Acidophilic green algal genome provides insights into adaptation to an acidic environment

Significance

Extremely acidic environments are scattered worldwide, and their ecosystems are supported by acidophilic microalgae as primary producers. To understand how acidophilic algae evolved from their respective neutrophilic ancestors, we determined the draft genome sequence of the acidophilic green alga Chlamydomonas eustigma and performed comparative genome analyses between C. eustigma and its neutrophilic relative Chlamydomonas reinhardtii. The results suggest that higher expression of heat-shock proteins and H ⁺-ATPase, loss of some metabolic pathways that acidify cytosol, and acquisition of metal-detoxifying genes by horizontal gene transfer have played important roles in the adaptation to acidic environments. These features are also found in other acidophilic green and red algae, suggesting the existence of common mechanisms in the adaptation to acidic environments.

Abstract

Some microalgae are adapted to extremely acidic environments in which toxic metals are present at high levels. However, little is known about how acidophilic algae evolved from their respective neutrophilic ancestors by adapting to particular acidic environments. To gain insights into this issue, we determined the draft genome sequence of the acidophilic green alga Chlamydomonas eustigma and performed comparative genome and transcriptome analyses between C. eustigma and its neutrophilic relative Chlamydomonas reinhardtii. The results revealed the following features in C. eustigma that probably contributed to the adaptation to an acidic environment. Genes encoding heat-shock proteins and plasma membrane H ⁺-ATPase are highly expressed in C. eustigma. This species has also lost fermentation pathways that acidify the cytosol and has acquired an energy shuttle and buffering system and arsenic detoxification genes through horizontal gene transfer. Moreover, the arsenic detoxification genes have been multiplied in the genome. These features have also been found in other acidophilic green and red algae, suggesting the existence of common mechanisms in the adaptation to acidic environments.