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Plants & Soil, Bulletin No. 1

Robert J. Palmer Jr. rjpalmer at MAILHOST.CAS.UTK.EDU
Mon Jun 2 07:17:06 EST 1997

Some thoughts about soil biofilms:

>Our objective in initiating this discussion is to stimulate focused debate
>on specific questions related to the role of biofilms in the behavior of
>bacteria in the phytoshpere and in soil.  (Some questions concerning the
>importance of biofilm formation for plant pathogenic bacteria, for example,
>are evoked in Morris C.E., Monier J.-M. and Jacques M-A. 1997. Appl.
>Environ. Microbiol. 63:1570-1576.)  Via these discussions we hope to
>identify research priorities and foster collaborations.  Biofilms associated
>with plants and soil have several particularities - compared to biofilms in
>other environments - that justify a focused discussion.  

>Firstly, biofilms
>associated with (terrestrial) plants and soil are in environments that are
>unsaturated or only occasionally water-saturated.

I feel that, unless one is speaking about arid regions, soils are not such
a bad place for bacteria as far as water relations go.  It is true that
soils often do not contain much free liquid water, however water potential
within soils, particularly agricultural soils, is normally much higher than
many other "terrestrial" environments, e.g., the surfaces of rocks or of
detritus on the soil surface.  The key factor is that the water potential
is usually controlled by matric factors rather than osmotic factors, and
this is really quite an interesting area.  A fair amount of bacterial
physiology research suggest that, unless relative matric potentials
approach 0.96 (1.0 = saturation, i.e., free liquid water or 100% RH),
bacteria are inactive.  Fungi, on the other hand, can be active at relative
matric potentials of 0.75.  One must also take into account that the
potential measured within a soil is not necessarily the potential at the
surface of a soil particle.  Also, bacteria in soils experience regular and
sometimes rapid fluctuations in water potential over time.  Clearly,
bacteria are quite active in soils, and the evidence for low or zero
activity at reduced water potential suggests that bacteria do experience
relatively decent conditions in the soil.

>plant-associated biofilms are on living surfaces.  This property may
>complicate studies of biofilm behavior in their natural environment compared
>to studies realized for biofilms on inert surfaces.

Yes - is it not true that most plant root systems excrete substances into
the soil?  This exudate is a source of nutrients for the bacterial
community that lives in that plant's rhizosphere and makes for a much more
complicated and interesting situation than biofilms on inanimate substrata
(and even for biofilms on mammalian cells).  What about bacteria on
above-ground surfaces?  Aren't aerial tissues (in contrast to roots)
covered by a waxy cuticle that would greatly reduce the chances that they
are directly nourished by the plant?

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