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The unicellular, green algal genus, Chlorococcum, is distinguished from other genera in the following attributes: asexual reproduction by zoospores, non-motile vegetative cells with a parietal, hollow spherical chloroplast, and vegetative cells with at least one pyrenoid. Various species in this group exhibit a wide range of ecological and physiological variability as shown from findings that numerous members in this genus are found in habitats of low pH. Recent molecular phylogenetic studies revealed two Chlorococcum lineages. Thus, the goal of this investigation is to determine if the ability to grow at low pH is correlated with the natural history of the organism. Molecular data from 18S and 28S rRNA genes were collected from fifteen different isolates of Chlorococcum in order to reconstruct the natural history of the genus. In addition, controlled ecological studies were used to verify or determine the pH growth optima of different species/isolates in this genus. Results from these experiments indicate that Clc. echinozygotum, Clc. elkhartiense (293), Clc. ellipsoideum, Clc. loculatum, Clc. macrostigmatum, Clc. refringens, Clc. sphacosum, and Clc. tatrense are capable of growth at low pH. Comparing phylogenetic trees based on 18S and 28S rRNA data with the pH growth experiments reveals that the ability to grow at low pH is not correlated with the natural history of the organism. These results indicate that Chlorococcum-like morphology has evolved at least three times and that the ability to grow at low pH has evolved at least three times as well.
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