In dense stratified systems such as microbial mats, photosynthesis and respiration are coupled due to a tight spatial overlap between oxygen-producing and -consuming microorganisms. We combined microsensors and a membrane inlet mass spectrometer with two independent light sources emitting in the visible (VIS) and near infrared (NIR) regions to study this coupling in more detail. Using this novel approach, we separately quantified the activity of the major players in the oxygen cycle in a hypersaline microbial mat: gross photosynthesis of cyanobacteria, NIR light-dependent respiration of Chloroflexus-like bacteria (CLB) and respiration of aerobic heterotrophs. Illumination by VIS light induced oxygen production in the top approximately 1 mm of the mat. In this zone CLB were found responsible for all respiration, while the contribution of the aerobic heterotrophs was negligible. Additional illumination of the mat with saturating NIR light completely switched off CLB respiration, resulting in zero respiration in the photosynthetically active zone. We demonstrate that microsensor-based quantification of gross and net photosyntheses in dense stratified systems should carefully consider the NIR light-dependent behaviour of CLB and other anoxygenic phototrophic groups.
Epilithic and endolithic biofilms were found to control the formation of stromatolites in karst‐water creeks. We used microsensors to determine the influence of biological processes on chemical conditions within the microenvironment of crystal nucleation sites: the stromatolite surface. Phototrophic members of the biofilms consisted of mainly cyanobacteria and diatoms. Oxygen, pH, calcium, and carbonate concentration microprofiles at the stromatolite surface and boundary layer showed a strong diurnal rhythm of calcium carbonate precipitation. During illumination, photosynthesis caused oxygen production, a marked increase in pH and CO32‐ concentrations, and a decrease in Ca2+ concentration at the stromatolite surface due to calcium carbonate precipitation. The opposite occurred in the dark, indicating decalcification. Calcite was approximately 16 times oversaturated in the bulk water, photosynthesis induced an increase of the supersaturation to <27 at the stromatolite surface under illumination, and respiration induced a decrease of the supersaturation to >10 in the dark. Photosynthetically stimulated calcium carbonate precipitation was confirmed by radioactive isotope (45Ca2+) uptake studies. Over a 24 h light : dark cycle, biofilms showed net calcification. Biotic activity within the stromatolite has a large effect on conditions at its surface and, therefore, contributes considerably to the stromatolite precipitation process.
Variations in morphology, fatty acids, pigments and cyanobacterial community composition were studied in microbial mats across intertidal flats of the arid Arabian Gulf coast. These mats experience combined extreme conditions of salinity, temperature, UV radiation and desiccation depending on their tidal position. Different mat forms were observed depending on the topology of the coast and location. The mats contained 63 fatty acids in different proportions. The increased amounts of unsaturated fatty acids (12-39%) and the trans/cis ratio (0.6-1.6%) of the cyanobacterial fatty acid n-18:1omega9 in the higher tidal mats suggested an adaptation of the mat microorganisms to environmental stress. Chlorophyll a concentrations suggested lower cyanobacterial abundance in the higher than in the lower intertidal mats. Scytonemin concentrations were dependent on the increase in solar irradiation, salinity and desiccation. The mats showed richness in cyanobacterial species, with Microcoleus chthonoplastes and Lyngbya aestuarii morphotypes as the dominant cyanobacteria. Denaturing gradient gel electrophoresis patterns suggested shifts in the cyanobacterial community dependent on drainage efficiency and salinity from lower to higher tidal zones. We conclude that the topology of the coast and the variable extreme environmental conditions across the tidal flat determine the distribution of microbial mats as well as the presence or absence of different microorganisms.
Photosynthetically driven calcification was investigated in diatom-dominated carbonate sediments from Bait Reef, Australia. Laboratory measurements conducted over complete diel cycles, using O 2 , pH, CO 2{ 3 , and Ca 2+ microsensors, confirmed that photosynthesis and respiration drive calcification and calcium release via their respective effects on the local pH. However, the dark situation does not simply mirror the light situation. Profiles showed that calcification and calcium release are not necessarily tightly coupled to the light cycle and that mass transfer phenomena need to be considered in diel chemical dynamics. The magnitude and timing of pH and CO 2{ 3 concentration changes did not simply follow the light cycle. The pH in the upper 3 mm of the sediment changed more rapidly upon illumination than upon darkening. Consequently, photosynthetically induced calcification began shortly (within 1 h) after illumination, but the pH remained elevated and calcification continued for ,7 h after darkening. Thus, calcification in marine phototrophic sediments is not limited to light periods, but may continue for extended periods after darkening. This decoupling of light, photosynthesis, and calcification has profound consequences for estimates of daily calcification rates, which have previously been made from measurements assuming close to steady states and 12 : 12 h light : dark calcification and decalcification. In Bait Reef sediments, such an assumption underestimates daily calcification rates by two-to threefold.
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