Miyagishima Group / Symbiosis and Cell Evolution Laboratory

Heterotrophic unicellular eukaryotes feeding on the unicellular red alga Cyanidiococcus sp. in moderately hot geothermal sulfuric springs

Yuki Sunada, Dai Tsujino, Shota Yamashita, Wei-Hsun Hsieh, Kei Tamashiro, Jin Izumi, Fumi Yagisawa, Baifeng Zhou, Shunsuke Hirooka, Takayuki Fujiwara, Shin-ya Miyagishima

FEMS Microbiology Ecology (2025) 101, Article fiaf048, DOI:10.1093/femsec/fiaf048

Sulfuric acidic hot springs (<pH 4.0, >37°C) are found in volcanic regions worldwide, where various bacteria, archaea, and the unicellular red algae Cyanidiophyceae dominate. Regarding heterotrophic eukaryotes, the only known species was the thermophilic amoeboflagellate Tetramitus thermacidophilus (class Eutetramitea, phylum Heterolobosea), which feeds on surrounding bacteria and archaea. In this study, we investigated three sulfuric hot springs (34.7–50°C, ~pH 2.0) in Japan to determine whether other heterotrophic eukaryotes inhabit these environments. As a result, we isolated and identified cultures of four species capable of surviving at pH 2.0 and 40°C: Allovahlkampfia sp. (Eutetramitea, Heterolobosea); Nuclearia sp. and Parvularia sp (Nucleariidea, Cristidiscoidea); and Vannella sp. (Discosea, Amoebozoa). Phylogenetic analyses suggest that these four species independently evolved from mesophilic and neutrophilic ancestors, separate from each other. Additionally, Platyophrya sp. (Colpodea, Ciliophora) and two species of Neobodo (Euglenozoa, Kinetoplastea) were also found in the same environment, while their maximum survival temperatures were 35°C and 30°C, respectively. Among these, all species except Neobodo were confirmed to grow exclusively by feeding on Cyanidiococcus sp., a dominant species of Cyanidiophyceae in the environment. Thus, various lineages of heterotrophic unicellular eukaryotes have independently developed acidophilic and thermotolerant traits, allowing them to colonize sulfuric hot springs.

Figure: Unicellular eukaryotes isolated and cultured from a sulfuric acidic hot spring
(A) The environment of a sulfuric acidic hot spring; (B) Mr. Sunada collecting samples on site; (C) Microscopic images of unicellular eukaryotes discovered and successfully cultured in this study (the green structures inside the cells are microalgae Cyanidiophyceae being ingested and digested intracellularly).

A single episode of sexual reproduction can produce large variation in population growth rates under dual stressors

Yawako W. Kawaguchi, Masato Yamamichi

Journal of Evolutionary Biology (2025) DOI:10.1093/jeb/voaf041

Press release (In Japanese only)

Sexual reproduction has been a central topic in evolutionary biology because of its many costs: why have organisms evolved sexual reproduction despite the many costs of sex? To answer the question, researchers have conducted laboratory experiments to measure population growth rates with and without sexual reproduction under a stressor. Here we show that a single episode of sexual reproduction can produce a large amount of variation in population growth rates under dual stressors by laboratory experiments of a green alga, Closterium peracerosum–strigosum–littorale complex. We observed population dynamics of the alga under dual stressors and confirmed that high salinity and low pH decreased growth rates. By comparing parental and their hybrid F₁ populations, we observed larger variation in growth rates of F₁ populations (i.e., transgressive segregation) when pH was low. Interestingly, even when parental populations had negative growth rates, some F₁ populations showed positive growth rates in severe environmental conditions due to the large variation in population growth. By utilizing the recently obtained genomic information of the alga, we conducted a gene ontology (GO) enrichment analysis and found that genes with copy number variations between parental strains were more frequently associated with pH stress-related terms than salt stress-related terms. Our results suggest that recombination and variation in the number of gene copies might produce large genetic variation in the F₁ generation. This will be an important step toward a better understanding of evolution of sex and evolutionary rescue where rapid contemporary evolution prevents population extinction in changing environments.