Desiccation stress and tolerance in green algae: consequences for ultrastructure, physiological and molecular mechanisms

Abstract

Although most green algae typically occur in aquatic ecosystems, many species also live partly or permanently under aeroterrestrial conditions, where the cells are exposed to the atmosphere and hence regularly experience dehydration. The ability of algal cells to survive in an air-dried state is termed desiccation tolerance. The mechanisms involved in desiccation tolerance of green algae are still poorly understood, and hence the aim of this review is to summarize recent findings on the effects of desiccation and osmotic water loss. Starting from structural changes, physiological, and biochemical consequences of desiccation will be addressed in different green-algal lineages. The available data clearly indicate a range of strategies, which are rather different in streptophycean and non-streptophycean green algae. While members of the Trebouxiophyceae exhibit effective water loss-prevention mechanisms based on the biosynthesis and accumulation of particular organic osmolytes such as polyols, these compounds are so far not reported in representatives of the Streptophyta. In members of the Streptophyta such as Klebsormidium, the most striking observation is the appearance of cross-walls in desiccated samples, which are strongly undulating, suggesting a high degree of mechanical flexibility. This aids in maintaining structural integrity in the dried state and allows the cell to maintain turgor pressure for a prolonged period of time during the dehydration process. Physiological strategies in aeroterrestrial green algae generally include a rapid reduction of photosynthesis during desiccation, but also a rather quick recovery after rewetting, whereas aquatic species are sensitive to drying. The underlying mechanisms such as the affected molecular components of the photosynthetic machinery are poorly understood in green algae. Therefore, modern approaches based on transcriptomics, proteomics, and/or metabolomics are urgently needed to better understand the molecular mechanisms involved in desiccation-stress physiology of these organisms. The very limited existing information is described in the present review.

Ecophysiological and ultrastructural effects of dust pollution in lichens exposed around a cement plant (SW Slovakia)

“…A higher number of plastoglobules was also observed in plants exposed to several types of stress, e.g. heavy metals for macrophytes (Basile et al 2012a, b) and the lichen X. parietina (Paoli et al 2013(Paoli et al , 2014a, drought for mosses and green algae (Fernández-Marín et al 2013;Holzinger and Karsten 2013) and light stress for algae (Holzinger et al 2004;Holzinger and Lütz 2006). Therefore, the increase in plastoglobules cannot be regarded as a plant-or stress-type specific response.…”

Photosynthetic pigment changes and adaptations in biofilms in response to flow intermittency

“…An extensive classical literature about tolerance and recovery to desiccation already exists (Stevenson et al 1996;Holzinger and Karsten 2013). There is good knowledge of the resistant structures such as zygotes, akinetes, or cysts that algae produce under unfavourable conditions and that allow cells to be viable upon rewetting even after months, years and decades of desiccation (Davis 1972).…”

“…In a recent review, Holzinger and Karsten (2013) found that some of these physiological strategies in green algae already adapted to desiccation favour a quick recovery when rewet, whereas aquatic species were more sensitive to drying. It has been shown that cyanobacterial mats in Antarctic environments recover photosynthetic activity, similar to rates before desiccation, in few minutes (Vicent and Howard-Williams 1986;Hawes et al 1992;McKnight et al 2007).…”

Bioaerosol emissions from open microalgal processes and their potential environmental impacts: what can be learned from natural and anthropogenic aquatic environments?

“…However, the resistance to severe dehydration stress, known as desiccation tolerance, has been well documented for cyanobacteria and lichens. Several studies also highlight phylogenetic lineages of green-algae that are able to develop efficient survival strategies against desiccation [5,43].…”