggregates of the flocculent layer. Investigations of the light climate and the oxygen penetration within aggregates by microelectrodes showed that, even under dark conditions, no anoxic zones could be detected. The dominance of photoautotrophic processes in the flocculent layer and its particles was indicated at both study sites. However, taken into account the different morphometry and hydrography of the study sites, it became clear that the high trophic status of study site A had a negative impact on the benthic primary production rates and changed species composition.