Abstract
Closely related strains of thermophilic Synechococcus were cultivated from the microbial mats found in the effluent channels of Mushroom Spring, Yellowstone National Park (YNP). These strains have identical or nearly identical 16S rRNA sequences but are representative of separate, predicted putative ecotype (PE) populations, which were identified by using the more highly resolving psaA locus and which predominate at different vertical positions within the 1-mm-thick upper-green layer of the mat. Pyrosequencing confirmed that each strain contained a single, predominant psaA genotype. Strains differed in growth rate as a function of irradiance. A strain with a psaA genotype corresponding to a predicted PE that predominates near the mat surface grew fastest at high irradiances, whereas strains with psaA genotypes representative of predominant subsurface populations grew faster at low irradiance and exhibited greater sensitivity to abrupt shifts to high light. The high-light-adapted and low-light-adapted strains also exhibited differences in pigment content and the composition of the photosynthetic apparatus (photosystem ratio) when grown under different light intensities. Cells representative of the different strains had similar morphologies under low-light conditions, but under high-light conditions, cells of low-light-adapted strains became elongated and formed short chains of cells. Collectively, the results presented here are consistent with the hypothesis that closely related, but distinct, ecological species of Synechococcus occupy different light niches in the Mushroom Spring microbial mat and acclimate differently to changing light environments.
Highlights
The cell counts reported here were nearly identical to those reported by Brock (1978), who found the cell densities to be constant at several temperatures between 57.2 and 68◦C in the Mushroom Spring mat
The reduction in PBP contents when cells were grown at high irradiance were of similar magnitude, but the levels were slightly lower for the strains representative of putative ecotype (PE) A4 and A14 than for the PE A1 strain. These results indicate that all three strains regulate their pigment content as a function of irradiance in a manner similar to other cyanobacteria (Bernstein et al, 2014), but that the PE A4 and A14 strains differ in their specific responses from that of the PE A1 strain
The results shown here establish that Synechococcus strains corresponding to PEs predicted from pyrosequencing distribution analyses of psaA sequences exhibit different light adaptation and acclimation patterns as hypothesized by Becraft et al (2015) in the first paper of this series
Summary
As reviewed by the first paper in this three-paper series on the molecular dimension of microbial species (Becraft et al, 2015), in the course of a long-term effort to understand the composition, structure, and function of microbial mat communities inhabiting alkaline, siliceous hot springs in Yellowstone National Park (YNP), we have focused on sequence variation in increasingly more divergent genes, progressing from 16S rRNA (Ferris et al, 1997) to the 16S–23S rRNA internal transcribed spacer region (Ferris et al, 2003) to protein-encoding genes (Becraft et al, 2011; Melendrez et al, 2011). Becraft et al (2015) combined pyrosequencing analysis of the gene encoding psaA, a core subunit of the photosystem I reaction center, with analysis of the psaA sequences by Ecotype Simulation, an algorithm based on the Stable Ecotype Model of species and speciation that predicts ecological species populations from sequence variation (Koeppel et al, 2008). These hypothetical species are called putative ecotypes (PEs) until they are shown to exhibit properties expected of ecological species (Becraft et al, 2015). The predicted A-like PEs exhibit identical or nearly identical 16S rRNA sequences (see Olsen et al, 2015)
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