Re deep biofilms you should look up the work by people such as Todd
Stevens(?) and the group at the Pacific NorthWest National Laboratory in
Washington State, US. They are still looking at the organisation of
biofilms deep in the basalt under the former Hanford nuclear development
site. These grow as a result of groundwater interaction with the rock,
which releases hydrogen as the primary energy source, with subsequent
secondary growth on the organics so produced. There are a number of others
working on such sub-surface biogeochemistry, including Joe Suflita on SRBs
There are many other primary examples. One of the problems in getting an
answer to your question might be your limitation to freshwater. In terms of
really mature biofilms you cannot get past some of the streamers formed in
acid mine drainage. There are classic examples in the UK eg Wales. The
person to contact is Barrie Johnson at Bangor. These streamers can be
metres long, perhaps a metre wide and thickness measured in (I'm guessing) a
few centimetres. These represent a fascinating biogeochemical cycle where
the top layers are red - due to aerobic oxidation, and precipitation, of
iron at acid pH (high sulphate) - the middle sections are "colourless" - and
the bottom sections are black, due to anaerobic oxidation of said sulphate
to H2S, with precipitation of black metal deposits at neutral pH.
Barrie is pretty well versed on the phylogeny of these species, including
tracing back to common ancestors. But I think you'll find that between
aerobic and acidophilic iron and sulphur oxidisers on the one hand, and
anaerobic and neutrophilic iron and sulphate reducers on the other there
ain't much in the way of genetic convergence!
Something very similar happens at hydrothermal vents at black smoker
chimneys. Here the matrix is seawater, though what is bubbling out is
geothermal. The exact same situation arises as per above, except you have
thermophilic sulphur oxidisers providing primary resources to mesophilic and
marine SRBs in the space of nanometres, and within the concretions of the
I would be guessing that freshwater systems per se are probably too
oligotrophic to support anything close to either example of a mature biofilm
(though the chimneys are hard to classify given their concretions).
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