Nitrogen Limitation and Slow Drying Induce Desiccation Tolerance in Conjugating Green Algae (Zygnematophyceae, Streptophyta) from Polar Habitats

Abstract

BackgroundFilamentous Zygnematophyceae are typical components of algal mats in the polar hydro-terrestrial environment. Under field conditions, they form senescent vegetative cells, designated as pre-akinetes, which are tolerant to desiccation and osmotic stress.Key FindingsPre-akinete formation and desiccation tolerance was investigated experimentally under monitored laboratory conditions in four strains of Arctic and Antarctic isolates with vegetative Zygnema sp. morphology. Phylogenetic analyses of rbcL sequences revealed one Arctic strain as genus Zygnemopsis, phylogenetically distant from the closely related Zygnema strains. Algae were cultivated in liquid or on solidified medium (9 weeks), supplemented with or lacking nitrogen. Nitrogen-free cultures (liquid as well as solidified) consisted of well-developed pre-akinetes after this period. Desiccation experiments were performed at three different drying rates (rapid: 10% relative humidity, slow: 86% rh and very slow); viability, effective quantum yield of PS II, visual and ultrastructural changes were monitored. Recovery and viability of pre-akinetes were clearly dependent on the drying rate: slower desiccation led to higher levels of survival. Pre-akinetes survived rapid drying after acclimation by very slow desiccation.ConclusionsThe formation of pre-akinetes in polar Zygnema spp. and Zygnemopsis sp. is induced by nitrogen limitation. Pre-akinetes, modified vegetative cells, rather than specialized stages of the life cycle, can be hardened by mild desiccation stress to survive rapid drying. Naturally hardened pre-akinetes play a key role in stress tolerance and dispersal under the extreme conditions of polar regions, where sexual reproduction and production of dormant stages is largely suppressed.

Molecular and morphological diversity of Zygnema and Zygnemopsis (Zygnematophyceae, Streptophyta) from Svalbard (High Arctic)

“…Kim et al ., 2008, 2011; Holzinger et al ., 2009). In this study, we confirmed the occurrence of Zygnemopsis (Pichrtová et al ., 2014 b ), and further determined the species as Zygnemopsis lamellata by morphological characterization of a fertile sample observed in the field.…”

“…2013; Pichrtová et al . 2013, 2014 b ). In another filamentous conjugating green algal genus, Spirogyra , molecular diversity analysis of sterile strains from Germany and Austria revealed 53 different SSU rDNA genotypes within 130 isolated strains, and the genus was split into eight independent lineages (Chen et al ., 2012).…”

“…Capital letters were assigned to individual genotypes based on our recent publications on polar Zygnema spp. (Kaplan et al ., 2013; Pichrtová et al ., 2013, 2014 b ). …”

“…These polar hydro-terrestrial algal mats are often formed by algae of the genus Zygnema C. Agardh that are very well adapted to the harsh conditions of the polar climate, as demonstrated by various ecophysiological studies (Hawes, 1990; Holzinger et al ., 2009; Pichrtová et al ., 2013, 2014 a , 2014 b ; Vilumbrales et al ., 2013; Choi et al ., 2015). Nevertheless, both floristic and ecophysiological publications refer only to ‘ Zygnema spp.’ because species diversity within these mats remains unexplored.…”

“…Molecular methods have also been used in two recent experimental studies of polar Zygnema strains. The results showed sufficient variability in rbc L gene sequences among strains isolated from Svalbard and Antarctica (Pichrtová et al ., 2013, 2014 b ). …”