Abstract
BackgroundSeveral motile, filamentous cyanobacteria display the ability to self-assemble into tightly woven or twisted groups of filaments that form macroscopic yarns or ropes, and that are often centimeters long and 50–200 µm in diameter. Traditionally, this trait has been the basis for taxonomic definition of several genera, notably Microcoleus and Hydrocoleum, but the trait has not been associated with any plausible function.Method and FindingsThrough the use of phylogenetic reconstruction, we demonstrate that pedigreed, rope-building cyanobacteria from various habitats do not form a monophyletic group. This is consistent with the hypothesis that rope-building ability was fixed independently in several discrete clades, likely through processes of convergent evolution or lateral transfer. Because rope-building cyanobacteria share the ability to colonize geologically unstable sedimentary substrates, such as subtidal and intertidal marine sediments and non-vegetated soils, it is also likely that this supracellular differentiation capacity imparts a particular fitness advantage in such habitats. The physics of sediment and soil erosion in fact predict that threads in the 50–200 µm size range will attain optimal characteristics to stabilize such substrates on contact.ConclusionsRope building is a supracellular morphological adaptation in filamentous cyanobacteria that allows them to colonize physically unstable sedimentary environments, and to act as successful pioneers in the biostabilization process.
Highlights
It has long been known that certain filamentous cyanobacteria can form tightly-woven, rope-like bundles of trichomes that remain held together in a common, tubular, extracellular polysaccharide sheath [1,2], even though this trait has not been related to any particular ecophysiological function (Fig. 1)
Of rope building To test our hypothesis that rope-building evolved independently in several groups of cyanobacteria, we reconstructed the phylogenetic relationships of bona fide rope-building strains and field samples based on sequence comparisons of two genes (16S rRNA and kaiC), newly obtained for this work or from traceable sequences available in public databases
These two genes were chosen because the number of sequences available from other genes in relevant cyanobacterial representatives is very limited
Summary
It has long been known that certain filamentous cyanobacteria can form tightly-woven, rope-like bundles of trichomes (cohesive rows of cells) that remain held together in a common, tubular, extracellular polysaccharide sheath [1,2], even though this trait has not been related to any particular ecophysiological function (Fig. 1). It has, been used in taxonomy as the main morphological character to define several traditional genera, such as Microcoleus, Schizothrix or Hydrocoleum [3], with the tacit phylogenetic implication that rope-building is a synapomorphy, a case of shared inheritance of this trait in the present-day descendants of a common ancestor that built such ropes. This trait has been the basis for taxonomic definition of several genera, notably Microcoleus and Hydrocoleum, but the trait has not been associated with any plausible function
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