Phylogeny, morphology, and physiology of Micractinium strains isolated from shallow ephemeral freshwater in Antarctica

Abstract

SUMMARYCryotolerant eukaryotic microalgae were isolated from meltwater streams on Ardley Island and King George Island in Antarctica, and their morphological, molecular, and physiological characteristics were investigated. Owing to their simple morphology, distinctive characters were not observed with neither light microscopy nor transmission electron microscopy. However, molecular phylogenetic inferences drawn from the concatenated small subunit rRNA and internal transcribed spacer sequence data indicated that these microalgal strains belonged to the genus Micractinium. All the Micractinium strains showed cryotolerant properties, while their optimum growth temperature was around 20°C. Similar to other cryotolerant organisms, these Antarctic microalgae also contained a higher ratio of polyunsaturated fatty acids to saturated fatty acids. In this study, new Antarctic Micractinium spp. were discovered and added to the culture collection. These cryotolerant strains may serve as a promising source of nutritionally important linoleic (C18:2 ω6) and α-linolenic (C18:3 ω3) acids.

Morphology and phylogenetic relationships of <italic>Micractinium</italic> (Chlorellaceae, Trebouxiophyceae) taxa, including three new species from Antarctica

“…KSF0085 belonged to the same clade as Micractinium sp. (KNUA034), which is an Antarctic strain without bristles (Hong et al 2015). KSF0085 was sister to KSF0094 with strong to moderate support in the analyses.…”

“…The strains (KSF0100, KSF0112, KSF0114, and KSF0127) formed first clade were collected from four sites of the South Shetland Islands and were identical to Micractinium sp. (KNUA029) (Hong et al 2015). KSF0085 belonged to the same clade as Micractinium sp.…”

“…There are members of the genus Micractinium that are distributed worldwide, but no species has been reported yet in Antarctica (Guiry and Guiry 2019). The Antarctic strains show tolerance to lower temperature and produced polyunsaturated fatty acids in cold condition (Hong et al 2015). It suggests an evolutionary evidence of their adaptation to the Antarctic environment.…”

“…However, culturing Micractinium strain with zooplankton does not always produce bristles and colonies (Pröschold et al 2011). M. conductrix, M. inermum, and other Antarctic strains (KNUA029 and KNUA034) of the genus are non-bristle and similar to Chlorella morphologically (Pröschold et al 2011, Hoshina and Fujiwara 2013, Hong et al 2015. Species delimitation in Micractinium has always been problematic because there are few diagnostic morphological characters and they are not easily observed.…”

“…2). Hong et al (2015) reported that KNUA029 and KNUA034 were collected from two sites in West Antarctica. Morphological and molecular characteristics of KNUA029 are identical with four Antarctic strains (KSF0100, KSF0112, KSF0114, and KSF0127) and those of KNUA034 are same with strain KSF0085.…”