Inconsistency over the comparison of influence degree on the bacterial community in mangrove sediments between marine and terrestrial environments
Phyla Cyanobacteria and Chloroflexi  and class gamma-Proteobacteria  are typical bacterial communities in marine environments, and phyla Actinobacteria, Acidobacteria, and Verrucomicrobia [19-21] and classes alpha- and beta-Proteobacteria  are mainly found in terrestrial environments. The relative abundances of these communities could assess the influence degree on the bacterial community in mangrove sediments from marine and terrestrial environments .
Mangrove eco-systems, as typical intertidal wetlands, could be affected by the land and ocean. Marine environment has strong effect on the bacterial and viral communities in mangrove sediments, indicating the higher influence from oceans than that from land [7, 24]. For 16 out of 22 samples in this study, the relative abundances of marine bacteria were greater than those from land, thus supporting former studies (Fig. 1B and Additional file 1: S2). However, for the six remaining samples, the relative abundances of terrestrial communities were greater than those from oceans (Figs. 1B and Additional file 1: S2). This result is inconsistent with previous research, indicating that the comparison of influence degree between oceans and land could be more variable than previously observed. For example, anthropogenic factors can complicate this variation .
Microbial community structures in mangrove sediments showed spatial variability and temporal stability
Location affected the alpha diversities of archaeal (Additional file 1: Fig. S5) and fungal communities (Additional file 1: Fig. S6), and ANOSIM, Adonis (Additional file 2: Table S2), LEfSe (Fig. 3A), and PCoA (Fig. 2A) results supported the significant influence of the location on bacterial, archaeal, or fungal community structure. Depth could also significantly affect the alpha diversities and structures of bacterial and fungal communities (Additional file 1: Figs. S4, S6, Figs. 2B, 3B and Additional file 2: Table S2). To avoid the effects of rhizosphere in mangroves , we only collected the bulk sediments away from the plant roots. However, the location and depth were still the significant spatial dimensions that shaped the microbial communities in mangrove sediments, suggesting that the distribution of microbial communities in mangrove sediments was highly territorialized in a fairly close sampling range (e.g., in the same city for sampling).
Our work failed to detect significant differences among the seasonal samples. The influence of spatial dimensions supports previous studies [7, 26, 27]. However, the weak effect from temporal dimension, namely, seasonal change, is unusual, especially for fungi with dynamic community structure in mangrove sediments . We attribute this difference to the stable abiotic factors, except SAL and PS (Fig. S1), among the seasonal samples from the tropics, as previously reported .
Generally, our work demonstrated the variability in spatial dimensions and stability in temporal dimension of the microbial communities in tropical mangroves in Sanya City. This characteristic not only ensures the diversity of microbial functions in the different areas of mangrove sediments, but also guarantees the stability of the microbial functions against time variation, indicating a versatile but stable environment in tropical mangrove sediments.
PS served as the key abiotic factor for shaping the microbial communities in mangrove sediments
Limited studies focus on the key abiotic factors that shape the microbial communities in mangrove sediments. Among at least 13 abiotic factors that we collected in this work, PS was the key abiotic shaping factor for the bacterial, archaeal, or fungal community according to the results of linear fitting models (Fig. 4B, Additional file 1: Figs S8 and S10) and Mantel test (Table 2).
To the best of the authors’ knowledge, this study first demonstrated PS as a key shaping factor for the microbial communities in mangrove sediments. The influence of PS on mangrove microbial diversity has been reported before, which showed that PS could affect the diversity and abundance of laccase-like bacteria, and their abundance decrease in the order of sand > clay > silt in Mai Po mangrove sediments of China . Our work enhanced the value of PS for the assembly of not only the laccase-like bacteria but also three typical microbial communities in mangrove sediments.
The possible reasons of the PS influence on microbial structures are as follows: (i) High nutritional content is present in the soils or sediments with specific PS [30, 31]. The difference in nutritional contents resulted in the variation in the structures of microorganisms. (ii) Soils or sediments with small PS could provide microbes a protective habit from predators [32, 33]. Therefore, high abundance of microbes is often detected in samples with small PS. (iii) The crucial enzymes, such as urease, invertase, alkaline phosphatase, and xylanase, for bacterial survival are rich in sediments with specific PS [34-37] and could cause the abundance and diversity of microbes in the present samples. (iv) Permeability influenced microbial structures as confirmed by Probandt et al. , and PS is the sole variable that affects the permeability calculation of soils or sediments . Thus, PS could affect the microbial structure by controlling the permeability of sediment samples.
In addition to PS, SAL and HUM were also uniform significant abiotic factors for bacterial, archaeal, and fungal communities (Fig. 4A and Additional file 3: Table S4). Considering the element cycling functions of these microorganisms in mangrove sediments, the above factors might indirectly influence carbon, nitrogen, and sulfur cycles in mangrove sediments by affecting the microbial community structures.
Providers of bioavailable carbohydrates and ammonia nitrogen served as the key biotic factor for shaping the bacterial communities in mangrove sediments
Keystone taxa are known as “the engineers” for shaping the community structures in nature because of their high connectivity with other species in the co-occurrence structures. Therefore, the keystone taxa are crucial biotic factors shaping the microbial communities in mangrove sediments. We failed to obtain any information about archaeal and fungal communities because of the possible weak dependency among these microorganisms and the possible interactions of archaeal and fungal communities with other living organisms rather than the creatures of their own kinds. Unlike archaea and fungi, keystone taxa were found in bacterial community. Results showed that the OTUs belonging to phyla Ignavibacteriae, Proteobacteria, Bacteroidetes, Chloroflexi, and Acidobacteria served as the bacterial keystone taxa in Sanya mangrove sediments (Table 2).
Interestingly, 16 out of 18 of these OTUs provided nitrogen and carbohydrates (Table 2). In detail, OTU5273 and 5013, which belong to the order Ignavibacteriales, are photoautotrophic green sulfur bacteria [40, 41]. The order Rhodospirillales (OTU12246 and 11459) and the family Rhodobacteraceae (OTU12355) are photoautotrophic bacteria [42, 43] and nitrogen (N) fixers [44-46]. Erythrobacteraceae (OTU5037) is also an important photoautotrophic family . Desulfobulbaceae (OTU7966) is a sulfate‐reducing bacteria that can actively oxidize H2, reduce sulfate, fix CO2 , and mediate N fixation in the oceans . Some members of the phyla Chloroflexi (OTU10701, 16075, and 12101) and Acidobacteria (OTU2963) are also photoautotrophic bacteria . In addition, the families Flavobacteriaceae (OTU11086) [50, 51], Flammeovirgaceae (OTU6475) [52, 53], and Draconibacteriaceae (OTU11996) [54, 55] and the order Xanthomonadales (OTU5104 and 11887)  degrade stable polysaccharides into active monosaccharides.
Polysaccharides, including cellulose and seaweed polysaccharide, are important storage forms of organic C in mangrove sediments [57, 58]. However, recalcitrant polysaccharides are difficult to utilize for many microorganisms. Mangroves can also store considerable dissolved inorganic carbons that cannot be utilized by many microorganisms . Therefore, photo-autotrophic and polysaccharide-degrading bacteria can server as the keystone abiotic factors by providing bioavailable carbohydrates to other bacteria that interacted with them, as shown in the results of this work. Mangrove ecosystems also have a typical N-deficient environment . Therefore, the keystone OTUs with N-fixation ability in the present study can provide bioavailable ammonia nitrogen for the survival of other microorganisms and living organisms in mangrove sediments. Hence, these keystone species could serve as the centers for bacterial interaction in mangrove sediments because of the nutrition that they provide.
These keystone species drive the microbial structure and function  and predict changes in microbial community ; hence, their status, especially for bifunctional species that can fix both C and N, is important not only for themselves but also for many other bacteria that depend on them. Therefore, the changes of community structure of these keystone species provided in this work were potential important indicators for supervising the health of mangrove ecosystems.
Important contributions of the bacterial modules centered on photoautotrophic keystone taxa in mangrove sediments to the global climate
The abundances of keystone species were not dominant in natural environments and in our work, but these taxa are the centers of co-occurrence structures in bacterial community, suggesting that they could organize many bacterial species together and form considerable interaction modules . These bacterial modules might contribute to the ecological functions of mangroves, which are typical blue carbon ecosystems .
These bacterial co-occurrence modules centered on photoautotrophic keystone taxa could import inorganic CO2 into the sediments and support the survival of heterotrophic bacteria in the modules to form and store large amounts of organic carbons in microorganisms. Furthermore, the carbon fixed in mangrove sediments could be exported into the oceans , while some could be sealed up as recalcitrant dissolved organic matters in oceans for millions of years by the marine microbial carbon pump . N fixation is another key function for some of these keystone taxa. Ammonia nitrogen, the product of N fixation, is an essential nutrient for living organisms, including primary producers, and nitrate, the product of nitrification of ammonia nitrogen, is crucial for the photosynthesis of phytoplankton . The keystone taxa with N fixation ability could further increase the amount of C fixation by providing the limited nitrogenous nutrients for photosynthesis. In brief, these bacterial modules centered on photoautotrophic keystone taxa could be the additional C fixers and storage sites besides the large phytobenthos in mangroves and serve as potential vital participants in the storage of blue carbons and mitigation of climate warming.
Our work aimed to reveal the spatiotemporal influence and the key abiotic and biotic shaping factors for the microbial communities in mangrove sediments through amplicon sequencing and to reflect the structural information of mangrove microorganisms. However, we did not research and discuss the functional information about these communities. Hence, metagenomic and meta-transcriptomic sequencing should be conducted to describe the microbial communities for future works in mangrove ecosystems.