Microalgae monitoring in freshwater reservoirs intended for human consumption: application of innovative fluorimetric tools

During the past decades a higher concern about the occurrence of potential harmful algae in freshwater bodies has arisen due to the increase of toxic species reported worldwide. Among these organisms, cyanobacteria count several species with toxigenic potential being able to produce a wide range of cyanotoxins, whose presence in drinking water has been associated with different human health-risks. Monitoring the presence of both cyanobacteria and microalgae is required to determine the quality of water intended for human consumption. The qualitative and quantitative analysis of phytoplankton via microscopic observation is time-consuming with late results that could prevent a prompt response by quality control agencies. Several tools have been developed to obtain algae composition estimation in a water sample based on specific class characteristics (e.g. flow cytometry, pigments analysis via HPLC), but they usually have high costs or require advanced expertise. A German company, bbe Moldaenke GmbH, has developed a user-friendly fluorimeter that could discriminate four major algae classes (Chlorophyta, Cyanophyta, Diatoms and Cryptophyta) and recognize the cyanobacteria genus Planktothrix which includes some toxic species. The principle of the method is based on the specific algal fluorescence spectrum which represents a fingerprint of an algal class; in fact, the chlorophyll a spectrum of each algal group is influenced by the presence of different accessory pigments. In this study we compared the chlorophyll a content of algal cultures and natural freshwater samples using either portable and on-site bbe fluorimeters and traditional spectrophotometric determination. Preliminary results of an intercalibration performed among five distinct laboratories to compare the two methods, showed similar values of total chlorophyll a concentration in algal cultures. Identification at algal class level performed in natural samples with fluorimeters and compared with microscopic observations was in most of the cases accurate. Our data also confirm that the fluorimeter was able to discriminate the genus Planktothrix from other cyanobacteria (Microcystis aeruginosa) in culture. Data obtained in this preliminary work demonstrate that bbe fluorimeters could be a useful tool for a rapid algae screening in natural freshwater samples to underline the presence of cyanobacteria by quality control agencies.