Matching molecular diversity and ecophysiology of
benthic cyanobacteria and diatoms in communities
along a salinity gradient
Ulrich NuÈbel,1²* Ferran Garcia-Pichel,1³
Ester Clavero2 and Gerard Muyzer1§
1Max Planck Institute for Marine Microbiology,
Bremen, Germany.
2Departament QuõÂmica Ambiental, CID-CSIC,
Barcelona, Spain.
Summary
The phylogenetic diversity of oxygenic phototrophic
microorganisms in hypersaline microbial mats and
their distribution along a salinity gradient were investi-
gated and compared with the halotolerances of closely
related cultivated strains. Segments of 16S rRNA genes
from cyanobacteria and diatom plastids were retrieved
from mat samples by DNA extraction and polymerase
chain reaction (PCR), and subsequently analysed by
denaturing gradient gel electrophoresis (DGGE). Sequ-
ence analyses of DNA from individual DGGE bands
suggested that the majority of these organisms was
related to cultivated strains at levels that had pre-
viously been demonstrated to correlate with charac-
teristic salinity responses. Proportional abundances
of ampli®ed 16S rRNA gene segments from phyloge-
netic groupings of cyanobacteria and diatoms were
estimated by image analysis of DGGE gels and were
generally found to correspond to abundances of the
respective morphotypes determined by microscopic
analyses. The results indicated that diatoms accounted
for low proportions of cells throughout, that the cyano-
bacterium Microcoleus chthonoplastes and close rela-
tives dominated the communities up to a salinity of
11% and that, at a salinity of 14%, the most abundant
cyanobacteria were related to highly halotolerant culti-
vated cyanobacteria, such as the recently established
phylogenetic clusters of Euhalothece and Halospiru-
lina. Although these organisms in cultures had pre-
viously demonstrated their ability to grow with close
to optimal rates over a wide range of salinities, their
occurrence in the ®eld was restricted to the highest
salinities investigated.
Introduction
It is a grand challenge for microbial eco