Symbiosis and Cell Evolution Laboratory
Temperature-dependent photostasis and nitrogen limitation in streamlined-genome red algae Cyanidiophyceae from natural habitats
Dai Tsujino, Takayuki Fujiwara, Shota Yamashita, Kei Tamashiro, Jin Izumi, Fumi Yagisawa, Baifeng Zhou, Shunsuke Hirooka, Yuki Sunada, Kintake Sonoike, Shin-ya Miyagishima
The ISME Journal, Volume 20, Issue 1, January 2026, wrag105 DOI:/10.1093/ismejo/wrag105
Sulfuric acidic hot springs in volcanic regions around the world are inhabited by unicellular red algae, Cyanidiophyceae. These algae are extremophilic eukaryotes that thrive under highly acidic conditions around pH 2 and across a remarkably wide temperature range approaching 20–50°C. In this study, we revealed how these algae maintain photosynthesis without losing their blue-green pigmentation under dramatically different temperature conditions.
In general, photosynthetic organisms experience reduced growth and metabolic activity at low temperatures. In contrast, light energy absorption itself is relatively insensitive to temperature. As a result, cells accumulate excess energy that cannot be fully utilized. This excess energy generates reactive oxygen species (ROS), which damage the photosynthetic machinery and even the cells themselves. To avoid such damage, many plants and algae reduce their photosynthetic pigments under low-temperature conditions, thereby limiting light absorption.
However, Cyanidiophyceae inhabiting Kusatsu Hot Springs behaved differently. Field observations and laboratory culture experiments showed that even at 20–25°C, where cellular growth was nearly halted, the levels of photosynthetic pigments such as chlorophyll and phycocyanin remained largely unchanged. These cells maintained light-harvesting capacities comparable to those of actively growing cells near their optimal temperature of 40°C. In other words, they remained in what appears to be a highly risky state: continuing to absorb light energy despite almost no growth.
Our analyses revealed that these algae dissipate excess energy not primarily through “regulated photoprotection,” but rather through more nonregulated energy dissipation pathways. At the same time, they redirect energy toward ROS detoxification and cellular repair, thereby maintaining a balance between light energy absorption and energy processing to minimize low-temperature damage. In contrast, at higher temperatures, increased growth rates led to limitations in nitrogen availability required for growth, indicating that the cells face distinct challenges depending on temperature. Thus, Cyanidiophyceae maintain photosynthesis across a wide temperature range while coping with different stresses: oxidative stress at low temperatures and nutrient limitation at high temperatures.
This study advances our understanding of the survival strategies of extremophilic eukaryotes and provides insights into how simplified photosynthetic systems adapt to fluctuating environments. These findings may also help us understand how early photosynthetic eukaryotes adapted to changing environments throughout Earth’s history.
This study was supported by JSPS KAKENHI (22H02651, 23H04961, 24H00579, 24KJ0224, and 25KJ1330), the JST SPRING program (JPMJSP2104), and the JST MIRAI Program (JPMJMI22E1).
Figure. Cyanidiophyceae inhabiting Kusatsu Hot Springs under low- and high-temperature conditions.
(A) Cyanidiophyceae mats actively proliferating under high-temperature conditions in Kusatsu Hot Springs (left) and mats whose growth is largely arrested under low-temperature conditions (right). During colder periods, the mats are reduced by water flow; however, the micrograph in the right panel shows that Cyanidiophyceae cells remain alive within rock crevices. (B) Graduate student Tsujino-san conducting a field survey of Cyanidiophyceae habitats. (C) Microscopic images of cells cultured at different temperatures. Cell morphology and pigmentation remain largely unchanged at both low temperature (20°C) and the optimal temperature (40°C).