Effect of temperature and light on growth of and photosynthesis by Synechococcus isolates typical of those predominating in the octopus spring microbial mat community of Yellowstone National Park

Abstract

Previous molecular analysis of the Octopus Spring cyanobacterial mat revealed numerous genetically distinct 16S rRNA sequences from predominant Synechococcus populations distantly related to the readily cultivated unicellular cyanobacterium Synechococcus lividus. Patterns in genotype distribution relative to temperature and light conditions suggested that the organisms contributing these 16S rRNA sequences may fill distinct ecological niches. To test this hypothesis, Synechococcus isolates were cultivated using a dilution and filtration approach and then shown to be genetically relevant to natural mat populations by comparisons of similarities of 16S rRNA genes and 16S-23S internal transcribed spacer (ITS) regions. Most isolates were identical or nearly identical at both loci to predominant mat genotypes; others showed 1- to 2-nucleotide differences at the 16S rRNA locus and even greater difference in ITS sequences. Isolates with predominant mat genotypes had distinct temperature ranges and optima for growth that were consistent with their distributions in the mat. Isolates with genotypes not previously detected or known to be predominant in the mat exhibited temperature ranges and optima that were not representative of predominant mat populations and also grew more slowly. Temperature effects on photosynthesis did not reflect temperature relations for growth. However, the isolate with the highest temperature optimum and upper limit was capable of performing photosynthesis at a higher temperature than other isolates. Growth rate and photosynthetic responses provided evidence for light acclimation but evidence of, at best, only subtle light adaptation.

FIG. 1.

Phase-contrast photomicrographs of Synechococcus isolates of different genotypes. Arrows identify contaminant organisms.

FIG. 2.

View from above of the apparatus used for temperature-controlled microsensor measurement of oxygenic photosynthesis in small volumes of Synechococcus cultures. Inset shows detail of triangular culture chamber with sleeve to accommodate microsensor.

FIG. 3.

Comparison of Synechococcus growth rates with respect to temperature: (a) Octopus Spring isolates B′, B″, and B‴; (b) Octopus Spring isolates with genotypes A, B′, and B predominating in situ; (c) Synechococcus groups II (B-like), III (A-like), and IV (A′-like) from Hunter’s Hot Springs, Oregon (modified from reference 14). Error bars correspond to ±1 standard error.

FIG. 4.

Microsensor measurements of gross (left panels) and net (right panels) oxygenic photosynthesis as a function of light intensity and temperature for A, B′, and B isolates.

FIG. 5.

Gross photosynthetic rate of oxygenic photosynthesis at 440 μmol photons m⁻² s⁻¹ as a function of temperature measured for A, B′, and B isolates by use of an oxygen microsensor.