Using integrating sphere spectrophotometry in unicellular algal research

Abstract

Scanning spectrophotometers equipped with integrating spheres were once highly specialized spectrophotometric equipment but are now more readily available commercially. They are particularly useful for objectively assessing the spectral absorption and reflectance of algal cell suspensions. Ordinary dual-beam spectrometers do not give valid measurements of the spectral properties of cells due to light scattering. Spectra of unicellular algae using integrating sphere spectroscopy can measure the absorbance (Abs), transmissions (%T) and reflectance (%R) and hence the actual absorptance (%Abt) of turbid cell suspensions and hence the in vivo pigment absorption properties of photosynthetic organisms. These results were compared with those obtained using conventional dual-beam spectrophotometry scans on turbid cell suspensions and the in solvent spectra of photosynthetic pigments. The common unicellular green alga, Chlorella sp., is used as an example of an oxygenic photo-organism with chlorophyll a as the primary photosynthetic pigment and comparisons made to other unicellular algae such as a cyanobacterium (Synechococcus), Acaryochloris and a diatom (Chaetoceros). Photosynthetic bacteria, such as Rhodopseudomonas palustris, are photosynthetic but do not produce oxygen, and their photosynthesis is usually based on bacteriochlorophyll a. Comparisons are made of integrating sphere vs. dual-beam transmission spectroscopy of BChl a and BChl b organisms in solvent and in vivo of anoxygenic photosynthetic bacteria (Afifella & Thermochromatium [BChl a], Blastochloris [BChl b]) and with oxygenic organisms.