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Small streams are ecosystems mainly controlled by physical factors. Minor differences in these factors can affect periphyton, which are key functional communities in these ecosystems. Eight different environmental conditions combining two types of current, two flow velocities and two light intensities were produced and controlled in artificial channels. Their impact on young and mature periphyton was investigated during a 6-week exposure period. The two different levels of light intensity produced early effects on the algal community. In young periphyton, the lower level of light intensity enhanced the number of algal cells, and this community appeared to be significantly structured by light. As the periphyton matured, the effects of physical factors became more marked. At this later stage, both the bacterial and algal communities began to be affected. Both function (primary production) and structure began to respond to differences in light and in flow velocity. Small differences in low-level environmental factors, such as light and flow, had an effect both on the structure of periphyton and its functional capacities. Keeping in mind the close link between diversity and function in microbial communities, periphyton confronted to various environmental stresses (pollution, flooding) in the field may behave differently due to minor differences in physical factors.

Cyanobacteria-dominated microbial mat communities thrive widely and year round in coral reefs and tropical lagoons, with periodic massive development of benthic blooms. We studied the diversity and spatiotemporal variation of the cyanobacterial dominance in mats of the shallow lagoon of La Réunion Island in the Indian Ocean by means of denaturing gradient gel electrophoresis and cloning-sequencing approaches targeting the 16S rRNA gene, combined with macromorphological and micromorphological characterization of corresponding phenotypes. The mat-forming cyanobacteria were highly diversified with at least 67 distinct operational taxonomic units identified in the lagoon, encompassing the entire morphological spectrum of the phylum Cyanobacteria, but with striking dominance of Oscillatoriales and Nostocales. It appeared also that selective pressures acting at different geographical scales have an influence on the structure and composition of these mats dominated by cyanobacteria. First, large changes were observed in their diversity and composition in relation to local changes occurring in their environment. Second, from the data obtained on the richness and composition of the mats and from the comparison with similar studies in the world, tropical mats seem to display wider cyanobacterial richness than in temperate and cold areas. Moreover, these tropical mats share more species with mats in other tropical regions than with those in temperate and cold climatic regions, suggesting that marine cyanobacteria in biofilms and mats display a biogeographic structure.

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1. The development of periphytic algae and bacteria is controlled by a combination of interacting biotic processes and abiotic factors. Distinguishing between the selection pressure resulting from pollution and that of natural environmental factors is therefore one of the most critical aspects of assessing the impact of pollutants on the diversity and function of benthic microbial communities in natural ecosystems. 2. We studied how current velocity and season affect the ability of river biofilms to cope with complex chemical pollution. We compared the diversity, structure and production of periphytic algae and bacteria from four sampling zones with differing chemical water quality levels and different flow velocities over the course of two seasons (summer and winter).3. The three factors tested all influenced biofilm development, but this depended on the biological variable being measured. Bacterial and algal densities were highly dependent on season and chemical water quality. Algal density was lower in summer than in winter, but bacterial density and production increased from upstream (reference) to downstream (polluted), and this increase was more marked in winter. The impact of chemical water quality was also dependent on the season. 4. An interaction between current velocity and pollution was also detected. During the summer, there was no difference in bacterial density or production between the upstream and downstream segments in the fast current zones, whereas both variables were higher downstream in the slow current zones. Such interactions between environmental factors and the impact of water quality on biofilms must be taken into account in assessments of the effects of chemicals on biofilm community structure and functioning in rivers.

Marine benthic cyanobacteria in tropical areas have recently been associated with several human poisoning events. To enhance the characterization of these microorganisms and their potential toxicity, benthic cyanobacterial communities were sampled in the lagoons of two islands (Raivavae and Rurutu) located in French Polynesia where human poisoning events by seafood had been reported. The morphological appearance of the mats was used to identify four types of cyanobacterial mat. By a 16S rRNA sequencing approach, it appeared that these mats were usually dominated by a restricted number of operational taxonomic units (OTUs), which were closely related to Leptolyngbya, Oscillatoria, Hydrocoleum, and Anabaena sequences, as previously reported in other tropical lagoons. Interestingly, we determined that these dominant filamentous OTUs were associated in the mats with other cyanobacteria, including unicellular species. By using a population genetic approach based on the sequencing of the internally transcribed spacer (ITS) of the rRNA operon, we found a very restricted genetic diversity in the most common OTU, which displayed a high sequence similarity with Leptolyngbya sp. In addition, there was no geographic differentiation at various spatial scales in the distribution of the different genotypes, suggesting that this species is able to spread over large distances. Finally, PCR screening of genes involved in the biosynthesis of known cyanotoxins revealed the presence of the saxitoxin gene (stxG) in two mats containing a mix of filamentous and unicellular cyanobacterial species.

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