Chlorophyll extraction methods for the quantification of green microalgae colonizing building facades

The occurrence of microalgae on man-made surfaces such as roof tiles, concrete, building facades and other artificial surfaces in urban areas, where it causes aesthetically unacceptable discolouration of the surfaces, represents a conspicuous problem because of its contribution to biodeterioration and accelerated weathering. So far, microbial colonization has been recorded mainly macroscopically, and hence semi-quantitatively evaluated by the degree of discolouration, and the visual data assigned to a colour scale for algal infestation intensity. However, these assessments lack inter-calibration and suffer from colour modifications because of co-occurring fungi or background properties. By using chlorophyll a as a specific biomarker of aeroterrestrial microalgae, an extraction method was therefore developed to quantify biomass. Two green microalgae, Stichococcus sp. and Chlorella sp., were isolated from facades of buildings, established as monocultures and used in the present study. Algal samples are best preserved by freezing in liquid nitrogen followed by lyophilisation and subsequent storage at −18 °C. Dimethyl formamide (DMF) proved to be the best extraction solvent. Mechanical homogenisation improved extraction by up to 20%. However, chlorophyll a extraction efficiency was strongly species-specific and further influenced by the growth conditions, i.e. whether cells were in logarithmic or stationary growth phase. To achieve maximum values for chlorophyll a from samples of unknown algal species composition and physiological state it was found necessary to make at least two 24-h extractions in DMF. If the second extraction step contributed >20% to the total chlorophyll a concentration, a third DMF extraction was applied. Using this biomarker assay, up to 313 mg chlorophyll a m−2 was obtained from building facades, equivalent to c. 100 g algal fresh weight, which represents a high organic load.