Photosynthesis is the process by which plants, algae, some bacteria, and some protistans absorb solar energy to produce sugar, a form of energy that can be utilised by all organisms. The conversion of unusable sunlight energy into usable chemical energy is associated with the actions of the green pigment chlorophyll, which includes a number of variations, i.e. chlorophyll-a, b, c, d, e. All photosynthetic organisms, including algae, contain chlorophyll-a. The variations of chlorophyll (b, c, d and e) along with other compounds such as phycocyanin and carotenes, are accessory pigments, absorbing energy that chlorophyll-a does not.
Chlorophyll-a absorbs its energy from the violet-blue and orange-red wavelengths, and little from the intermediate (green-yellow-orange) wavelengths (Figure 1). The accessory pigments essentially increase the range of wavelengths that can be absorbed and hence utilised (Figure 1). While all algae contain chlorophyll-a, the accompanying accessory pigments are characteristic for each algal division.
Figure 1. Absorption spectra of plant pigments
In general, the amount of chlorophyll-a in a collected water sample is used as a measure of the concentration of suspended algae and microalgae (phytoplankton). High concentrations of phytoplankton can lead to algal blooms. Chlorophyll-a concentration can be used as an indicator of overall water quality. The most accurate method of determining the quantity of chlorophyll at a particular site is to collect a sample of water and transfer it to a laboratory for analysis, which uses a filtration and extraction technique. However, this method does not lend itself to continuous monitoring of chlorophyll, as regular collection of samples (e.g. hourly intervals) would be labour-intensive.
An alternative method for measuring chlorophyll does not require the collection and extraction of samples, and can therefore be carried out on site. This alternative method provides an estimate of the chlorophyll concentration, utilising the fluorescence of chlorophyll and accessory pigments. When irradiated with light of a particular wavelength, chlorophyll or pigments emit light of a higher wavelength. For example, chlorophyll-a fluoresces when irradiated with blue light with a wavelength of approximately 450 nanometres. Fluorometers induce chlorophyll and accessory pigments to fluoresce by beaming light at a specific wavelength into a sample, and measuring the higher wavelength light that is emitted. Fluorometers utilise Light Emitting Diodes (LEDs) as the source of the light.
Figure 2. Excitation and Emission wavelengths for Chlorophyll-a
Figure 3. Simplified operation of Chlorophyll Probe