ALBERT

Notes: Abstract A small number of prokaryotic species have a unique physiology or ecology related to their development of unusually large size. The biomass of bacteria varies over more than 10 orders of magnitude, from the 0.2 mum wide nanobacteria to the largest cells of the colorless sulfur bacteria, Thiomargarita namibiensis, with a diameter of 750 mum. All bacteria, including those that swim around in the environment, obtain their food molecules by molecular diffusion. Only the fastest and largest swimmers known, Thiovulum majus, are able to significantly increase their food supply by motility and by actively creating an advective flow through the entire population. Diffusion limitation generally restricts the maximal size of prokaryotic cells and provides a selective advantage for mum-sized cells at the normally low substrate concentrations in the environment. The largest heterotrophic bacteria, the 80 x 600 mum large Epulopiscium sp. from the gut of tropical fish, are presumably living in a very nutrient-rich medium. Many large bacteria contain numerous inclusions in the cells that reduce the volume of active cytoplasm. The most striking examples of competitive advantage from large cell size are found among the colorless sulfur bacteria that oxidize hydrogen sulfide to sulfate with oxygen or nitrate. The several-cm-long filamentous species can penetrate up through the ca 500-mum-thick diffusive boundary layer and may thereby reach into water containing their electron acceptor, oxygen or nitrate. By their ability to store vast quantities of both nitrate and elemental sulfur in the cells, these bacteria have become independent of the coexistence of their substrates. In fact, a close relative, T. namibiensis, can probably respire in the sulfidic mud for several months before again filling up their large vacuoles with nitrate.

Notes: Abstract The vertical zonation of light, O2, H2S, pH, and sulfur bacteria was studied in two benthic cyanobacterial mats from hypersaline ponds at Guerrero Negro, baja California, Mexico. The physical-chemical gradients were analyzed in the upper few mm at ≥ 100 μm spatial resolution by microelectrodes and by a fiber optic microprobe. In mats, where oxygen produced by photosynthesis diffused far below the depth of the photic zone, colorless sulfur bacteria (Beggiatoa sp.) were the dominant sulfide oxidizing organisms. In a mat, where the O2–H2S interface was close to the photic zone, but yet received no significant visible light, purple sulfur bacteria (Chromatium sp.) were the dominant sulfide oxidizers. Analysis of the spectral light distribution heare showed that the penetration of only 1% of the incident near-IR light (800–900 nm) into the sulfide zone was sufficient for the development of Chromatium in a narrow band of 300 μm thickness. The balance betweem O2 and light penetration down into the sulfide zone thus deterined in mcro-scale which type of sulfur bacteria becamed dominant.

Notes: Abstract Bacterial sulfate reduction and transformations of thiosulfate were studied with radiotracers in a Microcoleus chthonoplastes-dominated microbial mat growing in a hypersaline pond at the Red Sea. The study showed how a diel cycle of oxygen evolution affected respiration by sulfate-reducing bacteria and the metabolism of thiosulfate through oxidative and reductive pathways. Sulfate reduction occurred in both oxic and anoxic layers of the mat and varied diurnally, apparently according to temperature rather than to oxygen. Time course experiments showed that the radiotracer method underestimated sulfate reduction in the oxic zone due to rapid reoxidation of the produced sulfide. Extremely high reduction rates of up to 10 μmol cm−3 d−1 were measured just below the euphotic zone. Although thiosulfate was simultaneously oxidized, reduced and disproportionated by bacteria in all layers of the mat, there was a shift from predominant oxidation in the oxic zone to predominant reduction below. Concurrent disproportionation of thiosulfate to sulfate and sulfide occurred in all zones and was an important pathway of the sulfur cycle in the mat.

Notes: Abstract The role of complementary spectral utilization of light for the zonation of different groups of oxygenic phototrophic organisms in sediments was studied. The marine sediment was covered by a dense population of diatoms with an underlying population of cyanobacteria. Action spectra for photosynthesis and spectral scalar irradiance, E0, were measured directly in the sediment at a spatial resolution of 0.1 mm by the use of oxygen and light microsensors. The action spectrum for the diatoms was similar to the attenuation spectrum of the scalar irradiance, K0, in the diatom layer with Chl. a and carotenoids being the major photosynthetic pigments. The action spectrum of the cyanobacteria showed photosynthesis maxima at the absorption regions of Chl. a and phycocyanin. The measured depth distribution of spectral scalar irradiance and the action spectra of diatoms and cyanobacteria were used to calculate the spectral quality for photosynthesis of the 400–700 nm light to which the two populations were exposed. This spectral quality was compared to that of the light incident on the sediment surface. Due to preferential extinction of wavelengths, at which their photosynthetically active pigments had maximal absorption, the relative light quality for diatoms was reduced to 85% of the quality of incident light at a similar total quantum flux. This effect was partly due to spectral alterations of light backscattered from the underlying sediment with cyanobacteria. The cyanobacteria at the bottom of the euphotic zone, in contrast, experienced a light spectrum which was favorably altered, to 107% in quality, due to absorption by the overlying diatoms. It was concluded that these changes in spectral light quality can be considered as only one of more factors explaining the zonation of the two phototrophic populations, and that total light intensity and the chemical microenvironment are probably more important factors.

Notes: Abstract The manufacture of a new fiber-optic irradiance microsensor with cosine collecting properties is described. The 70 μm wide light collector, cast on the tip of a tapered optical fiber, consisted of a 50 μm wide flat methacrylate diffuser surrounded by an opaque black coating to prevent light entry from angles 〉 90°. The collector had a directional sensitivity close to the theoretical cosine response, but the sensitivity was lower than the ideal at larger incident angles due to the inherent optical properties of the interface between the collector and the medium. The irradiance microsensor was used concurrently with fiber-optic microsensors for radiance and scalar irradiance in two cyanobacterial mats: a gelatinous laminated mat of Aphanothece sp. and Phormidium sp. and a compact marine intertidal mat of Microcoleus chthonoplastes. At the surface, the ratio of scalar irradiance to downward irradiance depended on spectral absorption characteristics of the sediment and ranged from 1.2 at 430 nm in the Microcoleus mat to 2.0 at 760 nm in the Aphanothece-Phormidium mat. As the light field became more isotropic with depth, the ratio of scalar irradiance to downward irradiance increased at all wavelengths and a maximum of 3.9 was reached in the gelatinous mat at both 675 and 760 nm. The results stress the importance of measuring the right light parameter when photobiological processes in sediments are investigated and the application of scalar irradiance and irradiance as quantitative measures of available light for photosynthesis parameter is discussed. From the distribution of scalar irradiance and irradiance, in situ absorption coefficients were calculated. Within the upper 3 mm of the gelatinous mat, the vertical attenuation coefficients of scalar irradiance, upward radiance and upward irradiance in the Aphanothece-Phormidium mat all increased due to scattering of the light away from the direction of the incident collimated light. Below 3 mm attenuation coefficients for scalar irradiance, irradiance and radiance in the near-infrared spectrum (NIR) became identical indicating that the light field approached an asymptotic radiance distribution. In the compact Microcoleus mat, a near-asymptotic radiance distribution of NIR was obtained at only 0.4 mm depth.

Notes: The direct temperature dependence of aerobic respiration was determined in sediment from Aarhus Bay, Denmark, in incubations shorter than 12 h at temperature intervals of 1.7°C. Oxygen consumption showed a bimodal distribution between −2 and 80°C indicating the presence of distinct non-thermophilic and thermophilic populations. The thermophilic oxygen consumption had minimum, optimum, and maximum temperatures of 40, 55, and 65°C, respectively, and accelerated strongly after a lag phase of 2–3 h, which may be due to an activation of spores. The source of this dormant thermophilic population is unknown. Oxygen consumption by the non-thermophilic population had minimum and maximum temperatures of 〈−1 and 45°C, respectively. The optimum temperature increased from a broad plateau of 20–30°C in late winter to 30–35°C in late summer, and the apparent activation energy in the natural temperature range (0–15°C) increased from ∼50 to ∼70 kJ mol−1, corresponding to Q10 values of ∼2.0 and ∼3.0, respectively. These changes indicated an adaptation of the aerobic population to seasonal temperature conditions. Due to the seasonal adaptation and to diffusive limitations, a relatively weak temperature dependence of the area-specific aerobic mineralisation rate in the sediment was calculated, Q10=1.8. Model calculations further demonstrated significant shifts in the relative importance of aerobic and anaerobic mineralisation due to seasonal temperature variation, with less importance of aerobic respiration and a larger fraction of benthic oxygen consumption coupled to the reoxidation of reduced inorganic compounds during summer than during winter.

Notes: Abstract Sulfate reduction was measured with the 35SO42− -tracer technique in slurries of sediment from Aarhus Bay, Denmark, where seasonal temperatures range from 0° to 15°C. The incubations were made at temperatures from 0°C to 80°C in temperature increments of 2°C to search for presence of psychrophilic, mesophilic and thermophilic sulfate-reducing bacteria. Detectable activity was initially only in the mesophilic range, but after a lag phase sulfate reduction by thermophilic sulfate-reducing bacteria were observed. No distinct activity of psychrophilic sulfate-reducing bacteria was detected. Time course experiments showed constant sulfate reduction rates at 4°C and 30°C, whereas the activity at 60°C increased exponentially after a lag period of one day. Thermophilic, endospore-forming sulfate-reducing bacteria, designated strain P60, were isolated and characterized as D esulfotomaculum kuznetsovii. The temperature response of growth and respiration of strain P60 agreed well with the measured sulfate reduction at 50°–70°C. Bacteria similar to strain P60 could thus be responsible for the measured thermophilic activity. The viable population of thermophilic sulfate-reducing bacteria and the density of their spores was determined in most probable number (MPN) dilutions. The density was 2.8·104 cells·.g−1 fresh sediment, and the enumerations suggested that they were all present as spores. This result agrees well with the observed lag period in sulfate reduction above 50°C. No environment with temperatures supporting the growth of these thermophiles is known in the region around Aarhus Bay.

Notes: Thioploca spp. are multicellular, filamentous, colorless sulfur bacteria inhabiting freshwater and marine sediments. They have elemental sulfur inclusions similar to the phylogenetically closely related Beggiatoa, but in contrast to these they live in bundles surrounded by a common sheath. Vast communities of large Thioploca species live along the Pacific coast of South America and in other upwelling areas of high organic matter sedimentation with bottom waters poor in oxygen and rich in nitrate. Each cell of these thioplocas harbors a large liquid vacuole which is used as a storage for nitrate with a concentration of up to 500 mM. The nitrate is used as an electron acceptor for sulfide oxidation and the bacteria may grow autotrophically or mixotrophically using acetate or other organic molecules as carbon source. The filaments stretch up into the overlying seawater, from which they take up nitrate, and then glide down 5–15 cm deep into the sediment through their sheaths to oxidize sulfide formed by intensive sulfate reduction. New major occurrences have been found in recent years, both in lakes and in the ocean, and have stimulated the interest in these fascinating bacteria.