| Research Abstract |
Recently, considerable attention has been focused on the environmental impact of fish culture in coastal areas. A large input of unconsumed feeds and fecal matter from fish farms into the environment results in an organic enrichment in sediments leading to an increased oxygen demand for microbial processes and ultimately highly reduced or anoxic conditions. Although microbial communities are principal playrs in decomposition processes, changes in microbial communities in areas of organic enrichment, which will indicate the environmental health, have not however been examined due to the paucity of suitable methods. Signature lipid biomarker (SLB) analysis, which is reported to be one of the more sensitive chemical methods, was used to examine the seasonal and regional variation in microbial biomass and its community structure along with sediment characteristics in fish culture areas and their ambient environment.
Eight and three stations from fish culture and ambient areas were selected,
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respectively from culture farms in Tashima and Yokoshima of Hiroshima Prefecture and surface sediment samples were collected during four seasons. Sediment samples were analyzed for temperature, pH,oxidation-reduction potential (Eh), acid volatile sulfide-sulfur (AVS-S), weight loss on ignition (IL), lipid phosphate, and phospholipid ester-linked fatty acids (PLFA). In laboratory, incubation experiments were conducted to evaluate the influence of feed on microbial biomass. Sediments from fish.culture areas were characterized by low Eh, and high IL and AVS-S,indicating that the sediments were either under highly reduced or anoxic condition. The characteristics of sediments from ambient areas were totally different from fish culture areas. Seasonal was less pronounced than regional variation of sediment parameters, except temperature. Lipid phosphate, an indicator of microbial biomass, showed a marked regional variation. Sediments from fish culture areas contained significantly higher concentrations of lipid phosphate than the ambient sediments, indicating the influence of high organic matter input from fish farms. Results of incubation experiments revealed that the levels of lipid phoshphate in sediments added with feed were always high in comparison with the control, supporting the influence of feed on the levels of organic matter and lipid phosphate. Marked regional variation of PLFA groups was observed with high levels in fish culture areas and low values in ambient areas. The proportions of eight microbial biomarkers PUFA (microeukaryotes), 16 : 1,17 : 1 and 18 : 1 (bacteria), 10Me 16 : 0 (SRB ; Desulfobacter sp.), br17 : 1 (Desulfovibrio sp.), iso and anteiso of 15 : 0 and 17 : 0 (anaerobes) were used to describe the microbial community structure. The microbial biomarkers, except 10Me 16 : 0 and br17 : 1, in fish culture areas were significantly higher than that in ambient areas.
Based on the annual mean values of lipid phosphate, the study area was divided into Level I (<=30 nmol/g) : low values of IL,AVS and microbial biomarkers, and high values of Eh (oxic) ; Level IV (>=100 nmol/g) : high values of IL,AVS and microbial biomarkers and low values of Eh (highly anoxic) ; and Level II (31-70 nmol/g) and Level III (71-99 nmol/g) : levels of those parameters were found between Levels I and IV.Sediment parameters and microbial biomarkers differed significantly among these areas, except between Level II and III,indicating the differences in microbial community structure of sediments. The variations of microbial biomarkers at a narrow range of IL (8-10%) and Eh (-200 to -100 mV) were significantly greater than that observed in a range of IL (3-8%) and Eh (>-100 mV). Effect of fish culture on ambient areas was not evident. These results suggest that changes which are not detectable from sediment parameters can be easily detected by biomarker analysis. Furthermore, the level of 100 nmol/g lipid phosphate can be considered as a critical level and a regular monitoring of lipid phosphate will provide information about the status of environmental health. Less
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