BSG/ BGRG Annual Conference: 50th Anniversary Meeting
‘Green Walls’:
Simulating algal growth on sandstone heritage
Khawaja. S
1
1
Oxford Rock Breakdown Laboratory, School of Geography and the Environment, University of Oxford, United Kingdom.
[email protected]
a
c
b
Week 3
Week 4
Figure 2: Relationship between colonisation patterns and moisture 
d
e
g
Why study algal biofilms?
From inoculation to colonisation...
The growth of green algal biofilms on important components of
our sandstone heritage has become a primary conservation concern. An increasingly warm and wet climate is likely to produce
increasingly favourable conditions for algal biofilm development.
This is of particular concern in Northern Ireland (NI) where the
rate and extent of greening is increasing. Whilst these growths often have a profound physical impact (see figures a, b and c:
Elmwood Hall, Belfast), their morphology is little understood and
their precise physical impact upon the underlying sandstone is uncertain. Consequently, there is a need for geomorphological studies to aid our understanding of the role of these green biofilms.
Understanding the environmental range required for algal
biofilm development is of great use given uncertain predictions of future climate change in the north-west of the UK.
Are they benign and bioprotective,
or are they
detrimental and biodeteriorative?
Weak algal solutions prepared in a general growth medium
(3N-BBM+V) were used to inoculate 50mm3 blocks of
Stanton Moor sandstone, a stone common to the UK building industry. Blocks were placed in trays of distilled water.
In response to this question green biofilms have been studied
simultaneously at a field site in Derrygonnelly, NI (see figures d
and e) as well as under controlled laboratory conditions at the
School of Geography, University of Oxford (see figures f , g & h).
This multi-disciplinary study funded by the EPSRC, aims to
answer this question using a biogeomorphological approach. By
simulating growth of green algal species, we can develop an
understanding of the bioreceptivity of selected sandstones from
the British Isles.
Week 2
f
In this pilot study air temperature (20°C), relative humidity
(80%), light levels (~1300lx maximum) and lighting regime
(16hours light: 8hours dark) were controlled to a high resolution in a specialised growth chamber. In so doing it has
been possible to encourage maximum algal yield of the isolated species, Chlorella vulgaris; a known coloniser of
British sandstones.
By measuring changes in moisture content, stone hardness,
colour and permeability prior to and following colonisation,
we can gain a better picture of sandstone bioreceptivity.
h
Figure 1: Comparison of algal growth patterns and subsurface moisture content
Block 1
Block 3
Base Measurement
%WME
%WME
Block 19
%WME
CONTROL
%WME
Legend
%WME
%WME
0
20
Week 1
Week 1
Block 13
40
60
Week 2
80
Week 3
Weight of Colonisation
Very Light,
Light in some areas, some patchy heavier growth,
Average % WME
Medium density,
Fairly heavy growth– stone visible beneath,
Week 4
Percent Algal
Cover
Legend
100
Heavy growth– no stone visible beneath.
Sandstone bioreceptivity- Key results from the pilot study
Patterns of growth and the role of moisture
 Optimum growth conditions for Chlorella vulgaris were maintained
Moisture content within the test blocks was measured weekly using a protimeter (measuring percentage
yet non-homogeneous patterns of colonisation were observed
Wood Moisture Equivalent (%WME); a non -destructive measure of conductivity in porous materials).
 Colonisation occurred in rings; but the morphology of these rings
Nine measurement points were taken across the exposed surface to ensure fine data resolution.
was highly variable between blocks.
 It is often assumed that green algal species are moisture loving;
despite this, maximum growth (cover and density) was observed
within a limited moisture range.
 Next Step: Compare changes in stone hardness and permeability
prior to colonisation and following maturation of growth.
 Next Step: Simulate growth under future climatic conditions in NI.
35-40%WME
In all 19 test blocks, a clear visual pattern was observed; maximum algal colonisation corresponding with
35-40%WME (see Figure 1). Colour measurements demonstrated that on all blocks the central (and interestingly the wettest) zone of the block remained entirely uncolonised, even after one month. Algal
cover was generally greater on the wetter blocks (see Figure 2) but there was significantly less correspondence between the density of colonisation and moisture regime. Interestingly, density of growth did not
display a clear linear trend over time but fluctuated in a number of cases.