Succession and seasonal variation in epilithic biofilms on artificial reefs in culture waters of the sea cucumber Apostichopus japonicus

Periphytic biofilms in aquaculture waters are thought to improve water quality, provide an additional food source, and improve the survival and growth of some reared animals. In the AsiaPacific region, particularly in China, artificial reefs are commonly used in the commercial farming of sea cucumbers. However, few studies have examined the epilithic biofilms on the artificial reefs. To gain a better understanding of the succession of epilithic biofilms and their ecological processes in sea cucumber culture waters, two experiments were conducted in culture waters of the sea cucumber Apostichopus japonicus in Rongcheng, China, using artificial test panels. On the test panels of succession experiment, more than 67 species were identified in the biofilms. On the test panels of seasonal variation experiment, more than 46 species were recorded in the biofilms. In both experiments, communities of epilithic biofilms were dominated by diatoms, green algae and the annelid Spirorbis sp. In the initial colonization, the dominant diatoms were Cocconeis sp., Amphora spp. and Nitzschia closterium in June, which were succeeded by species of Navicula, Cocconeis and Nitzschia(July to September), and then by Licmophora abbreviata, Nitzschia closterium and Synedra spp. in the following months. A diatom bloom in the autumn and filamentous green algae burst in the summer were also observed. Ecological indices well annotated the succession and seasonal changes in epilithic communities. Multidimensional scaling(MDS) analysis found significant differences in diatom community composition among months and seasons. Fast growth of biofilms was observed in the summer and autumn, whereas the biomass of summer biofilms was largely made up of filamentous green algae. Present results show that the components of epilithic biofilms are mostly optimal foods of A. japonicus, suggesting that biofilms on artificial reefs may contribute important nutritional sources for sea cucumbers during their growth seasons. Future works should include quantitative determination of the contribution of epilithic biofilms to the diet of A. japonicus, potential roles of epilithic biofilms in regulating the water quality of sea cucumber ponds, and the regulation of epilithic biofilms in sea cucumber culture ponds.

Numerical Modeling of Mass Transfer in the Interaction between River Biofilm and a Turbulent Boundary Layer

In this article dedicated to the modeling of vertical mass transfers between the biofilm and the bulk flow, we have, in the first instance, presented the methodology used, followed by the presentation of various results obtained through analyses conducted on velocity fields, different fluxes, and overall transfer coefficients. Due to numerical constraints (resolution of relevant spatial scales), we have restricted the analysis to low Schmidt numbers (Sc=0.1, Sc=1, and Sc=10) and a single roughness Reynolds number (Re*=150). The analysis of instantaneous concentration fields from various simulations revealed logarithmic concentration profiles above the canopy. In this zone, the concentration is relatively homogeneous for longer times. The analysis of results also showed that the contribution of molecular diffusion to the total flux depends on the Schmidt number. This contribution is negligible for Schmidt numbers Sc≥0.1, but nearly balances the turbulent flux for Sc=0.1. In the canopy, the local Sherwood number, given by the ratio of the total flux (within or above the canopy) to the molecular diffusion flux at the wall, also depends on the Schmidt number and varies significantly between the canopy and the region above. The exchange velocity, a purely hydrodynamic parameter, is independent of the Schmidt number and is on the order of 10% of in the present case. This study also reveals that nutrient absorption by organisms near the wall depends on the Schmidt number. Such absorption is facilitated by lower Schmidt numbers.

Epilithic biofilm as a reservoir for functional virulence factors in wastewater-dominant rivers after WWTP upgrade

Virulence factors(VFs)confer upon pathogens the ability to cause various types of damage or diseases.Wastewater treatment plants(WWTPs)are important point sources for the emission of pathogens and VFs into receiving rivers.Conventional WWTP upgrades are often implemented to improve the water quality of receiving ecosystems.However,knowledge on the pathogens,VFs,and health risks to receiving aquatic ecosystems after upgrade remains limited.In this study,we investigated detailed pathogenic information,including taxa,pathogenicity,and health risk,in two wastewater-dominant rivers after WWTP upgrade.Using 16S rRNA gene sequencing,we screened 14 potential pathogens in water and epilithic biofilm samples,though they were significantly more enriched in the biofilms.Combining 16S rRNA and metagenomic sequencing data,we identified Pseudomonas and Aeromonas as the dominant pathogenic taxa carrying functional VFs(e.g.,mobility and offensive)in the epilithic biofilm.Moreover,strong pathogen-specific VF-host co-occurrence events were observed in the epilithic biofilm samples,indicating the importance of biofilms as reservoirs and vehicles for VFs.Further,we demonstrated that mobility VF is crucial for biofilm formation and pathogens in biofilm carrying offensive VF may be highly invasive.Quantification and health risk assessment suggested that the skin contact risk of P.aeruginosa carrying VFs was higher than the acceptable probability of 10^(-4)in both water and epilithic biofilm samples,which may threaten ecological and human health.