Structural inflexibility of the rhizosphere microbiome in mangrove plant Kandelia obovata under elevated CO2

Response of bacterial diversity and community structure to metals in mangrove sediments from South China

Human activities have given rise to metal contamination in the constituents of mangrove ecosystems, posing a critical threat to sediment microorganisms; hence, it is of great importance to comprehend the effects of metals on the microbial communities in mangrove sediments. This study was the first to explore the response of the bacterial diversity and community structure to nine metals (As, Co, Cr, Cu, Mn, Ni, Pb, V and Zn) and organic matter fractions (including total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS)) in mangrove wetlands from Zhanjiang, China, using 16S rRNA high-throughput sequencing technology and Spearman correlation analysis. The results showed that these nine metals were scattered differently in different mangrove sediments, and the metals and organic matter fractions jointly affected the bacterial communities in the sediments. Several metals displayed significant positive correlations with the abundances of the phylum Bacteroidetes and the genera Actibacter and Sphingobacterium but significant negative correlations with the abundances of two genera Holophaga and Caldithrix. Furthermore, the abundances of the phylum Actinobacteria and many bacterial genera showed significant positive or negative responses to the levels of the three organic matter fractions. Interestingly, the levels of a number of bacterial genera that exhibited increased abundance with high levels of metals and TS might be reduced with high TOC and TN, and vice versa: the levels of genera that exhibited decreased abundance with high levels of metals and TS might be increased with high TOC and TN. Overall, many bacterial groups showed different response patterns to each metal or organic matter fraction, and these metals together with organic matter fractions influenced the bacterial diversity and community structure in mangrove sediments.

Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests

Mangroves store blue carbon (693 Mg CORG ha−1) disproportionate to their small area, mainly (74%) in deep soil horizons. Global stock estimates for mangroves (5.23–8.63 Pg CORG) are equivalent to 15–24% of those in the tropical coastal ocean. Carbon burial in mangrove soils averages 184 g CORG m−2 a−1 with global estimates (9.6–15.8 Tg CORG a−1) reflecting their importance in carbon sequestration. Extreme weather events result in carbon stock losses and declines in carbon cycling and export. Increased frequency and ferocity of storms result in increasingly negative responses with increasing strength. Increasing temperatures result in increases in carbon stocks and cycling up to a critical threshold, while positive/negative responses will likely result from increases/decreases in rainfall. Forest responses to sea-level rise (SLR) and rising CO2 are species- and site-specific and complex due to interactive effects with other drivers (e.g., temperature, salinity). The SLR critical threshold is ≈ 6 mm a−1 indicating survival only under very low-low CO2 emissions scenarios. Under low coastal squeeze, landward migration could result in sequestration and CO2 losses of 1.5 and −1.1 Pg C with net stock gains and losses (−0.3 to +0.5 Pg C) and CO2 losses (−3.4 Pg) under high coastal squeeze.

Genetic Elucidation of Quorum Sensing and Cobamide Biosynthesis in Divergent Bacterial-Fungal Associations Across the Soil-Mangrove Root Interface

Plant roots in soil host a repertoire of bacteria and fungi, whose ecological interactions could improve their functions and plant performance. However, the potential microbial interactions and underlying mechanisms remain largely unknown across the soil-mangrove root interface. We herein analyzed microbial intra- and inter-domain network topologies, keystone taxa, and interaction-related genes across four compartments (non-rhizosphere, rhizosphere, episphere, and endosphere) from a soil-mangrove root continuum, using amplicon and metagenome sequencing technologies. We found that both intra- and inter-domain networks displayed notable differences in the structure and topology across four compartments. Compared to three peripheral compartments, the endosphere was a distinctive compartment harboring more dense co-occurrences with a higher average connectivity in bacterial-fungal network (2.986) than in bacterial (2.628) or fungal network (2.419), which could be related to three bacterial keystone taxa (Vibrio, Anaerolineae, and Desulfarculaceae) detected in the endosphere as they are known to intensify inter-domain associations with fungi and stimulate biofilm formation. In support of this finding, we also found that the genes involved in cell-cell communications by quorum sensing (rhlI, lasI, pqsH, and lasR) and aerobic cobamide biosynthesis (cobG, cobF, and cobA) were highly enriched in the endosphere, whereas anaerobic cobamide biosynthesis (encoded by cbiT and cbiE) was dominant in three peripheral compartments. Our results provide genetic evidence for the intensified bacterial-fungal associations of root endophytes, highlighting the critical role of the soil-root interface in structuring the microbial inter-domain associations.

Heavy metal pollution in urban river sediment of different urban functional areas and its influence on microbial community structure

In this study, the Songgang River (SR) was selected as a typical contaminated urban river in a highly urbanized city (Shenzhen) that is extensively polluted by heavy metals (HMs). Five representative sampling sites were selected from different urban functional areas along the SR, and the spatial and vertical distributions of HMs and the related environmental risk were investigated. In addition, the distribution variability, composition, and abundance of microbial communities, as well as the correlation between the abundance of the operational taxonomic units (OTUs) and the HM contents were analyzed. The spatial distribution of HMs in the sediment revealed wide variation among the different urban functional areas. Industrial and residential areas had higher HM contents, following the order of Cu > Zn > Ni > Cr > Pb. In addition, the vertical characterization (5-300 cm) of HM content showed a decreasing trend with depth, with a distinct layer around 120-180 cm that might have been caused by anthropogenic activity. An ecological risk assessment indicated that Cu, Ni, and Cr pose high potential risks in these industrial and residential areas (at the depth of 5-180 cm). Furthermore, microbial community analysis indicated that some HM-tolerant bacteria (e.g., Gallionella, Acidovorax, Arenimonas, Curvibacter, and Sideroxydans) were dominant in the 5-120 cm layer, corresponding to high HM contents. A canonical correspondence analysis and co-occurrence network further confirmed that there was a strong correlation among the urban functional areas, HM contents, and the abundance of microorganisms in the urban river sediment. The results of this study have the potential to provide a bio-augmentation strategy for the in-situ bioremediation of sediment contaminated by HMs.

Variations of soil bacterial diversity and metabolic function with tidal flat elevation gradient in an artificial mangrove wetland

Understanding the sensitivity of soil bacteria to environmental fluctuations can enhance the management of microbial ecosystem services in artificial mangrove wetlands. In this study, the variation in bacterial diversity and metabolic functions in different compartments (bulk soil, rhizosphere soil, and rhizoplane) of the soil and mangrove plant along the tidal elevation gradient was studied in Xiatanwei (Xiamen China) mangrove wetland park, a Kandelia obovata-dominated artificial mangrove stand. With the increase of the tidal flat elevation, the soil pH, total organic matter, and soil moisture contents decreased significantly, while the soil electric conductivity and redox potential increased significantly. The bacterial diversity in the bulk soil and the rhizosphere soil both decreased with the elevation of tidal levels. The relative abundance of the dominant phyla in the bulk and rhizosphere soils decreased with the rise of the tidal flat level. A significant rhizosphere effect was observed in the roots of K. obovata that the rhizosphere soil had higher bacterial diversity and richness than that in the bulk soil nearby. The rhizosphere soil of K. obovata at the low-tidal flat was enriched with the genera Nitrospira and Planctomycetes, which are valuable for the mangrove ecosystem. The Chao1 estimator and Shannon index of the bacterial community in the rhizoplane of K. obovata were much lower than that in the rhizosphere and bulk soils. Results of Biolog-Eco assay show that the bacterial groups in low tidal flat bulk soil had the highest ability in utilizing the carbon sources, which was indicated by the high values of average well color development and the high McIntosh index, and the utilization ability of carbon source decreased with the increase of tidal flat levels. The variation of the soil humidity and Eh jointly shaped the diversity and metabolic function of soil bacterial communities along the tidal flat elevation gradient.