BIOFILMS IN UNSATURATED ENVIRONMENTS: PLANTS AND SOIL,
BULLETIN NO. 2 (29/10/97)
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Dear colleagues,
On 30 May this year we posted the first issue of "BIOFILMS IN UNSATURATED
ENVIRONMENTS: PLANTS AND SOIL" at this news and discussion site. For those
of you who haven't seen Bulletin N=B0 1, we stated that our objective was to
initiate discussion about the role of biofilms in the behavior of bacteria
in two unsaturated environments: the phytosphere and in soil. In parallel
to that bulletin, we tried to solicit interest from colleagues working
specifically with phyllosphere microorganisms who were not, at the time,
participating in this discussion group. After coming to grips with an
initial feeling of discouragement (due to lots of behind-the-scenes stuff,
and certainly NOT due to the interesting and rapid responses posted in
response to Bulletin N=B0 1) we are happy to post Bulletin N=B0 2.
What role do biofilms play in the ecology and physiology of bacteria in
unsaturated environments such as in soil and on leaf surfaces? Here is a
summary of the hypotheses that were proposed in response to Bulletin N=B0 1:
CONCERNING BACTERIAL SURVIVAL AND GROWTH IN THE SOIL OR ON PLANT SURFACES:
In soils, biofilms do not significantly contribute to the resistance of
bacteria to desiccation as water potentail in soils (other than arid soils)
is probably often high enough to support metabolic activity (see R.J.
Palmer, 02 June; I. Sutherland, 03 June). In fact, in xeric environments,
microorganisms are more likely to be restricted to microcolonies (see R.
McLean, 03 June). On leaves, water availability has a distinct diurnal
variation (except for rainy days). Noctural moisture may be sufficient for
bacterial growth. But during the dry periods, biofilms on leaves protect
bacteria from desiccation (see R. Veeh, 02 June; I. Sutherland, 03 June).
We would like to raise the point here that the importance of noctural
moisture for bacterial growth on leaves would depend on the length of the
lag phase and the generation time of epiphytic bacteria relative to the
duration of the moisture on the leaves. Furthermore, environmental
conditions probably vary much more rapidly in the phyllosphere than in the
soil: temperature variations, cycles of nutrient inputs and the alternance
of dry/wet periods are probably much more marked in the phyllosphere than
in soils. Hence, biofilms may effect bacterial growth and survival
differently in these two environments.
CONCERNING THE DIVERSITY OF THE MICROBIAL COMMUNITIES IN THESE ENVIRONMENTS:
There were some suggestions that the diversity of bacteria in cultivated
soils is different from (D.B. Hendrick, 05 June) or less than (R. Veeh, 02
June) that of fallow soils. However, these hypotheses don't specifically
concern the role of biofilms in these environments. We were looking for
something like this: On a leaf, for example, biofilms foster metabolic
exchanges and the establishment of chemical and physical gradients and
barriers among the microorganisms within a biofilm. Hence the diversity of
niches within a biofilm is greater than that on the remainder of the leaf
leading to a greater microbial diversity inside than outside the epiphytic
biofilms. On the other hand, we could argue that the density of cells
within a biofilm intensifies competitive interactions among microorganisms
allowing anitbiotic producers or bacteria with rapid growth rates and high
affinities for available nutrients to dominate the microbial population
within the biofilm more effectively than outside the biofilm. Does anyone
have any reactions to these suggestions ? Also, what role could biofilms
play in successional changes within a community in the soil or on a leaf?
CONCERNING BACTERIAL-PLANT INTERACTIONS:
Roots exude abundant chemicals that may stimulate specific microbial
populations (R.J. Palmer, 02 June; R. Veeh, 02 June). Leaves, on the other
hand, exude smaller quantities of chemicals due to the presence of a
relatively thick, waxy cuticle and hence their exudates have much less
influence on the population dynamics of phyllosphere bacteria than do root
exudates on rhizosphere bacteria (R.J. Palmer, 02 June). These
hypotheses, however, do not specifically concern biofilm bacteria. To
these hypotheses we add this propostion: Within a biofilm there are
metabolic exchanges and recycling of chemicals. This leads to the
formation and exchange of chemicals that are degredation products of
substances exuded from the host plant. These products of degradation
provide nutrient resources for organisms that are not able to use all/some
substances exuded by the plant. These metabolic exchanges have a greater
influence on the population dynamics of biofilm bacteria on leaves than on
roots. This is due to the greater abundance and diversity of exudates from
leaves than from roots. (But perhaps this difference is masked by all of
the exogenous sources of nutrients on leaves, i.e., pollen, honeydew, dead
spores, etc?.) Furthermore, many bacteria produce biosurfactants. Hence,
we might ask if exudation of nutrients from leaf or root surfaces is
stimulated BY biofilm formation as well as stimulating FOR biofilm
formation.
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=46OR THE NEXT PHASE OF THE DISCUSSION:
It would be interesting to have responses for the following questions.
Please try to indicate in your response which of the particular questions
you are addressing (this will help us summarize):
1) R. McLean (03 June) and D.B. Hendrick (05 June) assert that soil
bacteria spend much of their time 'dormant', waiting for favorable
conditions (food, water). If this is the case, what role do biofilms play
in the survival of soil-borne bacteria if they are already adapted to
handle these stresses?
2) Does anyone have any remarks about the hypotheses concerning diversity
(presented above)?
3) Concerning the bacteria-plant interaction: we have heard that the EPS
of some rhizoplane bacteria (Bacillus polymyxa, Xanthomonas spp.) may be
involved in protecting plants from desiccation by coating the roots during
their colonization by these bacteria. Are root exudates important in
determining the abundance and types of EPS produced? Are these bacteria
forming biofilms on the root surface and are these biofilms involved in
uptake of soil nutrients by roots? Does anyone have hypotheses to
formulate about these or related phenomena on roots or leaves?
Until next time.....
Cindy Morris
INRA, Station de Pathologie V=E9g=E9tale
Montfavet, France
morris at avignon.inra.fr
Jean-Michel Monier
ESPM Department
University of California-Berkeley, USA
monier at nature.berkeley.edu
