Gradient of microbial communities around seagrass roots was mediated by sediment grain size

Abstract

Although research on plant rhizosphere is extensive, studies focusing on rhizoplane, rhizosphere, and outer rhizosphere of seagrass remain rare. Here, we present a detailed characterization across root-associated microbiomes of the seagrass Enhalus acoroides at two locations (Xincun Bay and Li'an Harbor in Hainan, South China) with different sediment properties. The metabolic activity and composition of microbial communities within the rhizoplane (R), rhizosphere (SR), and outer rhizosphere (OR) compartments were compared using the Biolog-Eco method and high-throughput sequencing, respectively. These results showed R sediment had the highest metabolic ability in utilizing carbon substrates, followed by SR and OR in both locations, which indicated higher or more diverse carbon resources in sediment in proximity to the roots. With respect to bacterial diversity, our study showed that the OR region supported the highest biodiversity (α indices), followed by SR and R communities, which revealed the strongest selective effect of rhizoplane niche. With respect to community composition, three root-associated communities at Xincun Bay were characterized by a decreasing trend in relative abundance of Sulfurovum and Sulfurimonas (sulfide oxidizers belonging to Episilonproteobacteria) from R to OR, which revealed their better adaption to lower oxygen concentrations and lower available carbon conditions. Vibrio and Photobacterium assigned to Gammaproteobacteria were highly enriched in rhizoplane compared with SR and OR at Li'an Harbor, which may play an important role in nutrient cycling around the seagrass roots. On the other hand, the gradient of bacterial community structure and metabolic capability created by seagrass roots was more obvious at Li'an Harbor than that at Xincun Bay, which may be related to sediment grain size.