Combined effects of toxic Microcystis aeruginosa and high pH on antioxidant responses, immune responses, and apoptosis of the edible freshwater bivalve Corbicula fluminea.

Due to increasing anthropogenic perturbation and water eutrophication, cyanobacterial blooms (CYBs) have become a global ecological and environmental problem. Toxic CYBs and elevated pH are considered to be the two key stressors associated with eutrophication in natural waters, particularly in the event of CO2 depletion induced by dense blooms. However, previous research has been focused on investigating the impacts of toxic CYBs or pH changes in isolation, whereas the interactive effects of such stressors on edible bivalves that inhabit CYB waters still lack information. In this study, the combined effects of toxic Microcystis aeruginosa and pH shifts on the antioxidant responses, immune responses, and apoptosis of the edible freshwater bivalve Corbicula fluminea were explored. The results showed that the activity of antioxidant enzymes was significantly impacted by the interactive effects between toxic M. aeruginosa exposure and time course, yet pH shifts showed no significant effects on the activities of these antioxidant enzymes, implying that the antioxidant response in C. fluminea was mainly triggered by toxic M. aeruginosa exposure. Toxic M. aeruginosa also induced an increased production of reactive oxygen species and malondialdehyde in treated clams, particularly under high pH settings. The elevated lysosomal enzyme activity helped C. fluminea defend against toxic M. aeruginosa exposure under high pH conditions. The principal component analysis (PCA) and the integrated biomarker response (IBR) results suggested that the treated clams were subjected to the elevated toxicity of toxic M. aeruginosa in conditions of high pH. The heat shock proteins-related genes might be triggered to resist the oxidative damage in treated clams. Moreover, the upregulation of TNF and casp8 genes indicated the potential activation of the caspase8-mediated apoptotic pathway through TNF receptor interaction, potentially resulting in apoptosis. The TUNEL assay results further confirmed that apoptosis appeared in treated clams. These findings improve our understanding of the combined toxicological effects of harmful algae and pH shifts on bivalves, which will provide insights into a comprehensive ecological risk assessment of toxic CYBs to edible bivalve species.

Higher genotypic diversity and distinct assembly mechanism of free-living Symbiodiniaceae assemblages than sympatric coral-endosymbiotic assemblages in a tropical coral reef.

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.IMPORTANCESymbiodiniaceae 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.

Succession of diversity, assembly mechanisms, and activities of the microeukaryotic community throughout Scrippsiella acuminata (Dinophyceae) bloom phases.

Harmful algal bloom (HAB) is a rapidly expanding marine ecological hazard. Although numerous studies have been carried out about the ecological impact and the ecological mechanism of HAB outbreaks, few studies have comprehensively addressed the shifts of species composition, metabolic activity level, driving factors and community assembly mechanisms of microeukaryotic plankton in the course of the bloom event. To fill the gap of research, we conducted 18S ribosomal DNA and RNA sequencing during the initiation, development, sustenance and decline stages of a Scrippsiella acuminata (S. acuminata) bloom at the coastal sea of Fujian Province, China. We found that the bloom event caused a decrease in microeukaryotic plankton species diversity and increase in community homogeneity. Our results revealed that the RNA- and DNA-inferred communities were similar, but α-diversity was more dynamic in RNA- than in DNA-inferred communities. The main taxa with high projected metabolic activity (with RNA:DNA ratio as the proxy) during the bloom included dinoflagellates, Cercozoa, Chlorophyta, Protalveolata, and diatoms. The role of deterministic processes in microeukaryotic plankton community assembly increased during the bloom, but stochastic processes were always the dominant assembly mechanism throughout the bloom process. Our findings improve the understanding of temporal patterns, driving factors and assembly mechanisms underlying the microeukarytic plankton community in a dinoflagellate bloom.

In situ community transcriptomics illuminates CO2-fixation potentials and supporting roles of phagotrophy and proton pump in plankton in a subtropical marginal sea.

Lineage-wise physiological activities of plankton communities in the ocean are important but challenging to characterize. Here, we conducted whole-assemblage metatranscriptomic profiling at continental shelf and slope sites in the South China Sea to investigate carbon fixation potential in different lineages. RuBisCO expression, the proxy of Calvin carbon fixation (CCF) potential, was mainly contributed by Bacillariophyta, Chlorophyta, Cyanobacteria, and Haptophyta, which was differentially affected by environmental factors among lineages. CCF potential exhibited positive or negative correlations with phagotrophy gene expression, suggesting phagotrophy possibly enhances or complements CCF. Our data also reveal significant non-Calvin carbon fixation (NCF) potential, as indicated by the active expression of genes in all five currently recognized NCF pathways, mainly contributed by Flavobacteriales, Alteromonadales, and Oceanospirillales. Furthermore, in Flavobacteriales, Alteromonadales, Pelagibacterales, and Rhodobacterales, NCF potential was positively correlated with proton-pump rhodopsin (PPR) expression, suggesting that NCF might be energetically supported by PPR. The novel insights into the lineage-differential potential of carbon fixation, widespread mixotrophy, and PPR as an energy source for NCF lay a methodological and informational foundation for further research to understand carbon fixation and the trophic landscape in the ocean.IMPORTANCEMarine plankton plays an important role in global carbon cycling and climate regulation. Phytoplankton and cyanobacteria fix CO2 to produce organic compounds using solar energy and mainly by the Calvin cycle, whereas autotrophic bacteria and archaea may fix CO2 by non-Calvin cycle carbon fixation pathways. How active individual lineages are in carbon fixation and mixotrophy, and what energy source bacteria may employ in non-Calvin carbon fixation, in a natural plankton assemblage are poorly understood and underexplored. Using metatranscriptomics, we studied carbon fixation in marine plankton with lineage resolution in tropical marginal shelf and slope areas. Based on the sequencing results, we characterized the carbon fixation potential of different lineages and assessed Calvin- and non-Calvin- carbon fixation activities and energy sources. Data revealed a high number of unigenes (4.4 million), lineage-dependent differential potentials of Calvin carbon fixation and responses to environmental conditions, major contributors of non-Calvin carbon fixation, and their potential energy source.

The Combined Effects of Toxic Microcystis aeruginosa and Thermal Stress on the Edible Clam (Corbicula fluminea): Insights into Oxidative Stress Responses and Molecular Networks.

Cyanobacterial blooms (CYBs) have become a global environmental issue, posing risks to edible bivalves. Toxic cyanobacteria and thermal stress represent the two key co-occurring stressors to bivalves experiencing CYBs. To investigate the combined effects of these stressors on the edible bivalve Corbicula fluminea, the responses to oxidative stress and the molecular mechanisms of physiological adaptations in C. fluminea were examined under co-exposure to toxic Microcystis aeruginosa and thermal stress. The activity of antioxidant enzymes, including GST, SOD, CAT, GPx and GR, was significantly influenced by the interaction between temperature and M. aeruginosa (p < 0.05). A positive correlation was observed between toxic M. aeruginosa exposure and elevated SOD and GPx activities at 30 °C, demonstrating that SOD and GPx may help C. fluminea defend effectively against MCs under thermal stress. Furthermore, significant interactive effects between toxic M. aeruginosa and temperature were also observed in ROS and MDA (p < 0.05). The results of the PCA and IBR index also evidenced the apparent influence of toxic M. aeruginosa and thermal stress on oxidative stress responses of C. fluminea. The eggNOG and GO annotations confirmed that a substantial portion of differentially expressed genes (DEGs) exhibited associations with responses to oxidative stress and transporter activity. Additionally, KEGG analysis revealed that abundant DEGs were involved in pathways related to inflammatory responses, immune functions and metabolic functions. These findings improve our understanding of the mechanism of the physiological adaptation in bivalves in response to cyanotoxins under thermal conditions, potentially enabling the evaluation of the viability of using bivalves as a bioremediation tool to manage CYBs in eutrophic waters.

A comparative study reveals the relative importance of prokaryotic and eukaryotic proton pump rhodopsins in a subtropical marginal sea.

Proton-pump rhodopsin (PPR) in marine microbes can convert solar energy to bioavailable chemical energy. Whereas bacterial PPR has been extensively studied, counterparts in microeukaryotes are less explored, and the relative importance of the two groups is poorly understood. Here, we sequenced whole-assemblage metatranscriptomes and investigated the diversity and expression dynamics of PPR in microbial eukaryotes and prokaryotes at a continental shelf and a slope site in the northern South China Sea. Data showed the whole PPRs transcript pool was dominated by Proteorhodopsins and Xanthorhodopsins, followed by Bacteriorhodopsin-like proteins, dominantly contributed by prokaryotes both in the number and expression levels of PPR unigenes, although at the continental slope station, microeukaryotes and prokaryotes contributed similarly in transcript abundance. Furthermore, eukaryotic PPRs are mainly contributed by dinoflagellates and showed significant correlation with nutrient concentrations. Green light-absorbing PPRs were mainly distributed in >3 µm organisms (including microeukaryotes and their associated bacteria), especially at surface layer at the shelf station, whereas blue light-absorbing PPRs dominated the <3 µm (mainly bacterial) communities at both study sites, especially at deeper layers at the slope station. Our study portrays a comparative PPR genotype and expression landscape for prokaryotes and eukaryotes in a subtropical marginal sea, suggesting PPR's role in niche differentiation and adaptation among marine microbes.

Eukaryotic plankton community assembly and influencing factors between continental shelf and slope sites in the northern South China Sea.

Eukaryotic plankton are pivotal members of marine ecosystems playing crucial roles in marine food webs and biogeochemical cycles. However, understanding the patterns and drivers of their community assembly remains a grand challenge. A study was conducted in the northern South China Sea (SCS) to address this issue. Here, 49 samples were collected and size-fractionated from discrete depths at continental shelf and continental slope in the northern SCS over a diel cycle. From high throughput sequencing of the 18S rDNA gene V4 region, 2463 operational taxonomic units (OTUs) were retrieved. Alveolata and Opisthokonta overwhelmingly dominated the assemblages in the abundance (44.76%, 31.08%) and species richness (59%, 12%). Biodiversity was higher in the slope than the shelf and increased with depth. Temperature and salinity appeared to be the most important deterministic drivers of taxon composition. Community structure was influenced by multiple factors in the importance order of: environmental factors (temperature + salinity) > spatial factor > water depth > sampling time. Furthermore, the neutral model explained more variations in the smaller-sized (0.22-3 µm) community (24%) than larger-sized (3-200 µm) community (16%) but generally explained less variations than did deterministic processes. Additionally, our data indicated that the larger plankton might be more environmentally filtered and less plastic whereas the smaller plankton had stronger dispersal ability. This study sheds light on the differential contributions of the deterministic process and stochastic process and complexities of assembly mechanisms in shaping the community assembly of micro-nano and pico-eukaryotic biospheres in a subtropical ocean.

Diversity Distribution, Driving Factors and Assembly Mechanisms of Free-Living and Particle-Associated Bacterial Communities at a Subtropical Marginal Sea.

Free-living (FL) and particle-associated (PA) bacterioplankton communities play critical roles in biogeochemical cycles in the ocean. However, their community composition, assembly process and functions in the continental shelf and slope regions are poorly understood. Based on 16S rRNA gene amplicon sequencing, we investigated bacterial communities' driving factors, assembly processes and functional potentials at a subtropical marginal sea. The bacterioplankton community showed specific distribution patterns with respect to lifestyle (free living vs. particle associated), habitat (slope vs. shelf) and depth (surface vs. DCM and Bottom). Salinity and water temperature were the key factors modulating turnover in the FL community, whereas nitrite, silicate and phosphate were the key factors for the PA community. Model analyses revealed that stochastic processes outweighed deterministic processes and had stronger influences on PA than FL. Homogeneous selection (Hos) was more responsible for the assembly and turnover of FL, while drift and dispersal limitation contributed more to the assembly of PA. Importantly, the primary contributor to Hos in PA was Gammaproteobacteria:Others, whereas that in FL was Cyanobacteria:Bin6. Finally, the PICRUSt2 analysis indicated that the potential metabolisms of carbohydrates, cofactors, amino acids, terpenoids, polyketides, lipids and antibiotic resistance were markedly enriched in PA than FL.

The effect of environmental contamination on the community structure and fructification of ectomycorrhizal fungi.

Ectomycorrhizal fungi are an essential component of forest ecosystems, most of which can form edible and medical fruiting bodies. Although many studies have focused on the fructification of ectomycorrhizal fungi in phenology, the impact of environmental contamination, especially living garbage, on the formation of fruiting body is still unknown. A field investigation, combined with a high-throughput sequencing method, was used to study the effect of living garbage pollution on the fructification and hypogeous community structure of ectomycorrhizal fungi symbiosing with cedar (Cedrus deodara (Roxb.) G. Don). The results showed that garbage significantly altered soil abiotic and biotic properties, increasing soil urease activity, decreasing the soil exchangeable metal content and phosphatase activity, and ultimately inhibiting the formation of fruiting bodies. The pollution of garbage also changed the community structure of hypogeous ectomycorrhizal fungi where ectomycorrhizal ascomycetes dominated. In unpolluted sites, the relative abundance of ectomycorrhizal ascomycetes and basidiomycetes were almost equal. Although no fruiting bodies were observed in that soil polluted by living garbage, the sequencing result showed that various ectomycorrhizal fungi were present underground, suggesting that these taxonomic fungi had the potential to cope with adverse conditions. This study not only provided a deeper understanding of the relationship between ectomycorrhizal fungal communities and prevailing environmental conditions, but provided a new pathway for the excavation and utilization of the resource of antistress ectomycorrhizal fungi.

Application of a fast electrical pulse in gated multichannel plate camera.

An eight-frame gated microchannel plate (MCP) camera and a gating electrical pulse are described in this article. The gating electrical pulse is obtained by first generating a high voltage fast step pulse using avalanche transistors in Marx bank configuration, and then shaping it using avalanche diodes. The high voltage fast step pulse is about 200 ps in fall time and 4 kV in amplitude. The gating pulse wave form with width of 160 ps and amplitude of 2.5 kV is achieved. Each frame photocathode coated with gold on the MCP is part of a 12 Omega transmission line with open circuit end driven by the gating electrical pulse. The camera is tested by illuminating its photocathode with ultraviolet laser pulses, 266 nm in wavelength, which shows exposure time as short as 120 ps.