摘要：

In the freshwater areas of the Everglades, Florida, U.S.A., carbonate is precipitated in dense cyanobacterial mats. Precipitation is linked with photosynthesis in the mats in a quantitative relationship.On ground of field observations and experiments a model for precipitation in the filamentous cyanobacteria Scytonema is proposed, which links precipitation to bicarbonate use in photosynthesis and subsequent release of OH ions.Besides supersaturation of the water with respect to carbonate and photosynthetic bicarbonate use, precipitation requires a suitable sheath structure and composition. The characteristics of the sheath seem to be responsible for a distinct crystal morphology in the two genera Scytonema and Schizothrix , as well as for the restriction of calcification to the outer sheath in Scytonema . In the immediate vicinity of the trichom precipitation seems to be inhibited.Comparison of this form of calcifying cyanobacteria with calcification in calcareous algae shows many similarities and rises the question of the biological significance of calcification or precipitation.The precipitated carbonate shows equilibrium precipitation in its δ oxygen values, while it is enriched in 13 C relative to the ambient water. This agrees with a model of precipitation in which the carbonate derives from the water immediately surrounding the filament. There the water is depleted in 12 C which is preferably taken up for photosynthesis. No respiratory carbon is involved in precipitation.From measurements of the amount of precipitation in the field and in experiments the annual sedimentation rate is estimated to be 0.024 to 0.24 mm. These values fall within the range of laminae thicknesses in fossil algal laminites.

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