Morphological variation in Staurastrum rotula (Zygnemaphyceae, Desmidiales) in the deepest natural Brazilian lake: essence or accident?

Abstract

For many decades, polymorphism and its consequences have only been studied from the taxonomic point of view. Presently, interest has switched to the environmental causes of morphological variation and its consequences in the form and essence of the species. This study aimed at evaluating desmids morphological modifications of Staurastrum rotula Nordstedt during inter-annual succession patterns in two warm monomitic tropical lakes: Dom 42°33'45"W) and Carioca (19°45'20"S, 42°37'12"W). The effect of thermal stability and light and nutrients availability was based on samples collected monthly from January 2002 to December 2006 compared the morphological modifications. Results indicated that morphological variation, asexual reproduction, theratological forms, mucilaginous envelope and fungal infection were highest in Lake Dom Helvécio and coincided with the biomass increase of species with complex morphology between September and March (stratification period). The Zmix oscillation, wind and rainfall occurring at the end of the mixing period and beginning of the stratification were suggested as autochthonous and allochthonous disturbance agents, respectively, identified as inducers of asexual reproduction and consequently of the morphological variation. It was suggested that incidence of parasitism may act as a potential controlling agent for the Staurastrum rotula population size. It was concluded that morphological variation represents accidents in the original form, i.e. in the desmid species essence, promoting the existence of ecoforms, not of new infraspecific taxa. Keywords

Aberrant Forms of Algae and Bioindication of Aquatic Ecosystem State

“…Most often, the authors mention the existence of aberrant forms, unable to establish their causes due to the multifactorial effect on the morphology of cells in nature. In the same cases, when observations are made under controlled experimental conditions, gigantism and ugliness are observed in desmids and chlorococcoid algae under the influence of cultivation conditions such as glucose addition, elevated of temperature [7,8], or decreasing in cells size of diatoms as a consequence of the vegetative fission [9][10][11][12][13]. Deformities are sometimes associated with illness or epiphytism [14,15] or are caused by acidification of the medium to a pH of less than 3.0 [16,17].…”