The increase of particle size shifts the biogeochemical cycle functions of mineral-associated microorganisms and weakens the mineral-associated organic carbon sink in mangrove soils.

Mineral-organic matter-microbe interactions greatly impact the biogeochemical processes and biodiversity in soils. An increasing trend of particle size (PS) in mangrove soils has been observed because of the relative sea level rise. However, the impacts of PS increase on the microbial biogeochemical functions and carbon sink in the mineral-associated microcosms are exceedingly nebulous. This work showed a remarkable difference in the communities of mineral-associated microorganisms (MMOs) in various PS fractions. Heavy metal contents and urease activity were the factors that mostly driven the MMO community variation in different PS fractions. Large PS fraction attenuated the stability of MMO communities according to the co-occurrence network characteristics. The PS increase significantly (P < 0.05) lowered the gene abundances for carbon input (e.g., carbon fixation) and raised the gene abundances for carbon loss (e.g., aerobic respiration). Combined with the significant decrease of mineral-associated organic carbon (MOC) in large PS fraction (P < 0.05), this work inferred that the PS increase could weaken the MOC sink partially due to the MMO function shift for carbon cycle. The current work indicated unhealthy changes of MMO communities and MOC storage in mangrove soils, and PS was of significance as an indicator for predicting the carbon sink function, especially for the stable form, such as MOC, in the soils of mangroves under the ecological background of climate migration. IMPORTANCE: Carbon with stable forms, such as mineral-associated organic carbon (MOC), is crucial for the sink capabilities in mangrove soils, and mineral-associated microorganisms (MMOs) are important players for the formation and metabolism of MOC. Therefore, the future successions of the MMO functions and MOC contents under the background of climate change are of value for a deeper understanding of mangrove ecology. The relative sea level rise caused by the global warming results in the increase of mangrove soil particle size (PS), which provides distinct microcosms for MMOs and MOC. However, the responses of MMO functions and MOC content to the PS increase of mangrove soils are unknown. The current study aims to reveal the succession regulations of MMO functions and their potential ecological impacts for the storages of MOC in different PS fractions, therefore widening our knowledge of future function migration and promoting the research development of mangrove.

Structure and assembly process of fungal communities in the Yangtze River Estuary.

Marine fungi are essential for the ecological function of estuarine ecosystems. However, limited studies have reported on the structure and assembly pattern of the fungal communities in estuaries. The purpose of this study is to reveal the structure and the ecological process of the fungal community in the Yangtze River Estuary (YRE) by using the amplicon sequencing method. Phyla of Ascomycota, Basidiomycota, and Chytridiomycota were dominant in the seawater and sediment samples from YRE. The null model analysis, community-neutral community model (NCM), and phylogenetic normalized stochasticity ratio (pNST) showed that the stochastic process dominated the assembly of fungal communities in YRE. Drift and homogeneous dispersal were the predominant stochastic processes for the fungal community assembly in seawater and sediment samples, respectively. The co-occurrence network analysis showed that fungal communities were more complex and closely connected in the sediment than in the seawater samples. Phyla Ascomycota, Basidiomycota, and Mucoromycota were the potential keystone taxa in the network. These findings demonstrated the importance of stochastic processes for the fungal community assembly, thereby widening our knowledge of the community structure and dynamics of fungi for future study and utilization in the YRE ecosystem.

Stochastic processes shape the aggregation of free-living and particle-attached bacterial communities in the Yangtze River Estuary, China.

An estuary plays an important role in material and energy exchange between the land and sea, where complex physical, chemical, and biological processes occur. Here, we investigated the assembly processes of free-living (FL) and particle-associated (PA) bacterial communities in two seawater layers at five stations in the Yangtze River Estuary (YRE) by using 16S rRNA sequencing methods. The results indicated that Proteobacteria was the most abundant phylum in the YRE. The α-diversity of PA community was significantly higher than FL community, and analysis of similarity showed significantly different (Global R = 0.2809, p < 0.005). RDA revealed that phosphate (PO4 3- ) was significantly correlated with PA bacterial community abundance (p < 0.05). An ecological null model showed that both PA and FL bacterial communities were mainly influenced by stochastic processes (PA: 100%, FL: 70%), which PA attached to nutrient particles and are less affected by environmental filtration. Dispersal limitation (50%) was the main assembly process of the PA community, while homogeneous selection (30%) and drift (30%) were important processes in the FL community assembly. The available substrate for colonization limits the transformation from FL to PA bacteria. This study would improve our understanding of FL and PA bacterial community structure and factors affecting assembly process in estuarine environments.

The importance of conditionally rare taxa for the assembly and interaction of fungal communities in mangrove sediments.

The fungal communities provide the nutrients and drive the cycles of elements in nature, and the rare fungal taxa are proved to be crucial for these communities in many environments. However, the ecological functions of rare taxa for the fungal communities in mangrove ecosystems are poorly assessed until now. This work aims to reveal the importance of rare taxa for the assembly of fungal communities in mangrove sediments by using the amplicon sequencing analysis of different spatiotemporal samples collected from Sanya mangroves, China. The results showed that Ascomycota and Basidiomycota were the dominant phyla in the conditionally rare taxa (CRT). The fungal communities possessed outstanding stability against the spatiotemporal variation and most collected environmental factors. The CRT possessed narrower niches and were more affected by the environmental variables than the abundant taxa. The current work demonstrated that the CRT had significantly higher relative abundances, degrees (the number of adjacent edges), clustering coefficients, and closeness centralities in the top 8 modules of the co-occurrence network (p < 0.05), indicating the important role of the CRT for the interaction of fungal communities in mangrove sediments. These findings indicate the importance of the CRT for the fungal community structures in mangrove sediments, and would deepen our understanding of dynamic functions of mangrove fungi, thereby facilitating the management, utilization, and protection of mangrove ecosystems. KEY POINTS: • Fungal communities in mangrove sediments are stable against environment variations. • The conditionally rare taxa (CRT) possessed narrower niches than the abundant fungal taxa. • The CRT are central for the co-occurrence network and interaction of fungal communities.

Expression and Characterization of a Novel Cold-Adapted and Stable ß-Agarase Gene agaW1540 from the Deep-Sea Bacterium Shewanella sp. WPAGA9.

The neoagaro-oligosaccharides, degraded from agarose by agarases, are important natural substances with many bioactivities. In this study, a novel agarase gene, agaW1540, from the genome of a deep-sea bacterium Shewanella sp. WPAGA9, was expressed, and the recombinant AgaW1540 (rAgaW1540) displayed the maximum activity under the optimal pH and temperature of 7.0 and 35 °C, respectively. rAgaW1540 retained 85.4% of its maximum activity at 0 °C and retained more than 92% of its maximum activity at the temperature range of 20-40 °C and the pH range of 4.0-9.0, respectively, indicating its extensive working temperature and pH values. The activity of rAgaW1540 was dramatically suppressed by Cu2+ and Zn2+, whereas Fe2+ displayed an intensification of enzymatic activity. The Km and Vmax of rAgaW1540 for agarose degradation were 15.7 mg/mL and 23.4 U/mg, respectively. rAgaW1540 retained 94.7%, 97.9%, and 42.4% of its maximum activity after incubation at 20 °C, 25 °C, and 30 °C for 60 min, respectively. Thin-layer chromatography and ion chromatography analyses verified that rAgaW1540 is an endo-acting ß-agarase that degrades agarose into neoagarotetraose and neoagarohexaose as the main products. The wide variety of working conditions and stable activity at room temperatures make rAgaW1540an appropriate bio-tool for further industrial production of neoagaro-oligosaccharides.