Chlorella is a genus of unicellular, nonmobile green microalgae first described by Beijerinck in 1890, with Chlorella vulgaris being the type species. Commonly, Chlorella cells are spherical or ellipsoidal with sizes ranging from 2 to 10 pm in diameter. They are distributed in diverse habitats such as freshwater, seawater, and soil and are free-living or symbiotic with lichens and protozoa (Gors et al., 2010). Chlorella cells reproduce themselves through asexual autospore production. Autospores are simultaneously released through rupture of the mother cell wall, with the number varying from 2 to 16. Chlorella has a thick and rigid cell wall, the structure of which may differ greatly among species.

There have been more than 100 strains of Chlorella reported in the literature. Because they lack conspicuous morphological characters, the classification of Chlorella has been problem­atic. An attempt was made to classify Chlorella species based on certain biochemical and phys­iological characters, i. e., hydrogenase, secondary carotenoids, acid and salt tolerance, lactic acid fermentation, nitrate reduction, thiamine requirement, and the GC content of DNA (Kessler, 1976). By comparing these characters, Kessler (1976) assigned 77 strains of Chlorella from the Culture Collection of Algae at Gottingen (SAG, Germany) to 12 taxa and suggested that Chlorella represents an assembly of morphologically similar species of a polyphyletic origin. Afterward, Kessler and Huss (1992) examined 58 Chlorella strains from the Culture Collection of Algae at the University of Texas at Austin using the above-mentioned biochem­ical and physiological characters and assigned them into 10 previously established species. The sugar composition of cell walls (either glucosamine or glucose and mannose) was also used as a taxonomical marker for Chlorella classification (Takeda, 1991, 1993). Using a 18S rRNA-based phylogenic approach, Huss et al. (1999) revised the Chlorella genus and consid­ered it as a polyphyletic assemblage dispersed over two classes of Chlorophyta, i. e., Chlorophyceae and Trebouxiophyceae. Only four species were suggested to be kept in the Chlorella genus: Chlorella vulgaris, Chlorella sorokiniana, Chlorella kessleri, and Chlorella lobophora. Later, Krienitz et al (2004) excluded Chlorella kessleri from the Chlorella genus and reduced the number of species to three. Here we will regard Chlorella as Chlorella sensu lato and include the data obtained from those Chlorella species that may have been excluded from the Chlorella genus by the studies mentioned.