Microbial functional assemblages predicted by the FAPROTAX analysis are impacted by physicochemical properties, but C, N and S cycling genes are not in mangrove soil in the Beibu Gulf, China

Mangroves and their global significance in climate change based on the storage of blue carbon and release of CO2, CH4, and N2O have attracted great attention. Microbial communities and their functions are essential for the biogeochemical cycle of carbon (C), nitrogen (N), and sulfur (S). Therefore, the assessment of microbial functions in mangrove habitats, which suffer from different external environments, is necessary to evaluate their unique ecological service functions. In the present work, the functional profiles of microbial communities in four mangrove reserves (one sandy and three muddy, in Beibu Gulf, China) were detected by 16 s RNA gene amplicon-based Functional Annotation of Procaryotic Taxa (FAPROTAX) analysis as well as C, N, and S gene q-PCR chips. The FAPROTAX analysis showed that the metabolic potential in C, N, and S cycling indicated a difference in the four mangrove regions. Moreover, some functional groups involved in N, C, and S cycling were significantly correlated with physicochemical characteristics. However, N, C, and S metabolic genes detected by q-PCR chip did not present the same changes following the variation of environmental conditions as those predicted by the FAPROTAX analysis. Most of the N, C, and S cycling genes were equally distributed in the four mangroves. Several genes involved in N cycling (amoA, amoB, nirK, and nosZ), carbon degradation (amyA, apu, mnp, lig, and chiA), carbon fixation (aclB), and oxidation of H2S (SoxY) were significantly higher in sampling site in Beihai City (Bei), which was the only site with sandy soils. We hypothesized that soil texture (sandy or muddy) rather than physicochemical elements contributes more significantly to the abundance of N, S, and C metabolic genes in mangrove sediment. Our study demonstrated that the change of physicochemical property in mangrove soil was easier to modify the functional microbial groups than functional genes. We should pay attention to the functional redundancy of soil microbiota when using microorganisms as ecological indicator to access the soil activity in mangrove habitats.