Hypersaline microbial mats from Lake Chiprana, Spain, and secondary mats (grown in outdoor mesocosms) from Solar Lake, Israel were investigated with respect to organic carbon flow and calcification mechanisms. These microbial sediment ecosystems are considered as a model ecosystem for first complex microbial communities on early earth and recent complex (marine) ecosystems.
Mats are characterized by intensive internal C-cycling. Many microbial aspects of these mats have been investigated, but surprisingly little is known about heterotrophic aerobes. We identified for the first time aerobic heterotrophic bacteria in hypersaline microbial mats. The genera Rhodobacter, Roseobacter, Marinobacter and Halomonas as numerically important populations. Further physiological studies with isolated strains showed that Rhodobacter and Roseobacter specialize on metabolizing photosynthates that are produced by phototrophic community members.
The flow of carbon was studied in mats from Lake Chiprana, Spain, a permanent natural hypersaline lake. Community structure and function was investigated in a series of field and laboratory studies (Jonkers et al. 2003). These mats produce significant amounts of dissolved organic carbon (DOC). The high flux of DOC out of the mats may explain why particularly in this system photoheterotrophic bacteria Chloroflexus cover the mat.
We investigated which factors limit accretation of microbial mats. N and P additions did not increase net photosynthesis, additions of organics did not increase respiration, suggesting no limitation of these compounds for the functioning and growth of these mats. Only aminoacid additions increased gross photosynthesis, the explanation is probably found in enhanced restoration of an essential, but labile, enzyme in the PS II system, which recovery or resupply is enhanced by the supply of precursors.
The role of oxygen tolerant sulfate-reducing bacteria in the mineralization of organic carbon in the oxic surface layer of Lake Chiprana microbial mats was investigated. Results of this study revealed that daytime porewater concentration of low molecular weight organic carbon compounds could support aerobic respiration in these bacteria. The conclusion of this study therefore is that sulfate-reducing bacteria in the oxic zone of phototrophic mats can successfully compete with aerobic heterotrophic bacteria for organic substrates (Jonkers et al. submitted).
An interesting study was done in towards oxygenic photosynthesis in an anoxic and sulfidic spring (Zodletone Spring, Oklahoma, USA). Initial results show high rates of oxygenic photosynthesis in an anaerobic environment (except during illumination inside the mats). The system closely represents life during the early stages of transition from anoxic to oxic world, i.e. when oxygenic photosynthesis initiated the oxidation of the reduced environment to nowadays conditions.
Calcification and photosynthesis rates have been measured in stromatolites originating from Rio Mesquites, Cuatro Ciénegas (Coahuila, Mexico). Our data suggest that calcification was driven by photosynthesis. Despite very high calcification rates ( in the order of 3.41 0.35 mmol Ca2+ m-2 hr-1) accretion rates were very low, due to bioerosive impact of endemic hydrobiid gastropods. The fact that accretion barely exceeded bioerosion in an environment highly conducive to calcification supports the potential impact of faunal grazing as causal agent in the demise of stromatolites in the late Proterozoic.
In previous studies (using ß-imaging of 45Ca incubations and microsensors for O2 and Ca2+) calcification in Lake Chiprana mats was shown to be driven by photosynthetic increase of pH. However, sulphate reduction in the light is described to also promote calcification by degrading EPS and thereby liberating Ca2+ ions. While we can exclude such a mechanism in the dark, it could still be operative in the light if sulphate reduction is more efficient during daytime. However, we found that in light and dark the sulfate reduction rates were the same, therefore, we found no evidence for the hypothesis that sulfate reduction stimulates calcification.
The international DFG project on the role of cyanobacterial mats in the degradation of oil compounds is finished. The results were scientifically satisfying, the practical perspectives of photosynthetic benthic communities in fighting pollution seem very limited. Mats were collected from the Middle East (Eilat and Gaza, within the DFG project) and from Europe (Ebro Delta, Spain and Etang de Berre, France, within the EU project) were investigated. Our studies showed that cyanobacterial mats from polluted sites possess a strong potential to degrade oil compounds, mainly aerobically. Pollution with oil stimulated sulfate-reducing bacteria, however rates of anaerobic degradation were low. Microsensor analysis showed that biodegradation was accompanied by decreasing photosynthesis. Population analysis exhibited changes in bacterial communities, including cyanobacteria, in the favor of tolerant and oil-degrading microorganisms. Detailed experiments on isolates and intact communities showed that cyanobacteria do not possess degradative capacities towards oil compounds. Aerobic heterotrophic are responsible for degradation. The role of cyanobacteria is supporting the activity of aerobic heterotrophs by providing oxygen and organics.

Henk Jonkers, Raeid Abed, Stefan Grötszchel, Ami Bachar, Susanne Hinck