No 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.

Ellipsoidal Chlorella-like species are very common in all kinds of aquatic and terrestrial habitats, and often identified as Chlorella saccharophila or C. ellipsoidea. However, the taxonomic status of these species remains unclear, because they are not related to the type species of the genus, Chlorella vulgaris. In this study, 23 strains isolated from different habitats, were investigated using a polyphasic approach, i.e. morphology and reproduction, ecophysiology, and combined SSU and ITS rDNA sequences. Phylogenetic analyses clearly demonstrated that these isolates formed a monophyletic lineage within the green algal class Trebouxiophyceae. All strains were characterized by ellipsoidal cell shape, unequal autospores during reproduction, and parietal chloroplasts, as well as by the biochemical capability to synthesize and accumulate the rather unusual polyol, ribitol. Although ribitol is a typical stress metabolite involved in osmotic acclimation, it can also be used as a chemotaxonomic marker. Comparative growth measurements under different temperature regimes indicated similar optimum growth temperatures and maximum growth rates in all studied Chlorella-like species. However, these were different from those of C. vulgaris. We therefore propose to transfer all Chlorella-like strains related to Chlorella saccharophila and C. ellipsoidea to the genus Chloroidium Nadson and to emend its diagnosis. We propose four new combinations: Chloroidium saccharophilum comb. nov., Chloroidium ellipsoideum comb. nov., Chloroidium angusto-ellipsoideum comb. nov. and Chloroidium engadinensis comb. nov. In contrast, Chlorella ellipsoidea sensu Puncˇocha´rova´, which has other morphological and ecophysiological characters, should be assigned to the genus Pseudochlorella (P. pringsheimii comb. nov.).

In this study the distribution pattern of UV-absorbing mycosporine-like amino acids (MAAs) was identified and quantified in Antarctic macroalgae and correlated with habitat as well as with the radiation climate in air and under water. In addition, specific amounts of MAAs from selected species collected at different depths, from different parts of the thallus and developmental stages were investigated. Seven different MAAs were detected in 17 out of 28 red algal species, whereas in all brown and 2 green algal species only traces of MAAs were found. In the green alga Prasiola crispa ssp. antarctica a high concentration of an unknown UV-absorbing substance with an absorption maximum at 324 nm was detected. MAA content was negatively correlated with water depth. Higher concentrations of UV-absorbing substances were found in the marginal tissues of thalli than in the basal parts. Tetrasporophytes and gametophytes exhibited similar MAA values. After transplantation from deep to shallow water, the MAA content remained unchanged for 8 d after transplantation. The data presented indicate 3 physiologically different groups of algae in terms of MAA values: (1) species with no capability for MAA biosynthesis; (2) species with a basic MAA concentration which is adjusted relative to changes in environmental radiation; (3) species with a constant relatively high MAA composition and concentration irrespective of environmental conditions. KEY WORDS: Antarctica · Macroalgae · Mycosporine-like amino acids · UV-absorbing compounds · UV radiation Resale or republication not permitted without written consent of the publisher

No abstract