K. Sahm et al., Sulphate reduction and vertical distribution of sulphate-reducing bacteriaquantified by rRNA slot-blot hybridization in a coastal marine sediment, ENVIRON MIC, 1(1), 1999, pp. 65-74
In the past, enumeration of sulphate-reducing bacteria (SRB) by cultivation
-based methods generally contradicted measurements of sulphate reduction, s
uggesting unrealistically high respiration rates per cell. Here, we report
evidence that quantification of SRB rRNA by slot-blot hybridization is a va
luable tool for a more realistic assessment of SRB abundance in the natural
environment. The distribution of SRB was investigated in a coastal marine
sediment by hybridization of membrane-immobilized rRNA with oligonucleotide
probes. As represented by general probe-target groups, SRB rRNA contribute
d between 18% and 25% to the prokaryotic rRNA pool. The dominant SRB were r
elated to complete oxidizing genera (Desulphococcus, Desulphosarcina and De
sulphobacterium), while Desulpho-bacter could not be detected. The vertical
profile and quantity of rRNA from SRB was compared with sulphate reduction
rates (SRR) measured with (SO42-)-S-35 tracer in whole-core incubations. W
hile SRB abundance was highest near the surface, peaking at around 1.5cm, m
easured sulphate reduction rates were lowest in this region. A second peak
of SRB rRNA was observed at the transition zone from oxidized to reduced se
diment, directly above the sulphate reduction maximum. Cell numbers calcula
ted by converting the relative contribution of SRB rRNA to the percentage o
f DAPI-stained cells indicated a population size for SRB of 2.4-6.1 x 10(8)
cells cm(-3) wet sediment. Cellular sulphate reduction rates calculated on
the basis of these estimated cell numbers were between 0.01 and 0.09 fmol
SO42- cell(-1) day(-1), which is below the rates that have been determined
for pure cultures (0.2-50fmol SO42- cell(-1) day(-1)) growing exponentially
at near-optimal temperature with a surplus of substrates.