What controls algal greening of sandstone heritage? An experimental approach

Recent observations have shown that many sandstone buildings, including important components of the UK’s cultural heritage, are becoming covered with green algal growths. This is likely to result from recent changes in air quality and the impacts of a changing climate. The northern regions of the UK in particular have an abundance of sandstone heritage and, given the likelihood of warmer, wetter winters here, algal growth on vulnerable monuments is likely to become a primary conservation concern over the next 50 years. Observations of sandstone monuments in the northern regions of the UK, in particular in Belfast (Northern Ireland), Sheffield and Edinburgh have highlighted that algal greening is notably patchy. This is likely due to the array of factors which affect the bioreceptivity of host substrates such as sandstone. The bioreceptivity of a substrate (its ability to become colonised by microbes such as green algae) is dependent on inherent, external and architectural factors. The role of these factors and the interrelationships between them requires further study.
This thesis aims to investigate the inherent, external and architectural factors which encourage colonisation of sandstone by green algae through an integrated programme of laboratory and field experimentation. The primary objectives of this study are: to develop improved laboratory experimental methods to control and monitor algal growth, to investigate the role of external, inherent and architectural factors and to explore the fundamental role of moisture in the development of algal greening.
In order to address these objectives, laboratory and field experiments have been linked within an integrated overall methodology. Short-term laboratory experiments have investigated the bioreceptivity of four British sandstones (Peak Moor, Dungannon, St Bees and ‘baluster stone’) to single and mixed green algal treatment with Stichococcus bacillaris, Chlorella vulgaris and Desmococcus olivaceus, under controlled conditions. Two field experiments have also been conducted. The first exposed unweathered blocks of Dungannon sandstone in the wet environment of Derrygonnelly, Northern Ireland for 30 months. The second exposed reclaimed sandstone balusters in a shaded and exposed site in central Oxford for 12 months. The laboratory and field experiments presented utlilise a range of simple and accessible methods to monitor biofilm development (for example novel methods to map biomass) and changes in substrate condition (such as monitoring surface moisture movements with weight change and hand-held moisture meters, and using light microscopy to help visualise the impact of green algal biofilms).
The results presented in this thesis confirm that moisture plays a fundamental role in the development of green algal biofilms. In laboratory experiments, colonisation often occurred within a consistent moisture zone and preferential greening in field experiments was observed in areas of frequent moisture movement. External factors have been shown to have a strong influence, in laboratory experiments where marine salts were applied, these were found to delay colonisation by around seven days. Furthermore, salts resulted in inhomogeneous patterns of colonisation, similar to those observed in scoping studies conducted in Sheffield. Laboratory experiments have also demonstrated that inherent substrate factors such as high porosity and presence of certain minerals (such as clay laminations in Dungannon) can increase the primary bioreceptivity of sandstone surfaces.
Field experiments have demonstrated that architectural factors such as aspect and geometry can increase the bioreceptivity of exposed samples. In particular, preferential greening was observed on the dynamically wetted south west facing blocks in Derrygonnelly and on exposed compared with shaded balusters in Oxford. Greening was also concentrated in areas of rainwater flows and stores.
Investigation of the role of external, inherent and architectural factors in the development of algal greening as provided by this project, supplies useful information for those managing our sandstone cultural heritage. This will enable more informed decisions to be made over appropriate management and conservation strategies for the future.