Microbial Distributions Across Wide-Ranging Temperature Gradients of Hot Springs in Thailand: A Review of 35 Years of Research on Hot Spring-Associated Microorganisms.

Although Thailand harbours > 100 hot springs with distinct physicochemistry for diverse microbial communities, an extensive biogeography of hot spring microbial communities has not previously been explored. Here we summarised the progression of previous research and their findings on microbial distributions using data gathered from 62 hot springs (temperature = 32°C-90°C, pH = 6.2-10.0), retrieved from 51 documents (from 1990 to 2025), spanning across 21 provinces along the northern, central, eastern, western and southern regions of Thailand. Cyanobacteriota communities were the most characterised, with Synechococcus and Phormidium as frequently reported genera among studies. Several cyanobacterial members, for example, Pseudanabaena, Synechococcus, Chroococcus and Chroococcidiopsis were reported at remarkably wide temperature ranges. For non-photosynthetic bacteria, Pseudomonadota and Bacteroidota were the majority of the communities, while members from Bacillota were the most isolated. Pseudomonadota also harbour the widest distribution range, spanning ~70°C across sites. However, temperature measurements from some studies might not accurately reflect in situ conditions. Additionally, archaea were rarely noted in the community likely due to methodological limitations. Microbial eukaryotes, for example, green algae and diatoms, were occasionally isolated and classified but their communities remain largely uncharacterised. This review provides the first integrated overview and reference database of hot spring-associated microorganisms in Thailand.