While in hospite Symbiodiniaceae dinoflagellates are essential for coral health, ambient free-living counterparts are crucial for coral recruitment and resilience. Comparing free-living and in hospite Symbiodiniaceae communities can potentially provide insights into endosymbiont acquisition and recurrent recruitment in bleaching recovery. In this study, we studied coral-endosymbiotic and ambient free-living Symbiodiniaceae communities in the South China Sea. We collected samples from 183 coral and ambient plankton samples and conducted metabarcoding to investigate the diversity distribution, driving factors, and assembly mechanisms of the two groups of Symbiodiniaceae. Results revealed Cladocopium C1 and Durusdinium D1 as dominant genotypes. We detected a higher genotypic diversity in free-living than in hospite symbiodiniacean communities, but with shared dominant genotypes. This indicates a genetically diverse pool of Symbiodiniaceae available for recruitment by corals. Strikingly, we found that the cooler area had more Symbiodiniaceae thermosensitive genotypes, whereas the warmer area had more Symbiodiniaceae thermotolerant genotypes. Furthermore, in hospite and free-living Symbiodiniaceae communities were similarly affected by environmental factors, but shaped by different assembly mechanisms. The in hospite communities were controlled mainly by deterministic processes, whereas the ambient communities by stochastic processes. This study sheds light on the genetic diversity of source environmental Symbiodiniaceae and differential assembly mechanisms influencing Symbiodiniaceae inside and outside corals. IMPORTANCE: Symbiodiniaceae dinoflagellates play a pivotal role as key primary producers within coral reef ecosystems. Coral-endosymbiotic Symbiodiniaceae communities have been extensively studied, but relatively little work has been reported on the free-living Symbiodiniaceae community. Conducting a comparative analysis between sympatric coral-endosymbiotic and free-living Symbiodiniaceae communities can potentially enhance the understanding of how endosymbiont communities change in response to changing environments and the mechanisms driving these changes. Our findings shed light on the genetic diversity of source environmental Symbiodiniaceae and differential assembly mechanisms shaping free-living and in hospite Symbiodiniaceae communities, with implications in evaluating the adaptive and resilient capacity of corals in response to future climate change.

Soil diatoms are essential components of microalgae communities in aquatic-terrestrial ecotones, contributing to primary production, organic matter production, nutrient cycling, and ecosystem stability. However, the functional traits and community assembly processes of diatoms in these ecotones remain poorly understood. This study used 18S rDNA high-throughput sequencing and functional trait analysis to investigate the dynamics, driving factors, and assembly processes of diatom communities in the aquatic-terrestrial ecotone of the Danjiangkou Reservoir. Significant differences in diatom diversity and composition were observed between waterward (frequently submerged) and landward zones (exposed to alternating wet and dry conditions). Functional traits revealed that larger, highly motile species dominated waterward communities, while smaller, less motile taxa were prevalent in landward zones. Total phosphorus (TP), soil organic matter (SOM), and magnesium (Mg ²⁺ ) were key environmental drivers of taxonomic and functional traits. Community assembly in both zones was primarily driven by random processes; however, deterministic processes, particularly heterogeneous selection, had a stronger influence in the landward zone, reflecting significant environmental filtering. These findings enhance understanding of biodiversity and community assembly in aquatic-terrestrial ecotones and provide valuable insights into their ecological dynamics.

Aquatic–terrestrial ecotones are highly dynamic biogeochemical hotspots where hydrological fluctuations profoundly influence microbial community structure and ecosystem functioning. However, the mechanisms underlying microbial community responses across hydrological gradients remain insufficiently understood. In this study, 16S rRNA gene sequencing was used to comparatively analyze bacterial communities in the waterward and landward zones of the drawdown area of the Danjiangkou Reservoir. The results showed that bacterial community composition differed significantly between the two zones, and waterlogging markedly increased bacterial α-diversity. Community variation was primarily associated with key environmental factors, including total phosphorus (TP), soil moisture content (SMC), and nitrate nitrogen (NO₃⁻-N). Compared with the landward zone, stochastic processes contributed more to community assembly in the waterward zone, which also exhibited higher network complexity and topological stability. In addition, several keystone taxa were identified, suggesting their potential roles in maintaining network structure and ecological stability. Functional prediction further revealed distinct metabolic potentials between zones, with enhanced anaerobic and redox-related functions in the waterward zone and predominantly aerobic metabolism in the landward zone. These findings suggest that hydrological fluctuations reshape bacterial community structure and potential ecological functions by jointly regulating water availability and nutrient dynamics. This study provides new insights into microbial ecological processes in reservoir riparian zones and offers a scientific basis for the management of aquatic–terrestrial ecotone ecosystems.

Suspended particulate matter (SPM) is a key environmental driver in aquatic ecosystems and plays a significant role in shaping microbial communities, particularly in sediment-rich rivers. Dam construction alters hydrological regimes and creates distinct SPM gradients; however, the response mechanisms of microeukaryotic plankton communities remain poorly understood. In this study, we used 18S rRNA gene high-throughput sequencing to characterize microeukaryotic plankton communities across riverine, lacustrine, and transitional zones of the Xiaolangdi Reservoir on the Yellow River (China). Our results revealed distinct community compositions in the lacustrine zone, with SPM identified as the primary factor driving community differentiation. Alpha diversity was highest in the riverine zone, while beta diversity differences among zones were dominated by species turnover. Dominant taxa included Cryptophyta (44.71% ± 30.79%), Metazoa (18.98% ± 17.71%), Perkinsea (7.97% ± 9.78%), Chlorophyta (7.06% ± 5.80%), and Dinophyta (6.06% ± 6.73%). Metazoa, Dinophyta, and Phaeophyta were enriched in high-SPM riverine waters, whereas Alveolata dominated low-SPM lacustrine zones. Community assembly was primarily deterministic, governed mainly by homogeneous selection, with stochastic processes exerting stronger influence in riverine zones. Network analysis indicated that riverine zones exhibited more complex and stable networks, lacustrine zones showed higher local but lower global connectivity, and transitional zones displayed stronger interactions but lower stability. These findings advance our understanding of microeukaryotic plankton responses to dam-induced environmental changes and provide a basis for assessing biodiversity impacts in regulated river systems.

Large reservoirs are human-managed ecosystems where microbial communities play pivotal roles in nutrient cycling and ecological stability. However, the mechanisms underlying microbial multistability and stress-induced functional shifts remain inadequately understood. Here, we employed a one-year high-resolution time-series 16S rRNA gene amplicon high-throughput sequencing dataset from the Danjiangkou Reservoir(China) to characterize the alternative stable states(ASS) of microbial communities. Two distinct ASSs were identified: State A, dominated by Cyanobacteria and Actinobacteriota, is characterized by higher alpha diversity, strong local connectivity, and phototrophy-driven energy acquisition. In contrast, State B, dominated by Bacteroidota and Proteobacteria, exhibits enhanced network modularity and functional resilience, supported by robust nitrogen and carbon cycling. Dissolved oxygen, temperature, and ammonia nitrogen were identified as the key environmental stressors regulating state transitions, with distinct thresholds and hysteresis phenomena elucidated via stability-landscape analysis. Functional prediction and structural equation modeling further confirmed that environmental stress reorganizes microbial metabolism pathways from light-dependent processes to nitrogen-cycling- dominated processes, which is mediated by alterations in microbial diversity and network stability. Collectively, these findings provide critical empirical evidence for microbial multistability and stress-driven regime shifts in large reservoirs. This work thereby informs adaptive water quality management strategies for large drinking water reservoirs, pivotal for mitigating nutrient pollution, safeguarding water security, and promoting sustainable freshwater ecosystem management.