Abstract
We present an image analysis routine to determine
the contribution of distinct morphotypes to the
total abundance and biomass (carbon) of freshwater bacterioplankton and to the fraction of cells detected by
fluorescence in situ hybridization via catalyzed reporter
deposition (CARD-FISH). The method was tested on
bacterial assemblages from an alpine lake (Piburger See,
Austria) at characteristic time points during the limnological year. Although on average 51% of 4’,6’-diamidino-2-phenylindole (DAPI)-stained objects were hybridized with the oligonucleotide probe EUB I-III, we
detected on average 80% of total biomass determined
from DAPI staining. The assemblage was numerically
dominated by cocci and rods <0.6 μm (mean cell volume
= 0.024 μm3). Only a minor part of these morphotypes
could be hybridized (18 and 50%, respectively). In contrast,
larger rods (0.087 μm3), cocci (0.155 μm3) and
vibrio-shaped cells (0.073 μm3) showed much higher
probabilities to be detected by CARD-FISH. These morphotypes
per se formed the highest contribution to total
biomass, which explained the high detection efficiency
of biomass with CARD-FISH. In addition, we determined
the seasonal dynamics of morphotype distributions
within 3 distinct phylogenetic lineages. Actinobacteria
were predominately small rods and cocci, whereas bacteria
from the Cytophaga–Flavobacterium–Bacteroides
group formed mainly large rods, cocci and filaments. Betaproteobacteria showed the highest morphological variability. Within all lineages, distinct spatio-temporal dynamics of dominant morphotypes were observed. Thus,
the approach presented here will allow for more detailed
studies of the amount of carbon bound by different bacterial
taxa. This is of relevance as distinct lineages can
contribute much more to total bacterial biomass than to
total bacterioplankton abundance.
Posch, T