Spring tufa forms by precipitation of calcium carbonate from supersaturated waters. Supersaturation is mostly attained, or is highest in some distance downstream the emergence of the spring or within streams in areas of rapids or water falls. The most important factor for tufa formation is degassing of carbon dioxide, so the water reaches the necessary degree of supersaturation for precipitation of calcium carbonate. Degassing can be achieved by physical processes, like highly turbulent flowing water, or biological processes, by presence of vegetation or microorganisms.
Riffle-pool sequence, the water flows fast and loosing his dissolved carbonate deposits tufa layers, the calcification of organic debris is very strong.
Rivularia sp. encrusting a tufa coated boulder.
Plants or autotrophic organisms, especially very small ones, like algae, can influence the hydrochemistry by changing the amount of dissolved carbon dioxide by their metabolism.
Also secreted extracellular substances, for example by Cyanobacteria, common in creeks and ponds, can profoundly modify the shape of crystallized calcium carbonate. Microbial induced tufa is widespread and can comprise the major part of total carbonate volume found in the surroundings of a spring.
Near “Teufelskanzel” – a small spring in a carbonatic catchement examined in 2006 - especially on the rock wall rigth of two small waterfalls modern tufa precipitation takes place. On the left side of the waterfall the scarp is overgrown by the moss species Eucladium verticillatum and Cratoneuretum sp., in contrast on the rigth part of the scarp, an algae mat was established.
In Tab.1 the found algae are listed following a transsect that stretches from the vicinity of the waterfall to the distant, respectively from to wet to dry: Locations 1 and 2 are located directly under the waterfalls, number 3 is influenced by spume of the waterfall and number 4 is already dry. Under wet conditions Phormidium incrustans and P. autumnale were quite common. In sample 2 Leptolyngbia perforans was recognised, in the green layer, incrusting the moss tufa. Also Chamaesiphon rostafinski was found attached to a Pseudoscytonema.
The observed algae mats, growing on 1,5 – 2,5 mm thick tufa layers are approximately 1000 μm thick layers, composed of Scytonema sp., Petalonema sp. and Crooccocales, including rhombohedral shaped cristales. The aerophilic Gloeocapsa rupestris and also Nostoc sp. were found under dry conditions in the algal mat.
In microscopic mounts of the algae mat, two types of cristal structures were observed in contact/related to the algae species Scytonema sp.:
(1) Rhombohedric (euhedral = with cristal faces) single cristals. Some of them are rounded, likely by chemical and mechanical erosion. In many cristaly wholes were observed, which may resemble with imprints of algal filamtents. Thus it can be supposed, that the nucleation of the cristal grain starts at the algal filament.
(2) As a second observed type, consistet of aggregate of cristals, that grew radially around the ending of some filaments.
This first observations shows that algae can play an important role for the nucleation and growth of cristal growth and so for tufa formation.
SANDERS, D.; UNTERWURZACHER, M. & RÜF, B. (2006): Microbially induced calcium carbonate in tufas of the western Eastern Alps: a first overview. Geo.Alp, Vol.3: 167-189
SCHLETTERER, M.; PERNEGGER, L. & BRESSAN, D. (2006): Tuff production at the "Teufelskanzel", Mühlauer Klamm. Unpublish. report "Cyanobacteria and calcification, an introductory course for Biologists and Earth Scientists."