Diversity and ecology of small coccoid green algae from Lake Itasca, Minnesota, USA, including Meyerella planktonica, gen. et sp. nov

摘要

We characterized 81 isolates of picoplankton and other small planktonic coccoid green algae from Lake Itasca, Itasca State Park, MN, USA. Sequence analysis and examination by light microscopy revealed three common algae: Pseudodictyosphaerium/Korshpalmella spp. (Chlorophyceae), Choricystis spp. (Trebouxiophyceae) and a previously undescribed autosporic coccoid. The Pseudodictyosphaerium and Korshpalmella species that were evaluated all possessed identical 18S ribosomal DNA (rDNA) and rbcL sequences, but this 18S rDNA sequence differed from the published 18S rDNA sequence for P. jurisii isolated from Europe. These chlorophycean algae were isolated from samples collected from Lake Itasca during all seasons. The 18S rDNA sequence of some of the Itasca isolates of Choricystis was nearly identical to the published sequence for C. minor; however, six different rbcL sequences were detected among the Choricystis isolates and none of these rbcL sequences matched published sequences for isolates from several lakes in Europe. The different rbcL sequence types of Choricystis had distinct seasonal distributions in Lake Itasca. These results extend our knowledge of the distribution of Pseudodictyosphaerium and Choricystis and suggest that Choricystis is even more diverse than previously thought. In addition, the distinct sequences of marker genes for Lake Itasca isolates indicate that these organisms have diverged from the European isolates. The previously undescribed alga isolated from Lake Itasca is a sister taxon to the trebouxiophyte lineage that includes Chlorella vulgaris, but differs from true Chlorella species in that it lacks a pyrenoid. Based upon this difference and molecular analyses, these isolates are described as Meyerella planktonica, gen. et sp. nov. (Trebouxiophyceae, Chlorophyta). All M. planktonica isolates examined possessed identical 18S rDNA, rbcL and ribosomal internal transcribed spacer sequences. Meyerella planktonica was isolated almost exclusively from winter and spring samples.