Microdistribution of algae in an Italian thermogene travertine

Abstract

A new method has been devised to examine microdistribution of algae upon and within travertine (freshwater limestone). At the travertine surface a sparse diatom community possessed a contagious distribution. Most algae were endoliths consisting of the diatom Achnanthes minutissima and the filamentous cyanobacterium Phormidium laminosum. Maximum diatom numbers were found at a depth of 2 mm below the surface and maximum cyanobacterium numbers at 5-8 mm depth. Total biovolume per unit area was low (0.62 mm 3 /cm 2 ) and composed of 92 % diatoms, and 8 % cyanobacteria. Spatial distributions of diatoms at the travertine surface were fitted with negative binomial models using the Maximum Likelihood Equation. The models indicated that dense clumps of photosynthesising diatoms with potential to initiate calcite precipitation were scarce and unlikely to influence travertine formation. Morisita indices of dispersion showed that diatom clump size varied over a wide range with the diatoms often randomly distributed within clumps. Endoliths were exposed to low irradiance within travertine. They were associated with allochthonous organic matter, which might permit heterotrophic growth.