A comparison of strains of Xanthonema (= Heterothrix, Tribonematales, Xanthophyceae) from Antarctica, Europe, and New Zealand

Seventeen strains of Xanthonema Silva (Tribonematales, Xanthophyceae) are examined by light microscopy and transmission electron microscopy, and their salinity tolerance is investigated. Ten strains are from Antarctica where the genus is poorly known. These strains are compared with two strains from New Zealand and five from Europe, including the only existing authentic strains of three species. Stalk-like chloroplast lobes resembling pyrenoids are described for the first time for Xanthonema. There are two types: one projects from the end of the chloroplast and the other projects from the internal face of the chloroplast into an invagination of the nuclear envelope. The latter type is a new finding for the Xanthophyceae. Xanthonema sessile (Vinatzer) Ettl et Gartner and X. hormidioides (Vischer) Silva are new records for Antarctica. Other antarctic strains resemble seven other species, but difficulties in making confident identifications are discussed. Three of the strains were unable to tolerate salinity greater than 10 ppt, but two strains tolerated salinity as high as 35 ppt. Seven of the antarctic strains closely resemble species known from Europe. The remaining three antarctic strains have features previously undescribed in Xanthonema.