Characterization of glycoconjugates of extracellular polymeric substances in tufa-associated biofilms by using fluorescence lectin-binding analysis

Freshwater tufa deposits are the result of calcification associated with biofilms dominated by cyanobacteria. Recent investigations highlighted the fact that the formation of microbial calcium carbonates is mainly dependent on the saturation index, which is determined by pH, the ion activity of Ca(2+) and CO(3)(2-), and the occurrence of extracellular polymeric substances (EPS) produced by microorganisms. EPS, which contain carboxyl and/or hydroxyl groups, can strongly bind cations. This may result in inhibition of CaCO(3) precipitation. In contrast, the formation of templates for crystal nucleation was reported by many previous investigations. The purposes of this study were (i) to characterize the in situ distribution of EPS glycoconjugates in tufa-associated biofilms of two German hard-water creeks by employing fluorescence lectin-binding analysis (FLBA), (ii) to verify the specific lectin-binding pattern by competitive-inhibition assays, and (iii) to assess whether carbonates are associated with structural EPS domains. Three major in situ EPS domains (cyanobacterial, network-like, and cloud-like structures) were detected by FLBA in combination with laser scanning microscopy (LSM). Based on lectin specificity, the EPS glycoconjugates produced by cyanobacteria contained mainly fucose, amino sugars (N-acetyl-glucosamine and N-acetyl-galactosamine), and sialic acid. Tufa deposits were irregularly covered by network-like EPS structures, which may originate from cyanobacterial EPS secretions. Cloud-like EPS glycoconjugates were dominated by sialic acid, amino sugars, and galactose. In some cases calcium carbonate crystals were associated with cyanobacterial EPS glycoconjugates. The detection of amino sugars and calcium carbonate in close association with decaying sheath material indicated that microbially mediated processes might be important for calcium carbonate precipitation in freshwater tufa systems.

Biosynthetic Ca2+/Sr2+ Exchange in the Photosystem II Oxygen-evolving Enzyme of Thermosynechococcus elongatus

The thermophilic cyanobacterium, Thermosynechococcus elongatus, has been grown in the presence of Sr2+ instead of Ca2+ with the aim of biosynthetically replacing the Ca2+ of the oxygen-evolving enzyme with Sr2+. Not only were the cells able to grow normally with Sr2+, they actively accumulated the ion to levels higher than those of Ca2+ in the normal cultures. A protocol was developed to purify a fully active Sr(2+)-containing photosystem II (PSII). The modified enzyme contained a normal polypeptide profile and 1 strontium/4 manganese, indicating that the normal enzyme contains 1 calcium/4 manganese. The Sr(2+)- and Ca(2+)-containing enzymes were compared using EPR spectroscopy, UV-visible absorption spectroscopy, and O2 polarography. The Ca2+/Sr2+ exchange resulted in the modification of the EPR spectrum of the manganese cluster and a slower turnover of the redox cycle (the so-called S-state cycle), resulting in diminished O2 evolution activity under continuous saturating light: all features reported previously by biochemical Ca2+/Sr2+ exchange in plant PSII. This allays doubts that these changes could be because of secondary effects induced by the biochemical treatments themselves. In addition, the Sr(2+)-containing PSII has other kinetics modifications: 1) it has an increased stability of the S3 redox state; 2) it shows an increase in the rate of electron donation from TyrD, the redox-active tyrosine of the D2 protein, to the oxygen-evolving complex in the S3-state forming S2; 3) the rate of oxidation of the S0-state to the S1-state by TyrD* is increased; and 4) the release of O2 is slowed down to an extent similar to that seen for the slowdown of the S3TyrZ* to S0TyrZ transition, consistent with the latter constituting the limiting step of the water oxidation mechanism in Sr(2+)-substituted enzyme as well as in the normal enzyme. The replacement of Ca2+ by Sr2+ appears to have multiple effects on kinetics properties of the enzyme that may be explained by S-state-dependent shifts in the redox properties of both the manganese complex and TyrZ as well as structural effects.