Conserving Stone Heritage

Greening of Masonry Walls Research Project

What do algae do on walls? Lab experiments

When green algal biofilms begin to colonise sandstone surfaces they can have very different impacts. In some cases, it is hypothesised that algal biofilms cause damage to the stone surface and subsurface; they are said to be biodeteriorative. Conversely, these biofilms may also take on more of a protective role; in these cases they are bioprotective.

It may be however; that an algal biofilm causes no change to the stone at all, whereby the biofilm would be said to benign. To establish appropriate action for the conservation of our built heritage, it is necessary to identify the precise impacts of these green growths. In particular, it is necessary to identify if and how some organisms are causing damage to the stone. One particularly useful way of monitoring the impacts of growth, is by conducting simulation experiments.

Simulation experiments involve growing different species of green algae (identified from field sampling using molecular microbial methods) on the sandstones we commonly see used for constructing buildings around the UK. Experiments are conducted in a climatically controlled chamber under set conditions for relative humidity (%RH), temperature (°C) and light level. These conditions can be varied to simulate typical diurnal conditions in different parts of the UK, in different seasons and also, conditions projected under future climatic scenarios.

We can get a better picture of the impact algal growth has on the stone surface and subsurface by monitoring the stone prior to, during and following colonisation; this is bioreceptivity analysis. By monitoring changes in stone weight, stone hardness, surface colour and changes in moisture content we can begin to understand the impact of green algal growth. Is it biodeteriorative, bioprotective or indeed, benign?


Why are laboratory simulations important?

By conducting experiments in a laboratory, we can control the factors which affect algal growth. By keeping all but a single factor constant (such as temperature, relative humidity or light levels) we are able to test the impact of that factor on algal growth and therefore, the impact of biofilm development on the stone substrate.

Laboratory simulations allow us to simulate ideal growth conditions. We are also able to simulate future climatic conditions in the UK, therefore allowing us to assess the impact of a changing climate on the development of these biofilms.

Simulations enable close monitoring of algal growth. In laboratory experiments it is possible to test the impact of biofilm development before and after growth.

Why is it important to conduct field experiments?

Field experiments allow us to observe and measure natural algal biofilm development, in real-time.

The natural environment is complex; biofilm development relies on a range of growth conditions. Field locations also provide sites with a variety of conditions which are useful to compare.

We are able to assess building facades; noting where growth occurs. We are then able to conduct tests on the stone substrate to assess the impacts of growth on the stone. This is bioreceptivity analysis.

What is bioreceptivity?

Bioreceptivity refers to the properties of stone which affect its ability to be colonised by different microbial species like green algae.

How do you measure bioreceptivity?

Biofilm development may affect properties of the stone substrate in a number of ways. By comparing measurements of stone hardness, weight and moisture content before and after algal biofilms develop, we can gain a better understanding of the positive and negative impacts of growth, on stone quality. Visual observations using photography and measurements of colour change over time are useful methods of documenting the spatial extent of algal cover.

What are the positive (bioprotective) impacts of green algal biofilms?

It is hypothesised that algal biofilms may help to bind and harden friable stone surfaces. In additions, like some higher plant growth on buildings (such as species of ivy and mosses) green algal biofilms may help to regulate temperature at the stone surface- therefore protecting against extremes such as frost and drought.

What are the negative (biodeteriorative) impacts of green algal biofilms?

Green algal biofilms may encourage moisture retention in pores- this may lead to the dissolution of the stone over time. Some species of algae may also release harmful oxalic acids which may also lead to stone decay. In addition, where algae are physically binded to the surface, weathering may be caused through the expansion and contraction of cell- walls. Aesthetically, green biofilms may appear to be unsightly which is of key concern as these microbial communities often multiply and colonise surfaces quickly.

What are the most important climatic conditions for biofilm development?

Green algal biofilms require certain conditions for growth. These include a mild climate (ideally around 15-20° C) and enough light to photosynthesise. However, the most important condition for growth is moisture. Moist, mild conditions are ideal for growth; this explains why we often see patches of green algae on the northern faces of buildings and beneath architectural features such as string courses and window sills.

What's likely to happen in the future?

Under future climate scenarios Northern Ireland and Scotland are likely to experience a warmer and wetter climate over the next 50 years. These conditions are likely to favour green algal growth. This may lead to the increased 'greening' of sandstone buildings in this region.