Characterization of an UV- and VIS-absorbing, purpurogallin-derived secondary pigment new to algae and highly abundant in Mesotaenium berggrenii (Zygnematophyceae, Chlorophyta), an extremophyte living on glaciers

Abstract

Mesotaenium berggrenii is one of few autotrophs that thrive on bare glacier surfaces in alpine and polar regions. This extremophilic alga produces high amounts of a brownish vacuolar pigment, whose chemical constitution and ecological function is largely unknown until now. Field material was harvested to isolate and characterize this pigment. Its tannin nature was determined by photometric methods, and the structure determination was carried out by means of HPLC-MS and 1D- and 2D-NMR spectroscopy. The main constituent turned out to be purpurogallin carboxylic acid-6-O-β-d-glucopyranoside. This is the first report of such a phenolic compound in this group of algae. Because of its broad absorption capacities of harmful UV and excessive VIS radiation, this secondary metabolite seems to play an important role for the survival of this alga at exposed sites. Attributes and abundances of the purpurogallins found in M. berggrenii strongly suggest that they are of principal ecophysiological relevance like analogous protective pigments of other extremophilic microorganisms. To prove that M. berggrenii is a true psychrophile, photosynthesis measurements at ambient conditions were carried out. Sequencing of the 18S rRNA gene of this alpine species and of its arctic relative, the filamentous Ancylonema nordenskiöldii, underlined their distinct taxonomic position within the Zygnematophyceae.

Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

“…It is very likely that this pigment is responsible for the purple-brown colouring of the cells, and hence parts of the ice sheet surface in Greenland. We did not find this pigment in our high performance liquid chromatography analyses, probably because of our use of a standard extraction method that is not appropriate for this very polar pigment (Remias et al, 2012).…”

“…and sparse representation of cyanobacteria (Figures 2a-d). The algae are all members of the saccoderm desmid family Mesotaeniaceae, (Zygnematophyceae), a relatively specialised group capable of growth on bare glacial ice (Uetake et al, 2010;Remias et al, 2012). Ancylonema and Cylindrocystis were first recorded in 'vast quantities' on the GrIS in the 1870s (Berggren, 1871; Nordenskiö ld, 1872) and more recently by Gerdel and Drouet (1960).…”

“…Visually, green chlorophyll pigments of saccoderm desmids were typically masked by a brown-topurple coloured pigment located in vacuoles surrounding algal chloroplasts (Figures 2c and d). Isolates of a secondary pigment from the ice alga Mesotaenium berggrenii, showing absorption peaks in the UVA, UVB and visible wavelengths (400-700 nm), have recently been identified as purpurogallin carboxylic acid-6-O-b-D, glucopyranoside (Remias et al, 2012) and evidence suggests that this is the same secondary pigment as that found in the ice alga Ancylonema nordenskio¨ldii (Remias et al, 2011). It is very likely that this pigment is responsible for the purple-brown colouring of the cells, and hence parts of the ice sheet surface in Greenland.…”

“…Motility is known for some members of the Zygnematophyceae (Kim et al, 2005), but has not been observed in the algal species recorded on the GrIS (Remias et al, 2012). An alternative hypothesis is that cells utilise self-shading at higher irradiances, and that the inflexion points ( Figure 5 Figure 4 Primary and secondary production at the glacial surface in four nominal categories of ice pigmentation.…”

“…Other algae, for example, Mesotaenium berggerenii impart a less conspicuous grey tinge to snow and ice (Ling and Seppelt, 1990;Remias et al, 2012) and may easily be mistaken, by the naked eye, to be dust. However, the role played by the photosynthetic organisms that are retained on the bare ice surface on the GrIS has largely been ignored since the early pioneering work.…”