Abstract
The capacity of halophiles to thrive in extreme hypersaline habitats derives partly from the tight regulation of ion homeostasis, the salt-dependent adjustment of plasma membrane fluidity, and the increased capability to manage oxidative stress. Halophilic bacteria, and archaea have been intensively studied, and substantial research has been conducted on halophilic fungi, and the green alga Dunaliella. By contrast, there have been very few investigations of halophiles that are phagotrophic protists, i.e., protozoa. To gather fundamental knowledge about salt adaptation in these organisms, we studied the transcriptome-level response of Halocafeteria seosinensis (Stramenopiles) grown under contrasting salinities. We provided further evolutionary context to our analysis by identifying genes that underwent recent duplications. Genes that were highly responsive to salinity variations were involved in stress response (e.g., chaperones), ion homeostasis (e.g., Na+/H+ transporter), metabolism and transport of lipids (e.g., sterol biosynthetic genes), carbohydrate metabolism (e.g., glycosidases), and signal transduction pathways (e.g., transcription factors). A significantly high proportion (43%) of duplicated genes were also differentially expressed, accentuating the importance of gene expansion in adaptation by H. seosinensis to high salt environments. Furthermore, we found two genes that were lateral acquisitions from bacteria, and were also highly up-regulated and highly expressed at high salt, suggesting that this evolutionary mechanism could also have facilitated adaptation to high salt. We propose that a transition toward high-salt adaptation in the ancestors of H. seosinensis required the acquisition of new genes via duplication, and some lateral gene transfers (LGTs), as well as the alteration of transcriptional programs, leading to increased stress resistance, proper establishment of ion gradients, and modification of cell structure properties like membrane fluidity.
Highlights
Hypersaline environments are habitats for a variety of halophilic microorganisms that are adapted to the often-extreme conditions prevailing in these settings
We identify gene duplications and probable lateral gene transfer (LGT) events that potentially contributed to the halophilicity of H. seosinensis, to previous studies on halophilic yeast and the polyextremophile alga Galdieria sulphuraria (Lenassi et al, 2013; Schönknecht et al, 2013; Zajc et al, 2013)
There was good agreement between the analyses: 2,418 open-reading frames (ORFs) were identified as differentially expressed by all three analyses, and the great majority of the ORFs that were flagged as differentially expressed by EBSeq were identified by limma and DESeq2 (87 and 90%, respectively)
Summary
Hypersaline environments are habitats for a variety of halophilic microorganisms that are adapted to the often-extreme conditions prevailing in these settings. True halophilic microbes require the presence of salt to grow optimally and several cannot divide at salt concentrations under ∼9%, which is around three times the salinity of seawater (Gochnauer et al, 1975; Oren, 2002a; Park et al, 2006, 2007, 2009; Cho et al, 2008; Kuncicet al., 2010; Park and Simpson, 2011; Foissner et al, 2014) Challenges faced by these organisms include ionic stress (especially the toxicity of sodium and chloride ions), osmotic stress, dehydration/desiccation stress (induced by complete evaporation), and reduced solubility of metabolites including nutrients and oxygen. Adaptation to varying salinities involves adjustment of membrane lipid composition (Russell, 1989); H. werneckii maintains a fluid membrane over a wide range of salinities by keeping a low sterol-tophospholipid ratio and by decreasing both fatty acyl length and the saturation level of phospholipids (Turk et al, 2004, 2007)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
