Sylvie Cousin

The power of species sorting: Local factors drive bacterial community composition over a wide range of spatial scales

There is a vivid debate on the relative importance of local and regional factors in shaping microbial communities, and on whether microbial organisms show a biogeographic signature in their distribution. Taking a metacommunity approach, spatial factors can become important either through dispersal limitation (compare large spatial scales) or mass effects (in case of strongly connected systems). We here analyze two datasets on bacterial communities [characterized by community fingerprinting through denaturing gradient gel electrophoresis (DGGE)] in meso- to eutrophic shallow lakes to investigate the importance of spatial factors at three contrasting scales. Variation partitioning on datasets of both the bacterial communities of 11 shallow lakes that are part of a strongly interconnected and densely packed pond system <1 km apart, three groups of shallow lakes ≈100 km apart, as well as these three groups of shallow lakes combined that span a large part of a North-South gradient in Europe (>2,500 km) shows a strong impact of local environmental factors on bacterial community composition, with a marginal impact of spatial distance. Our results indicate that dispersal is not strongly limiting even at large spatial scales, and that mass effects do not have a strong impact on bacterial communities even in physically connected systems. We suggest that the fast population growth rates of bacteria facilitate efficient species sorting along environmental gradients in bacterial communities over a very broad range of dispersal rates.

Congruence of morphological, reproductive and ITS rDNA sequence data in some Australasian Eunotia bilunaris (Bacillariophyta)

Clones identified as Eunotia bilunaris were isolated from two freshwater habitats in New Zealand and one in Tasmania. Experimental studies of sexual reproduction showed that the mating system was strictly heterothallic. All clones were morphologically similar and, in mixtures of compatible clones, were able to produce a viable F1, regardless of geographical origin. There was nevertheless some geographically related variation in ITS rDNA sequences, suggesting that dispersal and concerted evolution are unable to prevent allopatric divergence. Isolates from two of the three localities also showed significant ITS polymorphisms. Auxosporulation followed a similar pattern to that in other Eunotia species, being isogamous with a single functional gamete per gametangium; plasmogamy was effected via two fusing papillae. Details of auxospore structure, initial cell size, and vegetative valve metrics (width, striation densities) show that conspecificity with a recently studied European E. bilunaris is doubtful. The first SEM observations of the perizonium of Eunotia are presented and show that the central transverse band is apparently a split ring as in araphid pennate diatoms, in contrast to other raphid diatoms, except some Bacillariaceae.

Phylogenetic Analysis of Partial Bacterial 16S rDNA Sequences of Tropical Grass Pasture Soil under Acacia tortilis subsp. raddiana in Senegal

We used direct recovery of bacterial 16S rRNA gene sequences to investigate the bacterial diversity under Acacia tortilis subsp. raddiana, a legume tree naturally growing in the dry land part of Senegal (West Africa). Microbial DNA was purified directly from soil samples and subjected to PCR with primers specific for bacterial 16S rRNA gene sequences. 16S rDNA clone libraries were constructed from two soil samples taken at two dates, i.e. June 25th 1999 (dry season) and August 28th 1999 (rainy season) at depths of 0.25-0.50 m and at 3 m distance from the stem. The 16S rDNA of 117 clones was partially sequenced. Phylogenetic analysis of these sequences revealed extensive diversity (100 phylotypes). Comparative sequence analysis of these clones identified members of the Gammaproteobacteria (35% of the phylotypes) as the most important group, followed by the Firmicutes division with 24%. Alphaproteobacteria, Betaproteobacteria, Acidobacteria and Actinobacteria were found to be less represented. Our data suggest that bacterial communities under Acacia tortilis subsp. raddiana might differ according to the season. The relative compositions of the populations is different in both samples: the Acidobacteria are present in a much higher percentage in the dry season than in the rainy season sample while the inverse effect is observed for the members of the other groups. Within the Gammaproteobacteria we found a shift between the dry season and the rainy season from pseudomonads to Acinetobacter and Escherichia related organisms.