Spatial scale and the diversity of benthic cyanobacteria and diatoms in a salina

We characterized the richness of benthic cyanobacteria and diatoms in a sal ina system using traditional and molecular biological methods. After determ ining the different morphotypes and 16S rRNA genes present in various local ities within this hypersaline system, an analysis of the increase of organi smal richness as a function of numbers of samples considered was carried ou t. We found that the spatial scales of sampling yielding significant increa ses in cumulative richness were those at which significant variations in en vironmental parameters (salinity, vertical microgradients) are known to exi st, indicating that the presence of environmental gradients contributes to increased biodiversity. Additionally, we could use this type of cumulative analysis for the estimation, through asymptotic extrapolation, of the total richness of oxygenic phototrophs present in the entire salina system, and for the estimation of the average degree of dissemination of community memb ers within the system. We found interesting differences between analyses ba sed on morphotypes or 16S rRNA genes. The cumulative number of rRNA gene se quences exceeded that of morphotypes by more than two-fold. This indicates that many organisms possessing distinct 16S rRNA gene sequences could not b e distinguished on the basis of morphology. Thus, some of the apparently wi dely distributed morphotypes may in fact conceal several ecologically indep endent genotypes.