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the Science of the
Tbtai l wirmment
The Science of the Total Environment 167 (1995) 215-220
Bioreceptivity: a new concept for building ecology studies
0. Guillitte
Unik d’Enseignem ent et de Recherche de Bioiogie Vigt tale, Facultk des Scien ces Agronomiques, Passag e des Dipo&s 2,
B-5030 Gemblowr, B elgium
Abstract
A definition of the concept of bioreceptivity as the ability of a material to be colonised by living organisms is given.
Related terms, such as primary, secondary, tertiary, intrinsic, extrinsic and semi-extrinsic bioreceptivity, and
bioreceptivity index are also expla ined. The usefulness, possible uses and methodological issues arising from this
concept are discussed.
Keywords: Bioreceptivity; Building ecology studies;Building material colonization
1. Introduction
Many building materials are prone to colonisa-
tion by living organisms. This colonisation causes
changes in colour and in the chemical or physical
properties of the materials. Since the late-60s,
these changes have been grouped under the terms
‘biodegradation’ or ‘biodeterioration’. The latter
seems to be used mainly in connection with mate-
rial degradation; it is missing in many specialised
dictionaries in favour of the word ‘biodegrada-
tion’ which applies more widely to the biological
degradation of substances or well-defined chemi-
cal compounds. These terms tend to give ‘col-
onisation’ negative and sometimes entirely sub-
jective connotations. Indeed, the invasion of ma-
terials by living organisms does not necessarily
lead to physical and chemical degradation but
simply to reversible colour changes that are per-
ceived differently according to the type of con-
struction, the location and the person studying
them. On the contrary, some authors consider the
colour changes to be aesthetically pleasing [l],
credit them with a protective role against man- or
weather-induced aggression [2-41 and suggest that
they have a cleansing effect which benefits the
environment [5].
Therefore, if one wishes to study the colonisa-
tion of materials without being biased by its ef-
fects on the materials, one should not limit one-
self to those characteristics affected by the
colonisation but should include those that allow
colonisation to take place. The precise role of the
building material characteristics in the colonisa-
tion process is not fully understood, with the
exception of acidity, whose influence on the tax-
onomic content of colonising organisms is well
known. In a previous work [5] on the kinetics of
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the colonisation of composite building materials
by plants, we have studied these characteristics
but have not been able to identify them all pre-
cisely. Thus, we have grouped them all under the
term ‘bioreceptivity’ as a means of elucidating
the impact of colonisation on the material with-
out having to resort to a full analytical approach.
2. Basic definitions and variants
In the medical field, the term ‘susceptibility” is
used to describe the vulnerability of an organism
to diseases, especially infectious diseases. The
term is also used in veterinary medicine. Toma et
al. [6] define susceptibility as the ‘aptitude of an
organism to harbour a pathogen, to allow its
development or multiplication, without necessar-
ily suffering’.
By extension, we would define the term ‘biore-
ceptivity’2 as the aptitude of a material (or any
other inanimate object) to be colonised by one or
several groups of living organisms without neces-
sarily undergoing any biodeterioration. The word
‘colonise’ is important since it indicates that con-
ditions for harbouring, development and multipli-
cation have to be met and excludes the ability of
a material to receive living organisms in a tran-
sient and fortuitous manner. It implies that there
is an ecological relationship between the material
and the colonising organisms. Thus, for example,
a joint of mortar is not bioreceptive to ants circu-
lating on it, even if it is their favourite trail on the
masonry. On the other hand, it can be highly
bioreceptive to others insects, such as the ichneu-
mons, if they are able to lay their eggs into it.
Seeds that are deposited on a material without
‘In French: r6ceptivi t6; in German: Empfanghchkeit.
*Our choice of the word ‘biorecept ivity’ as an alternative to
‘susceptibil ity’ is justified by an attempt to use a word that
translates in the same way into different languages after
adding the pretix ‘bio’. Furthermore, the word ‘receptivity’ is
used in English to describe the ability of a flower stigma to be
fertihsed by pollen grains through the pollen tube. There is a
clear similari ty with our concept. Therefore, we suggest using
the word ‘bioreceptivite’ in French, ‘Biorezeptivit lt’ in Ger-
man, ‘bioreceptiviteit’ in Dutch, ‘bioreceptividad’ in Spanish,
‘bioreceptividade’ in Portuguese and ‘biorecettivith’ in Italian.
being able to germinate and develop cannot be
related to the bioreceptivity of the material. How-
ever, if they are able to grow into plantlets and
survive for some time, one could probably say
that this material is bioreceptive to higher plants.
Therefore, bioreceptivity can also be defined as
the totality of material properties that contribute
to the establishment, anchorage and development
of fauna and/or flora. In stony materials, for
instance, it relates mainly to properties of the
area exposed to climatic elements, such as rough-
ness, porosity, moisture and the chemical compo-
sition of the surface layer. The capillary porosity
is a property of the core of the material that can
also affect colonisation.
When a material has not yet been exposed to
colonisation, the bioreceptivity wil l be expressed
only during the appearance of the first colonising
organisms. As long as the properties of the mate-
rial remain very similar or identical to those of its
initial state, we propose using the term ‘primary
bioreceptivity’ to indicate the initial potential of
colonisation. Characteristics of these properties
can evolve over time under the action of colonis-
ing organisms or other factors causing change,
and result in a new type of bioreceptivity, which
we call ‘secondary bioreceptivity’ (Fig. 1). For
practical purposes, secondary bioreceptivity is of-
ten more important than primary bioreceptivity.
Any human activity affecting the material - con-
solidation, coating with a biocide or surface
polishing - also modifies the initial or secondary
characteristics of the properties of the material,
inducing ‘tertiary bioreceptivity’. In principle,
efficient treatments should make this tertiary
bioreceptivity less important than primary and
secondary bioreceptivity.
Particles or substances that are not part of the
material, such as soil, dust or organic particles,
can deposit and accumulate on the material.
These exogenous deposits modify the initial con-
ditions of bioreceptivity. If they are substantial,
they can result in a type of colonisation which no
longer relates directly to the properties of the
material, i.e. those properties that allowed de-
posits to accumulate (Fig. 2). We suggest using
the word ‘extrinsic bioreceptivity’ to describe such
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Fig. 1. Primary, secondary and tertiary biorecept ivity in a stony material. White arrows, black arrows and discontinuous lines
represent the colonisation, physico-chemical deterioration and biodeterioration mechanisms, respectively.
a situation. Some elements of the colonising vege-
tation can, in turn, be colonised by epiphytes or
parasitised by other organisms. Thus, the vegeta-
tion can also be responsible for some extrinsic
bioreceptivity. In other cases, colonisation de-
pends directly and simultaneously on the proper-
ties of the material and on the deposits of ex-
ogenous substances (Fig. 3). We suggest using the
word ‘semi-extrinsic bioreceptivity’ to refer to this
phenomenon. Finally, when colonisation depends
mainly on the properties of the material, irrespec-
tive of exogenous contributions, one could use
the phrase ‘intrinsic bioreceptivity’. In fact, the
three types of bioreceptivity and their intermedi-
ate stages can occur on the same material.
3. Usefulness of the concept
The first advantage of the bioreceptivity con-
cept is that it completes the accessibility concept
developed by Heimans [7] to explain the colonisa-
tion process of materials involving other environ-
mental factors. Accessibility can be defined as the
characteristics of the environment that determine
the abundance of diaspore sources, proximity and
transport capabilities (anemochoria, myrmo-
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Fig. 2. Extrinsic biorecept ivity in a stony material (in the case
of primary biorecept ivity). The white arrow represents the
colonisation mechanism.
choria, avichoria, etc.), including the exposure of
the material to these sources and vectors.
Whereas this concept relates to the colonisation
potential of the environment, the bioreceptivity
concept expresses the colonisation potential as
defined by the characteristics of the material. It is
the combination of these potentials and particu-
lar environmental conditions, such as water, tem-
perature and light, that allows colonisation to
occur. Colonisation cannot occur in the absence
of one group of factors. Therefore, bioreceptivity
is the missing link that was required in the adop-
Fig. 3. Semi-extrinsic bioreceptivity in a stony material (in the
case of secondary biorecept ivity). White and black arrows
represent the colonisation and physico-chemical deterioration,
respectively.
tion o f an integrated approach to the colonisation
of materials. The bioreceptivity of a material will
be best expressed under maximum accessibility
and environmental conditions that are optimal for
the development of organisms.
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Based on this principle, one could consider the
possibility of assessing the bioreceptivity of a ma-
terial to an organism by artificially inoculating the
material with the diaspores of the organism and
placing them under optimal environmental condi-
tions (e.g. a growth chamber). A specific biorecep-
tivity index could thus be determined and in-
cluded in a bioreceptivity scale similar to biotic
indices. These indices would be complementary,
as biotic indices determined on similar substrates
allow a quality assessment of environmental con-
ditions. In this instance, the absence of colonising
cryptogams on the material would reflect a high
level of air pollution, whereas the same absence
in an experiment like that mentioned above would
mean that the material is not bioreceptive to
these cryptogams. A practical application of the
bioreceptivity index would be to provide weighted
biotic indices whenever they are determined from
different materials. For example, Seaward [8] has
shown that asbestos-cement slates are more likely
to be colonised by Lecanora muralis a lichen,
than natural slates or tiles, particularly in pol-
luted areas. Therefore, in spite of its ubiquity, this
lichen cannot be used as a pollution bioindicator,
unless the same roofing materials are found in
the areas under study. However, this exercise
could be carried out - if the bioreceptivity could
be determined accurately for each material - by
dividing the measurement data (number of occur-
rences, average size of the thallus, etc.) by the
bioreceptivity index.
The bioreceptivity index of a material would
also provide users with information on the coloni-
sation risk and help them choose an alternative
material or another use for the same material,
depending on whether or not colonisation is de-
sirable. It could also give information on the
effectiveness of various types of treatments of the
materials. Similarly, the influence of individual
properties or their synergetic effect on the coloni-
sation process could be assessed by measuring
bioreceptivity after a gradual change has occurred
in some of those properties.
Finally, the various types of bioreceptivity de-
fined above could be used to establish the se-
quence of events that lead to a potential or
observed colonisation. It also forces the observer
to conduct an analytical study of the pheno-
menon, thereby fostering a better understanding
of the factors involved in the colonisation process
and ways to prevent or enhance it. Among other
things, the distinction between primary and sec-
ondary bioreceptivity allows one the possibility to
assess the impact of biodeterioration.
4. Methodological problems arising from the
concept
The bioreceptivity of a given material can be
expressed only by subjecting it to various groups
of organisms under environmental conditions that
are optimal and specific for each group. The first
problem that needs to be overcome lies in the
lack of information on these conditions. The sec-
ond issue is how to get a material to be colonised
faster by colonising organisms such as lichens.
Finally, because many types of colonisation are
part of a synecological mechanism, colonisation
by a single type of organism can become either
impossible or completely atypical. In this case, it
is difficult to assess the respective contribution of
intrinsic and extrinsic bioreceptivity.
These problems in growing the colonising or-
ganisms are compounded by the selection of
parameters for measuring bioreceptivity or biore-
ceptivity indices (number of occurrences, biomass,
colonised area, appearance and growth rate of
colonising organisms, fertility, etc.). A practical
approach of these difficulties was illustrated by
the author [9]. At the current stage of concept
definition, it is interesting to note that the biore-
ceptivity of materials can be determined from a
set of relatively cosmopolitan species belonging to
the following major biological groups: autotrophic
bacteria, heterotrophic bacteria, microfungi,
macromycetes, cyanobacteria, green algae,
chrysophytes, endolithic lichens, epilithic lichens,
bryophytes, ferns and flowering plants.
5. Conclusion
Although the concept of bioreceptivity is at-
tractive, it requires additional methodological
studies before it can be used outside the area of
building materials. Multidisciplinary teams con-
sisting of biologists and building material special-
ists have to be set up to conduct integrated stud-
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ies under experimental conditions that are as
standard as possible to remove any subjectivity
attached to the concept. Specialised laboratories
will then be able to design bioreceptivity tests
similar to those used to determine the susceptibil-
ity to frost, and the hardness and the mechanical
strength of materials. These tests will provide an
additional tool for the selection of materials by all
users, including architects and those involved in
restoring buildings.
Acknowledgement
This paper was initiated with the financial sup-
port of the European Commission under the re-
search entitled ‘Interactive physical weathering
and bioreceptivity studies on building stones,
monitored by computerized X-ray tomography
(CT) as a potential non-destructive research tool’.
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