The Ecophysiological Performance and Traits of Genera within the Stichococcus-like Clade (Trebouxiophyceae) under Matric and Osmotic Stress

Abstract

Changes in water balance are some of the most critical challenges that aeroterrestrial algae face. They have a wide variety of mechanisms to protect against osmotic stress, including, but not limited to, downregulating photosynthesis, the production of compatible solutes, spore and akinete formation, biofilms, as well as triggering structural cellular changes. In comparison, algae living in saline environments must cope with ionic stress, which has similar effects on the physiology as desiccation in addition to sodium and chloride ion toxicity. These environmental challenges define ecological niches for both specialist and generalist algae. One alga known to be aeroterrestrial and euryhaline is Stichococcus bacillaris Nägeli, possessing the ability to withstand both matric and osmotic stresses, which may contribute to wide distribution worldwide. Following taxonomic revision of Stichococcus into seven lineages, we here examined their physiological responses to osmotic and matric stress through a salt growth challenge and desiccation experiment. The results demonstrate that innate compatible solute production capacity under salt stress and desiccation tolerance are independent of one another, and that salt tolerance is more variable than desiccation tolerance in the Stichococcus-like genera. Furthermore, algae within this group likely occupy similar ecological niches, with the exception of Pseudostichococcus.

Metabolite Profiling in Green Microalgae with Varying Degrees of Desiccation Tolerance

“…Proline, together with sorbitol, is among the key compatible solutes associated with protection from damage induced by various stress factors interfering with osmoregulation, such as dehydration [ 56 , 57 ]. The higher sorbitol and proline contents in D. epiphytica were confirmed using an HPLC assay and a spectrophotometric assay, respectively ( Table S2 ), in agreement with previous work on the same strain (SAG 11.88) and D. chodatii , showing that sorbitol is present at high concentrations [ 16 , 17 , 18 ]. Several sugars and sugar alcohols, including sorbitol, have been suggested to play a role in desiccation tolerance [ 30 , 32 ], but they also play a role in the lichen symbiosis [ 31 , 32 , 58 ].…”

“…Trebouxiophyceae are widespread globally, and occur in many aquatic and terrestrial ecosystems [ 9 , 10 , 11 ] with different lifestyles [ 2 , 12 ] either free-living or as lichen photobionts [ 13 , 14 , 15 ]. Ecophysiological traits of D. epiphtica and D. chodatii were recently reported [ 16 , 17 , 18 ]. In D. epiphtica , sorbitol and sucrose contents were used as chemotaxonomic markers [ 16 ], and the potential of Diplopshaera spp.…”

Pseudostichococcus Stands Out from Its Siblings Due to High Salinity and Desiccation Tolerance

“…The discovery of Stichococcus and Pseudostichococcus in a highly saline habitat is unusual, because Stichococcus has hitherto been considered euryhaline [36][37][38]. As with the other Stichococcus-like organisms, Pseudostichococcus are present in a variety of habitats, such as such as tree bark, surfaces of buildings and outdoor structures, diverse soils and are also components of biocrusts.…”