Seasonal changes and diversity of bacteria in Bohai Bay by RFLP analysis of PCR-amplified 16S rDNA gene fragments

Understanding the Variation of Bacteria in Response to Summertime Oxygen Depletion in Water Column of Bohai Sea

Aiming to reveal the variation in bacteria community under oxygen depletion formed every summer in water column of central Bohai Sea, a time-scenario sampling from June to August in 2018 at a 20-day interval along one inshore-offshore transect was settled. Water samples were collected at the surface, middle, and bottom layer and then analyzed by high-throughput sequencing targeting both 16S rRNA and nosZ genes. Compared to the surface and middle water, oxygen depletion occurred at bottom layer in August. In top two layers, Cyanobacteria dominated the bacterial community, whereas heterotrophic bacteria became dominant in bottom water of Bohai Sea. Based on the time scenario, distinct community separation was observed before (June and July) and after (August) oxygen depletion (p = 0.003). Vertically, strict stratification of nosZ gene was stably formed along 3 sampling layers. As a response to oxygen depletion, the diversity indices of both total bacteria (16S rRNA) and nosZ gene-encoded denitrification bacteria all increased, which indicated the intense potential of nitrogen lose when oxygen depleted. Dissolved oxygen (DO) was the key impacting factor on the community composition of total bacteria in June, whereas nutrients together with DO play the important roles in August for both total and denitrifying bacteria. The biotic impact was revealed further by strong correlations which showed between Cyanobacteria and heterotrophic bacteria in June from co-occurrence network analysis, which became weak in August when DO was depleted. This study discovered the variation in bacteria community in oxygen-depleted water with further effort to understand the potential role of denitrifying bacteria under oxygen depletion in Bohai Sea for the first time, which provided insights into the microbial response to the world-wide expanding oxygen depletion and their contributions in the ocean nitrogen cycling.

Disproportionate responses between free-living and particle-attached bacteria during the transition to oxygen-deficient zones in the Bohai Seawater

The Bohai Sea has recently suffered several seasonal oxygen-deficiency, even hypoxia events during the summer. To better understand effects of dissolved oxygen (DO) concentration on the bacterial composition in particle attached (PA) and free living (FL) fractions during the transition from oxic water to low oxygen conditions, the bacterial communities under three different oxygen levels, i.e., high oxygen (HO, close to 100% O₂ saturation), medium oxygen (MO, close to 75% O₂ saturation), and low oxygen (LO, close to 50% O₂ saturation) in the Bohai Sea were investigated using 16S rRNA amplicon sequencing. Fourteen water samples from 5 stations were collected during a cruise from August to September in 2018. The results showed that the sequences of Proteobacteria and Actinobacteriota jointly accounted for up to 74% across all 14 samples. The Shannon index in HO samples were significantly higher than in LO samples (P < 0.05), especially in PA communities. The composition of bacterial communities varied by oxygen concentration in all samples, and the effect was more pronounced in the PA fraction, which indicates that the PA fraction was more sensitive to the change in oxygen concentration, possibly due to the tighter interactions in this community than in the FL fraction. This study provides novel insights into the distribution of bacterial communities, and clues for understanding the responses of bacterial communities in the Bohai Sea during the transition from the oxic to oxygen-deficient zones.

Comparative Analysis of the Ecological Succession of Microbial Communities on Two Artificial Reef Materials

Concrete and wood are commonly used to manufacture artificial reefs (ARs) worldwide for marine resource enhancement and habitat restoration. Although microbial biofilms play an important role in marine ecosystems, the microbial communities that colonize concrete and wooden ARs and their temporal succession have rarely been studied. In this study, the temporal succession of the microbial communities on concrete and wooden AR blocks and the driving factors were investigated. The composition of the microbial communities underwent successive shifts over time: among the six dominant phyla, the relative abundances of Proteobacteria, Cyanobacteria and Gracilibacteria significantly decreased in wood, as did that of Cyanobacteria in concrete. Operational taxonomic units (OTU) richness and Shannon index were significantly higher in concrete than in wood. Non-metric multidimensional scaling ordination placed the microbial communities in two distinct clusters corresponding to the two substrate materials. The macrobenthic compositions of concrete and wood were broadly similar and shifted over time, especially in the first five weeks. The Shannon index of the microbial communities in concrete and wood increased significantly with the organism coverage. The results provide fundamental data on microbial community succession during the initial deployment of ARs and contribute to understanding the ecological effects of ARs.

Impact of artificial reefs on sediment bacterial structure and function in Bohai Bay

Artificial reefs have significantly altered ecological and environmental conditions compared with natural reefs, but how these changes affect sediment bacteria structure and function is unknown. Here, we compared the structure and function of the sediment bacterial community in the artificial reef area, the future artificial reef area, and the control area in Bohai Bay by 16S rRNA genes sequencing. Our results indicated that bacteria communities in the sediment were both taxonomically and functionally different between the reef area and control area. In the artificial reef area, the α-diversity was significantly lower, whereas the β-diversity was significantly higher. Functional genes related to chemo-heterotrophy, nitrate reduction, hydrocarbon degradation, and the human pathogens and human gut were more abundant, whereas genes related to the metabolism of sulfur compounds were less abundant in the artificial reef than in the control area. The differences in bacterial communities were primarily determined by depth in the artificial reef area, and by total organic carbon in the future reef area and control area. This study provides the first overview of molecular ecology to assess the impacts of artificial reefs on the bacteria community.