Selectivity and detrimental effects of epiphytic Pseudanabaena on Microcystis colonies

The cyanobacterium Microcystis aggregates into colonies with a mucilaginous sheath that constitutes a special microhabitat for many microorganisms that associate to it. Here, we examine the notorious, yet scarcely studied case of epiphytic association by the cyanobacterium Pseudanabaena sp. to colonial Microcystis. Co-cultivation of Pseudanabaena with different Microcystis strains evidenced strong specificity in the interaction, with dramatically different outcomes in each case, including (1) inability of Pseudanabaena to access the slime of Microcystis, (2) neutral co-existence of epiphytic Pseudanabaena and Microcystis, and (3) rapid epiphytic proliferation of Pseudanabaena, followed by lysis and rapid decay of Microcystis cells. Whereas strain-specific oligopeptide production could not explain the observed specificity, differences in slime microstructures among Microcystis strains revealed by low-temperature scanning electron microscopy suggest that slime structural features might initially determine the ability of Pseudanabaena to colonize Microcystis, subsequently driving the outcome of the interaction. Furthermore, even under "neutral" co-existence, Pseudanabaena proliferation results in an increase in density that leads to colony settling, implying potential selective losses under natural conditions. Both the selective and antagonistic characters of the interaction indicate that epiphytic Pseudanabaena have the potential to contribute to the dynamics of strains in natural Microcystis communities.