Diverse Patescibacteria assemblages and prevalence of ultra-small free-living Parcubacteria along a subterranean estuary.

Patescibacteria are a group of novel, mostly uncultivated bacteria characterized by ultra-small cell sizes and streamlined genomes. They are ubiquitous in diverse ecosystems, often prevailing in subsurface environments, yet basic aspects such as variability in cell size, abundance, and niche preferences of different taxa within Patescibacteria remain unknown, particularly along salinity gradients. Combining flow cytometry, IMPORTANCE: Patescibacteria are an enigmatic group of bacteria of ultra-small sizes and reduced genomes, commonly found in subsurface environments but largely unexplored in terms of their ecological roles. Despite being present in both freshwater and marine systems, no study has explored how they distribute along salinity gradients. This study provides new insights into their distribution, diversity, and niche partitioning along a Mediterranean subterranean estuary characterized by a strong salinity gradient. We show that Patescibacteria taxa seem to adapt to varying groundwater salinity conditions, displaying a remarkable capacity to occupy fresh, brackish, and saline niches through changes in composition. The identification of ultra-small coccoid cells and symbiotic-like associations highlights a diversity of lifestyles within these groups and provides one of the scarce visual proofs of Patescibacteria. With most detected taxa being highly novel, these findings point to an overlooked importance of Patescibacteria in coastal aquifers, biogeochemically active sites ubiquitous along most coastlines.