Ecological Distribution and Environmental Correlates of Far-red Light-utilizing Cyanobacteria in Taiwan’s Hot Springs

Far-red light-utilizing cyanobacteria (FRLCyano) extend oxygenic photosynthesis beyond the visible spectrum, yet their detection in natural environments remains challenging. Traditional pigment-based and culture-dependent methods are time-consuming and limited by factors such as inducible phenotypes, often underestimating FRLCyano abundance and distribution. Here, we applied the Far-Red Cyanobacteria Identification (FRCI), a customized 16S rRNA gene database and classification framework, to investigate FRLCyano communities in hot spring ecosystems. Using a comprehensive dataset from Taiwanese hot springs, we further validated this 16S-based approach against two functional lines of evidence: consistency with the FRL-specific marker gene apcE2 , which introduces a validation marker not included in the initial FRCI framework, and a significant positive correlation with chlorophyll f concentrations. The FRCI approach identified 11 distinct FRLCyano amplicon sequence variants, primarily affiliated with Leptolyngbya and Calothrix , and revealed that their relative abundance is driven by higher pH and lower temperatures. Extending this framework to global hot spring datasets revealed distinct biogeographical signatures, notably the rarity of Fischerella in Taiwan compared to other regions. Furthermore, the isolation of a novel FRL strain, Chroococcidiopsis sp. GD1, highlights the essential, complementary role of cultivation in comprehensive surveys. This study provides a robust, multi-method validated framework for assessing the distribution and ecological correlates of FRLCyano.