The unexpected extremophile: Tolerance to fluctuating salinity in the green alga Picochlorum

Abstract

The broadly halotolerant green alga, Picochlorum strain SENEW3, has a highly reduced nuclear genome of 13.5 Mbp that encodes only 7,367 genes. It was originally isolated from a shallow, mesophilic brackish-water lagoon that experiences extreme changes in temperature, light, and in particular, salinity (freshwater to 3-fold seawater). We challenged Picochlorum cells with high or low salinity shock and used transcriptomic and chlorophyll fluorescence analyses to elucidate tolerance to salinity fluctuation. The transcriptome analysis showed that one-half of the coding regions are differentially expressed in response to salinity changes. In addition, a significant number of co-expressed genes (usually from different metabolic pathways) are co-localized in the genome, forming 2-10 gene clusters. Whereas the overall salt stress response in Picochlorum SENEW3 is similar to that in other salt-tolerant algae, the "operon-like" structure in this species likely contributes to rapid recovery during salinity fluctuation. In summary, our work elucidates how evolutionary forces play out in a streamlined genome. Picochlorum SENEW3 relies on a broad array of adaptations from the reliance on horizontally transferred adaptive genes to the colocalization of stress response genes and a robust photosystem II to deal with a fluctuating environment. These attributes make Picochlorum SENEW3 of great biotechnological interest.

Microalgae community shifts during the biogas upgrading in an alkaline open photobioreactor

“…; Foflonker et al . ). Undesirable protozoa were also identified during the morphological screening approach.…”

“…emerge as prominent genera identified with the 16S rRNA and 18S rRNA primers respectively. These prominent genera can be found in marine environments as well as in freshwater (Henley et al 2004;Foflonker et al 2016). Undesirable protozoa were also identified during the morphological screening approach.…”

Spontaneous mutation rate as a source of diversity for improving desirable traits in cultured microalgae

“…It is therefore important that more estimates of spontaneous mutation rates are obtained in microalga of biotechnological interest. Algae with naturally high mutation rates, such as the strain RRC4223, for which several potential biotechnological applications have been documented [43][44][45][46][47][48][49][50][51], may constitute the best candidates for domestication.…”

“…Picochlorum RCC4223 has a small haploid genome of~13.5 Mb with 79.5% of coding sequences and 46% GC content [44]. Strains from the genus Picochlorum are versatile algae for large-scale culturing, capable of growing in a wide range of salinities and temperatures [45,46] with a few completely sequenced genomes already available, including the strain RCC4223 [46,47]. They also constitute interesting models in different fields, such as medicine [48], biofuels [49], food and aquaculture [50,51] and production of high value molecules [52].…”

Using complementary approaches to identify trans‐domain nuclear gene transfers in the extremophile Galdieria sulphuraria (Rhodophyta)

“…, Foflonker et al. , ). Significant efforts have been expended in assessing the extent of HGT on algal evolution and adaptation (e.g., Huang and Gogarten , Chan et al.…”