Genome of the halotolerant green alga Picochlorum sp. reveals strategies for thriving under fluctuating environmental conditions

Abstract

An expected outcome of climate change is intensification of the global water cycle, which magnifies surface water fluxes, and consequently alters salinity patterns. It is therefore important to understand the adaptations and limits of microalgae to survive changing salinities. To this end, we sequenced the 13.5 Mbp genome of the halotolerant green alga Picochlorum SENEW3 (SE3) that was isolated from a brackish water pond subject to large seasonal salinity fluctuations. Picochlorum SE3 encodes 7367 genes, making it one of the smallest and most gene dense eukaryotic genomes known. Comparison with the pico-prasinophyte Ostreococcus tauri, a species with a limited range of salt tolerance, reveals the enrichment of transporters putatively involved in the salt stress response in Picochlorum SE3. Analysis of cultures and the protein complement highlight the metabolic flexibility of Picochlorum SE3 that encodes genes involved in urea metabolism, acetate assimilation and fermentation, acetoin production and glucose uptake, many of which form functional gene clusters. Twenty-four cases of horizontal gene transfer from bacterial sources were found in Picochlorum SE3 with these genes involved in stress adaptation including osmolyte production and growth promotion. Our results identify Picochlorum SE3 as a model for understanding microalgal adaptation to stressful, fluctuating environments.

Genome-wide analyses of chitin synthases identify horizontal gene transfers towards bacteria and allow a robust and unifying classification into fungi

“…If the division 1 chs of the green algae (Trebouxiophyceae ) Chlorella [49] (Fig. 2) and Picochlorum [50] were vertically inherited from the Plantae ancestor, it would imply that these genes were independently lost in many lineages of the plantae. As mentioned for the CV and ESV classes of division 2 (Update of CHS fungi classification), chs gene exchanges might have occurred between algae-related viruses, including chloroviruses and fungi.…”

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

“…Also, the genome of Picochlorum SE3 has been sequenced revealing great adaptation advantages to stressful environments (Foflonker et al . ). This provides evidence of the versatility and adaptability of members of this genus to demanding and fluctuating environments in agreement with their prominent presence in the open photobioreactor (Fig.…”

The effect of the algal microbiome on industrial production of microalgae

“…SENEW3 has a wide salt tolerance from at least 0.35% to 10.8% . Compared to its less halotolerant sisters, the genome of the salt-tolerant strain was found to contain a suite of additional functional genes, 24 of which were derived from bacterial sources and were functional in response to salt stress (Foflonker et al, 2015). Although not a microalga, it is interesting to note that the transition of the brown macroalga Ectocarpus sp.…”