Salinity gradient differentiates potential novel ecotypes and diversity of Labyrinthulomycetes protists along the Haihe River, northern China

ABSTRACT: With a ubiquitous presence in marine ecosystems, Labyrinthulomycetes protists (LP) play critical ecological roles in oceanic habitats. Recently, some LP strains have been suggested to survive in low-salinity environments, but their distribution in freshwaters was largely unknown. This study investigated LP abundance and diversity dynamics along a fresh-saltwater gradient in 2 seasons. LP were detected in all samples. Although LP abundance in freshwaters (typically 10⁴ to 10⁵ copies l^-1) was significantly lower than that in saline waters, their abundance still corresponded to that of previously reported LP in some coastal waters, suggesting their potentially essential roles in riverine ecosystems. High-throughput sequencing analyses identified 110, 54, and 146 LP amplicon sequence variants (ASVs) in fresh, brackish, and saline waters, respectively. Canonical correspondence analysis and variance partitioning analysis further indicated that salinity and temperature were the most significant environmental factors to affect LP community structure. Notably, most of the dominant ASVs in fresh/brackish waters were annotated to a rarely reported Labyrinthulida family, Amphitraemidae, and a newly identified river cluster of the order Thraustochytrida, which were significantly different from those of saline waters. Finally, the metabolic capabilities of the detected LP genera suggest that LP likely play diverse ecological roles in riverine ecosystems.