Carbon cycling and calcification in hypersaline microbial mats

Abstract

Phototrophic microbial mats are laminated aggregations of microorganisms that thrive in extreme and oligotrophic environments. Primary production rates by oxygenic phototrophs are extremely high. Primary producers supply heterotrophic mat members with organic carbon, which in turn regenerate CO2 needed for autotrophic carbon fixation. Another potential source of CO2 is calcification, which is known to shift the carbonate equilibrium towards CO2. This thesis investigated the carbon cycle of microbial mats and stromatolitic oncolites, with special emphasis on oxygenic photosynthesis and calcification. Microbial mats from 'La Salada de Chiprana', which were used for three studies (chapters 2, 3&5), were characterised combining in situ and laboratory analyses (chapter 2). Maximal in situ gross photosynthesis rates of 0.44 nmol O2 cm-2 s-1 were only reached during early morning and late afternoon, higher light intensities around noon (2500 Ã ?Ã µmol photons m-2 s-1) inhibited gross photosynthesis. HPLC pigment analysis revealed that the phototrophs migrated downwards during the day, apparently to avoid high light intensities. It was calculated that up to 14% of carbon fixed by gross photosynthesis diffused to the overlying water in the form of low molecular weight fatty acids. The high abundances of Chloroflexus-like bacteria and sulphate-reducing bacteria found in the top layers of the mat are probably linked to the upward flux of these compounds, as they represent typical substrates for these bacteria. Lake Chiprana mats were also used to investigate why high primary production rates in microbial mats do not necessarily result in high growth rates (chapter 3). The response of gross photosynthesis and respiration rates to short-term additions of potentially rate-limiting compounds revealed that both processes were limited by phosphate and organic nitrogen. As net photosynthesis was not stimulated, it remains to be clarified whether the observed increased process rates did in fact induce biosynthesis, and if so, whether biosynthesis occurred at the expense of organic carbon excretion rates. Interestingly, gross photosynthesis and respiration were apparently closely coupled as both processes were always stimulated to the same extent. The experiment presented in chapter 4 revealed that certain physicochemical conditions might influence metabolism of planktonic and benthic ecosystems differently. High salinity limited oxygenic photosynthesis and respiration in benthic but not in planktonic cultures of Halothece sp. Evidence is presented that benthic cultures at high salinity reached inhibitory oxygen partial pressures (pO2) at lower photosynthesis rates due to the combination of decreased oxygen solubility and diffusive transport. The finding that high pO2 restricted photosynthesis indicates that the close coupling between photosynthesis and respiration, as was observed in chapter 3, might at least partially be due to the dependence of phototrophs on oxygen removal by heterotrophic respiration.Chapter 5 presents evidence that calcification in microbial mats from 'La Salada de Chiprana' is induced by photosynthesis and not by sulphate reduction. The ion concentration product (ICP) of calcium carbonate was only increased in the light and pore water analysis indicated that the ICP was mainly influenced by changes in CO32- concentration. The pH increase that shifted the carbonate equilibrium towards CO32- was found to be caused by oxygenic photosynthesis and not by sulphate reduction.Calcification in stromatolitic oncolites was also shown to be induced by a photosynthetically induced shift in the carbonate equilibrium (chapter 6). Microsensor measurements revealed that calcification was very high (1.49 kg CaCO3 m-2 a-1), which allowed accretion of oncolites despite grazing pressure (1.13 kg CaCO3 m-2 a-1) by metazoans.