Spatial patterns and links between microbial community composition and function in cyanobacterial mats

We imaged reflectance and variable fluorescence in 25 cyanobacterial mats from
four distant sites around the globe to assess, at different scales of resolution,
spatial variabilities in the physiological parameters characterizing their photosynthetic
capacity, including the absorptivity by chlorophyll a (Achl), maximum quantum yield of
photosynthesis (Ymax), and light acclimation irradiance (Ik). Generally, these parameters
significantly varied within individual mats on a sub-millimeter scale, with about 2-fold
higher variability in the vertical than in the horizontal direction. The average vertical profiles
of Ymax and Ik decreased with depth in the mat, while Achl exhibited a sub-surface
maximum. The within-mat variability was comparable to, but often larger than, the
between-sites variability, whereas the within-site variabilities (i.e., between samples from
the same site) were generally lowest. When compared based on averaged values of
their photosynthetic parameters, mats clustered according to their site of origin. Similar
clustering was found when the community composition of the mats’ cyanobacterial layers
were compared by automated ribosomal intergenic spacer analysis (ARISA), indicating
a significant link between the microbial community composition and function. Although
this link is likely the result of community adaptation to the prevailing site-specific
environmental conditions, our present data is insufficient to identify the main factors
determining these patterns. Nevertheless, this study demonstrates that the spatial
variability in the photosynthetic capacity and light acclimation of benthic phototrophic
microbial communities is at least as large on a sub-millimeter scale as it is on a global scale,
and suggests that this pattern of variability scaling is similar for the microbial community
composition.