Zostera marina (eelgrass) is classified as one of the marine angiosperms and is widely distributed throughout much of the Northern Hemisphere. The present study investigated the microbial community structure and diversity of Z. marina growing in Futtsu bathing water, Chiba prefecture, Japan. The purpose of this study was to provide new insight into the colonization of eelgrass leaves by microbial communities based on leaf age and to compare these communities to the root-rhizome of Z. marina, and the surrounding microenvironments (suspended particles, seawater, and sediment). The microbial composition of each sample was analyzed using 16S ribosomal gene amplicon sequencing. Each sample type was found to have a unique microbial community structure. Leaf-attached microbes changed in their composition depending on the relative age of the eelgrass leaf. Special attention was given to a potential microbial source of leaf-attached microbes. Microbial communities of marine particles looked more like those of eelgrass leaves than those of water samples. This finding suggests that leaf-attached microbes were derived from suspended particles, which could allow them to go back and forth between eelgrass leaves and the water column.

Onboard microcosm experiments were conducted to assess how bacterial growth pattern and community structure changed by the addition of labile organic compound during the KH-14-2 cruise of R/V Hakuho Maru (Atmosphere and Ocean Research Institute, the University of Tokyo and JAMSTEC) in May–June 2014. Seawater samples were collected from the three diversified oceanic environments, Kuroshio Current, North Pacific Sub-polar Gyre (SPG), and North Pacific Sub-tropical Gyre (STG) in the western North Pacific Ocean, filtered, supplemented with glucose, and incubated at 23 ± 1 °C, ~ 4 °C, and 23 ± 1 °C, respectively. Untreated control microcosms were also maintained for all the sample types. Significant increases in cell counts and cell sizes were observed in Kuroshio Current and STG waters, whereas in SPG neither the counts nor the sizes changed, even after 120 h of incubation. At early stages of incubation, the classes Bacteroidia, Alphaproteobacteria, and Gammaproteobacteria were dominant in the Kuroshio Current and SPG samples, while the phyla Cyanobacteria and Proteobacteria in the STG samples. Over incubation periods between 60 and 96 h, some members of the class Gammaproteobacteria gradually dominated within which the genera Vibrio and Alteromonas became dominant in the Kuroshio Current and STG, respectively. No growth was detected for the microcosms with seawater from SPG, regardless of glucose amendment. It is concluded that depending on the environmental condition, certain different bacterial groups proliferated quickly and modified the community structures. Temperature significantly influenced the growth and succession, and ultimately the community structure of bacteria.

As the seagrass leaves remain underwater, the primary source of leaf microbes is considered to be seawater heterotrophic bacterioplankton, which possess the ability to degrade biopolymers and are known to attach to surface and form biofilms. In this study, 16S rRNA gene amplicon sequencing was used to assess bacterial diversity and dynamics of the particle associated (PA) and free-living (FL) fraction of the seagrass-covering seawater (inside) and bulk seawater (outside) among different seagrass bed around Japan. Samples were collected from the three Zostera marinabeds (Ikuno-shima Is., Hiroshima; Nanao Bay, Ishikawa; Mutsu Bay, Aomori Prefecture) around Japan during summer (June-August 2015; July 2016). Prokaryotic DNA was extracted from samples using a FastDNA spin kit according to the manufacturer’s protocol. After extracting DNA, 16S ribosomal RNA (16S rRNA) genes were sequenced by Illumina Miseq platform. The Results showed that PA bacterial communities had a higher (p <0.001) diversity than FL ones. Compared to the outside of the seagrass bed, the inside had lower diversity both in PA and FL fraction. Taxonomic analysis revealed a different community composition between lifestyle (PA vs FL) and sampling point (inside vs outside). Differential abundance analysis showed that PA were significantly enriched in a diversity of Cyanobacteria (Synechococcaceae), Saprospiraceae and Hyphomonadaceae. Conversely, FL were more abundant in Gammaproteobacteria (including Halomonadaceae, Alteromonadaceae), Microbacteriaceae, Campylobacteraceae, Pelagibacteraceae,Acidimicrobiia(OCS155). The present data provide a comprehensive description of the PA and FL microbial community in the seagrass bed and can be useful for better understanding the seagrass microbe interactions.

Heavy metal contamination of fish is a global safety concern for the consumers. Atomic absorption spectrometry (AAS) was used to measure the concentration of three HMs (As, Pb, and Cr) in two commercially farmed fish species that are widely consumed: Oreochromis niloticus (tilapia) and Pangasianodon hypophthalmus (pangasius). This investigation was done to determine whether there was any potential health risk. The concentration of the studied heavy metals was within standard acceptable limits. As, Cr, and Pb had respective mean concentrations of 0.037 ± 0.023 mg/kg, 0.005 ± 0.002 mg/kg and 0.029 ± 0.015 mg/kg in the case of tilapia; and 0.049 ± 0.017 mg/kg, 0.007 ± 0.005 mg/kg and 0.024 ± 0.009 mg/kg, respectively, in the case of pangasius. Hierarchical clustering revealed that As may come through seepage and groundwater, while Cr and Pb are the products of contaminated feeds. ALP and ATPase activity varied significantly amongst the fishes' various organs. However, multiple regression analysis revealed that the current concentration of HMs in tilapia and pangasius was insufficient to predict enzymatic activity except for ALP in pangasius gill. The results of calculations for estimated daily intake (EDI), target hazard quotient (THQ), hazard index (HI), and carcinogenic risk (CR) indices made it abundantly evident that eating the fish under study posed no significant harm to the consumers' health. In conclusion, the contamination level of farmed tilapia and pangasius sold in the study area was within the permissible limit, however regular monitoring is needed to assure safe production.