| Summary: | The study of heterotrophic protists in the ocean is still in its early stages, compared to other microorganisms such as bacteria, archaea, and photoautotrophic protists. Labyrinthulomycetes protists (LP) are a type of unicellular protists that are widely distributed in global waters and have the potential to produce high-value products. In this study, the abundance, diversity, and community structure of LP in the coastal zone of Hainan Island in the South China Sea were investigated through quantitative PCR and high-throughput sequencing. The results showed that LP abundance varied by location and depth, with the highest levels (37.3 × 10<sup>3</sup> copies/L) found in the middle layer offshore and the lowest (0.386 × 10<sup>3</sup> copies/L) in the bottom layer offshore. The middle layer (chlorophyll maximum layer) had higher LP abundance both inshore and offshore than the surface and bottom layers. Interestingly, the highest LP richness and diversity was found in the inshore bottom. There was a significant difference in LP abundance between the offshore surface and bottom layers. The LP community was dominated by the genus <i>Aplanochytrium</i>, and four different ecotypes were identified. Additionally, the genus <i>Aurantiochytrium</i> had different cooperative and competitive strategies with bacteria in different habitats. This study sheds light on the abundance and community structure of LP in the coastal zone of Hainan Island, explores the potential interactions between LP and bacterial populations, and raises questions about the potential differentiation of LP ecotypes.
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