Lack of Phylogeographic Structure in the Freshwater Cyanobacterium Microcystis aeruginosa Suggests Global Dispersal

Abstract

BackgroundFree-living microorganisms have long been assumed to have ubiquitous distributions with little biogeographic signature because they typically exhibit high dispersal potential and large population sizes. However, molecular data provide contrasting results and it is far from clear to what extent dispersal limitation determines geographic structuring of microbial populations. We aimed to determine biogeographical patterns of the bloom-forming freshwater cyanobacterium Microcystis aeruginosa. Being widely distributed on a global scale but patchily on a regional scale, this prokaryote is an ideal model organism to study microbial dispersal and biogeography.Methodology/Principal FindingsThe phylogeography of M. aeruginosa was studied based on a dataset of 311 rDNA internal transcribed spacer (ITS) sequences sampled from six continents. Richness of ITS sequences was high (239 ITS types were detected). Genetic divergence among ITS types averaged 4% (maximum pairwise divergence was 13%). Preliminary analyses revealed nearly completely unresolved phylogenetic relationships and a lack of genetic structure among all sequences due to extensive homoplasy at multiple hypervariable sites. After correcting for this, still no clear phylogeographic structure was detected, and no pattern of isolation by distance was found on a global scale. Concomitantly, genetic differentiation among continents was marginal, whereas variation within continents was high and was mostly shared with all other continents. Similarly, no genetic structure across climate zones was detected.Conclusions/SignificanceThe high overall diversity and wide global distribution of common ITS types in combination with the lack of phylogeographic structure suggest that intercontinental dispersal of M. aeruginosa ITS types is not rare, and that this species might have a truly cosmopolitan distribution.

Genetic diversity of inorganic carbon uptake systems causes variation in CO2 response of the cyanobacterium Microcystis

“…This is in agreement with Janse et al (2004) and Van Gremberghe et al (2011), who did not find a relationship between the rDNA internaltranscribed spacer sequences of Microcystis strains and their geographical location, suggesting global dispersal of Microcystis strains. However, our six North-American strains contained both sbtAB and bicA (with a transposon insert in bicA of NIVA-CYA 140), the three Asian strains lacked complete bicA, strains from Europe included both genotypes and the three strains from Africa and Australia formed outgroups in the sbtAB tree.…”

Cyanobacteria

“…An in-depth comparative phenotypic and phylogenetic analysis of material collected from disparate geographical locations only showed very slight differences, if at all (Garcia-Pichel, Prufert-Bebout, and Muyzer, 1996). A similar global dispersal without clear differences between geographically separated populations, as based on sequence comparisons of the ITS (internal transcribed spacer) region between the 16S and the 23S rRNA genes, was observed for the freshwater planktonic Microcystis aeruginosa (Van Gremberghe et al, 2011). Metagenomic studies showed a remarkably low genomic diversity, with <1% nucleotide divergence in several genes, among geographically widely distributed populations and isolated strains of the marine unicellular nitrogen-fixing Crocosphaera watsonii (Zehr et al, 2007).…”

Species concepts and speciation factors in cyanobacteria, with connection to the problems of diversity and classification

“…Dvořák et al (2012) showed that episodic genetic isolation of the mat-forming cyanobacterium Microcoleus vaginatus may have led to the speciation events. On the other hand, 16S-23S ITS phylogenies of the freshwater, planktic cyanobacterium Microcystis aeruginosa revealed no connection between geographic position and a placement in phylogeny (van Gremberghe et al 2011). Further, no geographical patterning has been observed in polar cyanobacteria based on 16S rRNA (Jungblut et al 2010).…”