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Phylogenetic analysis of the '<i>Nannochloris</i>-like' algae and diagnoses of <i>Picochlorum oklahomensis</i> gen. et sp nov (Trebouxiophyceae, Chlorophyta)

TitlePhylogenetic analysis of the 'Nannochloris-like' algae and diagnoses of Picochlorum oklahomensis gen. et sp nov (Trebouxiophyceae, Chlorophyta)
Publication TypeJournal Article
Year of Publication2004
AuthorsHenley WJ, Hironaka JL, Guillou L, Buchheim MA, Buchheim JA, Fawley MW, Fawley KP
Keywords18s-, 2004, Aquatic-sciences, cell-wall, chlorella-, green-alga, life-cycle, marvania-geminata, morphology-, Nanochlorum-eucaryotum, position-, rcc, sbr_phyto, SBR_Phyto_DPO, SEQUENCES

A broadly halotolerant new isolate of a small asexual coccoid chlorophyte and six new, related freshwater isolates provided the impetus for a phylogenetic analysis of the so-called 'Nannochloris-like' algae within the Trebouxiophyceae. Previous taxonomic disagreements concerning this group had not been rigorously tested with molecular phylogenetic analyses. We show with 18S ribosomal DNA (rDNA) sequence phylogeny that 19 of 22 isolates previously assigned to either Nannochloris or Nanochlorum fall within a diverse sister clade to a clade including the four 'true' Chlorella species sensu loto. In addition, Marvania geminata, Gloeotila contorta, Chlorella sp. Yanaqocha RA1, Koliella spiculiformis, 'Chlorella minutissima' C-1.1.9. and new Koliella, Gloeotila and Marvania isolates were included in the Nannochloris-like clade. Distinct freshwater and marine or saline lineages comprise at least three major subclades, generally corresponding to cell division pattern. Seven of 14 marine or saline isolates are known (and the others presumed) to divide by autosporulation. Eight freshwater isolates divide by binary fission, including two Koliella, two Gloeotila, N. bacillaris, Chlorella sp. Yanaqocha RA1, and two new unassigned isolates. Four freshwater isolates divide by budding or autosporulation (three Marvania, including CCAP 251/1b. previously assigned to N. coccoides). The autosporic taxa N. eucaryotum UTEX 2502 (marine) and C. minutissima C-1.1.9 (freshwater), which have nearly identical 18S rDNA sequences, are deeper-branching than the freshwater and marine or saline lineages. We propose including the 13 marine or saline, autosporic taxa (excluding N. eucaryotum UTEX 2502) in the new genus Picochlorum until distinctive morphological or biochemical characters are identified that would indicate multiple genera corresponding to subclades. Such characters exist in the freshwater lineages, supporting retention of Koliella, Gloeotila, Marvania and Nannochloris as distinct genera, although each is currently represented by few isolates. Nannochloris at this time may be restricted to N. bacillaris and Chlorella sp. Yanaqocha RA1. We also describe halotolerant P. oklahomensis Hironaka sp. nov. Based on 18S rDNA sequence and lack of chlorophyll b, Nannochloris sp. UTEX 2379 should be reassigned to the Eustigmatophyceae.