Antioxidant Capacity, Enzyme Activities Related to Energy Metabolism, and Transcriptome Analysis of Crassostrea hongkongensis Exposed to Hypoxia.

Crassostrea hongkongensis (C. hongkongensis) is one of the three most commonly cultivated oyster species in China. Seasonal hypoxia is one of the most serious threats to its metabolism, reproductive behavior, and survival. To investigate the effects of hypoxia stress on the antioxidant capacity and energy metabolism of C. hongkongensis, the total antioxidant capacity (T-AOC), glycogen content, and enzyme activities (phosphofructokinase, PFK; pyruvate kinase, PK; phosphoenolpyruvate carboxykinase, PEPCK) of oysters were determined under normoxic (DO 6 ± 0.2 mg/L) and hypoxic (DO 1.5 mg/L) conditions at 0 h, 6 h, 48 h, and 72 h. We also determined the T-AOC, glycogen content, and enzyme activities of oysters under reoxygenation (recovered to normoxia for 24 h). To further examine the potential molecular regulatory mechanism of hypoxic adaptation, a transcriptome analysis was conducted on the gill of C. hongkongensis under normoxia (N, 72 h), hypoxia (H, 72 h), and reoxygenation (R). After being exposed to hypoxia for 6 h, the T-AOC, glycogen content, and enzyme activities of PK, PFK, and PEPCK in C. hongkongensis were significantly decreased. However, after prolonging the duration of hypoxia exposure for 72 h, the T-AOC, glycogen content, and enzyme activities increased compared to that of 48 h. After 24 h reoxygenation, the T-AOC, glycogen content, and enzyme activity of PK and PFK returned to close to initial levels. In addition, a transcriptome analysis discovered 6097 novel genes by mapping the C. hongkongensis genome with the clean reads. In total, 352 differentially expressed genes (DEGs) were identified in the H vs. N comparison group (235 upregulated and 117 downregulated genes). After recovery to normoxia, 292 DEGs (134 upregulated and 158 downregulated genes) were identified in the R vs. N comparison group, and 632 DEGs were identified (253 upregulated and 379 downregulated genes) in the R vs. H comparison group. The DEGs included some hypoxia-tolerant genes, such as phosphoenolpyruvate carboxykinase (PEPCK), mitochondrial (AOX), tyramine beta-hydroxylase (TBH), superoxide dismutase (SOD), glutathione S-transferase (GST), and egl nine homolog 1 isoform X2 (EGLN1). Additionally, DEGs were significantly enriched in the KEGG pathways that are involved in hypoxia tolerance, including the metabolism of xenobiotics by cytochrome P450 pathways and the HIF-1 signaling pathway. Then, we selected the five hypoxic-tolerant candidate DEGs for real-time quantitative polymerase chain reaction (RT-qPCR) validation, and the results were consistent with the transcriptome sequencing data. These discoveries have increased our understanding of hypoxia tolerance, recovery ability after reoxygenation, and molecular mechanisms governing the responses to hypoxia in C. hongkongensis.

Exploring the environmental influences and community assembly processes of bacterioplankton in a subtropical coastal system: Insights from the Beibu Gulf in China.

Due to rapid urbanization, the Beibu Gulf, a semi-closed gulf in the northwestern South China Sea, faces escalating ecological and environmental threats. Understanding the assembly mechanisms and driving factors of bacterioplankton in the Beibu Gulf is crucial for preserving its ecological functions and services. In the present study, we investigated the spatiotemporal dynamics of bacterioplankton communities and their assembly mechanisms in the Beibu Gulf based on the high-throughput sequencing of the bacterial 16 S rRNA gene. Results showed significantly higher bacterioplankton diversity during the wet season compared to the dry season. Additionally, distinct seasonal variations in bacterioplankton composition were observed, characterized by an increase in Cyanobacteria and Thermoplasmatota and a decrease in Proteobacteria and Bacteroidota during the wet season. Null model analysis revealed that stochastic processes governed bacterioplankton community assembly in the Beibu Gulf, with drift and homogenizing dispersal dominating during the dry and wet seasons, respectively. Enhanced deterministic assembly of bacterioplankton was also observed during the wet season. Redundancy and random forest model analyses identified the physical properties (e.g., temperature) and nutrient content (e.g., nitrate) of water as primary environmental drivers influencing bacterioplankton dynamics. Moreover, variation partitioning and distance-decay of similarity revealed that environmental filtering played a significant role in shaping bacterioplankton variations in this rapidly developed coastal ecosystem. These findings advance our understanding of bacterioplankton assembly in coastal ecosystems and establish a theoretical basis for effective ecological health management amidst ongoing global changes.

Seasonal dynamics of antibiotic resistance genes and mobile genetic elements in a subtropical coastal ecosystem: Implications for environmental health risks.

Antibiotic resistance poses a considerable global public health concern, leading to heightened rates of illness and mortality. However, the impact of seasonal variations and environmental factors on the health risks associated with antibiotic resistance genes (ARGs) and their assembly mechanisms is not fully understood. Based on metagenomic sequencing, this study investigated the antibiotic resistome, mobile genetic elements (MGEs), and microbiomes in a subtropical coastal ecosystem of the Beibu Gulf, China, over autumn and winter, and explored the factors influencing seasonal changes in ARG and MGE abundance and diversity. Results indicated that ARG abundance and diversity were higher in winter than in autumn, with beta-lactam and multidrug resistance genes being the most diverse and abundant, respectively. Similarly, MGE abundance and diversity increased in winter and were strongly correlated with ARGs. In contrast, more pronounced associations between microbial communities, especially archaea, and the antibiotic resistome were observed in autumn than in winter. The co-occurrence network identified multiple interactions between MGEs and various multidrug efflux pumps in winter, suggesting a potential for ARG dissemination. Multivariate correlation analyses and path modeling indicated that environmental factors driving microbial community changes predominantly influenced antibiotic resistome assembly in autumn, while the relative importance of MGEs increased significantly in winter. These findings suggest an elevated health risk associated with antimicrobial resistance in the Beibu Gulf during winter, attributed to the dissemination of ARGs by horizontal gene transfer. The observed seasonal variations highlight the dynamic nature of antibiotic resistance dissemination in coastal ecosystems, emphasizing the need for comprehensive surveillance and management measures to address the growing threat of antimicrobial resistance in vulnerable environments.

Transcriptional responses of Crassostrea hongkongensis under high and low salinity stress.

Salinity, a key limiting factor, affects the distribution and survival of marine species. The Hong Kong oyster (Crassostrea hongkongensis), a euryhaline species found along the coast of the South China Sea, has become a major aquaculture bivalve species. To determine the molecular mechanism by which oysters respond to coastal waters with varying salinity levels, we used RNA-seq to sequence the gill samples of oysters exposed to normal (25 ‰, S25), low (5 ‰, S5) and high (35 ‰, S35) salinity conditions for one month. The results revealed different expression transcriptome levels among oysters living under low and high salinity conditions. Using high-throughput sequencing, we identified 811 up-regulated genes and 769 down-regulated genes. As determined by KEGG pathway mapping, the differentially expressed genes (DEGs) were significantly enriched in the prion diseases, histidine metabolism, arginine and proline metabolism, and beta-alanine metabolism pathways in both the S5 vs. S25 and S35 vs. S25 group comparison. Several DEGs including heat shock 70 kDa protein 12B-like, poly (ADP-ribose) polymerase (PARP), and tripartite motif-containing protein 2 (TRIM2), and low-density lipoprotein receptor-like, as well as KEGG pathways, including arginine and proline metabolism, apoptosis, PPAR signaling pathway, the thyroid hormone signaling pathway, were concerning response to salinity stress. Additionally, eight DEGs involved in salinity adaptation were selected for RT-qPCR validation, and the results confirmed the credibility of the transcriptome sequencing data. Overall, we designed a one-month, medium-term experiment to examine the responses of C. hongkongensis exposed to different levels of salinity stress and performed transcriptome analysis using high-throughput sequencing. Our results enhance current understanding of the molecular mechanisms of salinity stress responses in C. hongkongensis and provided insights into the osmotic biology of oysters.

Whole genome sequencing of Crassostrea ariakensis (Mollusca: Ostreidae) and C. hongkongensis expands understandings of stress resistance in sessile oysters.

To understand the environmental adaptations among sessile bivalves lacking adaptive immunity, a series of analyses were conducted, with special emphasis on the widely distributed C. ariakensis. Employing Pacbio sequencing and Hi-C technologies, whole genome for each of a C. ariakensis (southern China) and C. hongkongensis individual was generated, with the contig N50 reaching 6.2 and 13.0 Mb, respectively. Each genome harbored over 30,000 protein-coding genes, with approximately half of each genome consisting of repeats. Genome alignment suggested possible introgression between C. gigas and C. ariakensis (northern China), and re-sequencing data corroborated this result and indicated significant gene flow between C. gigas and C. ariakensis. These introgressed candidates, well-represented by genes related to immunity and osmotic pressure, may be associated with environmental stresses. Gene family dynamics modeling suggested immune-related genes were well represented among the expanded genes in C. ariakensis. These outcomes could be attributed to the spread of C. ariakensis.

Survival and physiological energetics of highly invasive mussels exposed to heatwaves.

The South American mussel, Mytella strigata, is a highly invasive fouling species of great concern along intertidal shores in East and Southeast Asia, posing serious threats to native biodiversity and ecosystems. Intertidal areas, being increasingly attacked by heatwaves over the last decade, are among the most thermally challenging habitats, yet the fate of this highly invasive mussel under scenarios of hotter heatwaves remains unknown. Here, we investigated how M. strigata responded to intensifying heatwaves frequently occurring in the South China Sea. Over 97% of individuals survived the five-day-lasting heatwaves, suggesting their high ability to cope with short-term heatwaves. Virtually unaffected clearance rate and absorption efficiency throughout the course of heatwaves indicate the maintenance of energy acquisition, and significantly decreased respiration rate implies the depression of energy metabolism, generating significant decreases in the O:N ratio when heatwaves occurred. Scope for growth of heatwaves-stressed mussels significantly decreased during initial exposure and then increased over time. These findings indicate the remarkable ability of M. strigata to cope with heatwaves recorded in its invasive habitats and call the attention for the rapid spread of this highly invasive fouling species in the context of climate change.