| The colonization characteristics of bacterial communities on microplastics or plastic debris(PD)in the marine environments have generated great concern in recent years.However,the influence of environmental factors and polymer types on the formation of bacterial communities on PD in estuarine areas is less studied.To gain additional insights,five types of PD(polyvinyl chloride,polypropylene,polyethylene,polystyrene and polyurethane)were exposed for three-time periods(two weeks,four weeks and six weeks)at five interaction zones between the freshwater and seawater(F-S zones)and four special zones(Sp zones)deeply influenced by anthropogenic activities.16 S r RNA gene sequencing was used to identify the bacterial communities on PD,in seawater and sediment samples.In this study,we mainly investigate the spatial and temporal influences affecting the composition of bacterial communities on PD in Haihe Estuary and determine the key environmental factors.The results indicate that after the six-week exposure,the biomass on PD decreased by 41% from S1 to S5.In addition,the biomass on PS was significantly higher than on other polymer types,indicating that polymer type also influenced the biofilm formation.Nutrients(TN and TP)and salinity were the main factors influencing the average growth rate of biofilm.The nutrients(total nitrogen(TN)and total phosphorus(TP))showed a positive correlation with the average growth rate of biofilm,while salinity showed a negative correlation with the average growth rate of biofilm.The Chao 1index increased from S1 to S3,suggesting that the increase in bacterial species on PD from S1 to S3 could be attributed to the increase in salinity.Moreover,the variations in the Chao 1 index in the Sp zones(S6,S7,S8 and S9),which could be ascribed to the similar salinity at these sites,were not significant compared with the variation from F-S zones(from S1 to S5).Salinity should be the primary factor affecting the bacterial diversity,while temperature,dissolved oxygen and nutrients(TN and TP)may have a secondary effect under similar salinity conditions in the Haihe Estuary of Bohai Bay.The physiological characteristics of the bacterial communities on PD changed obviously with time.At first,the bacterial communities that could use various substances as carbon source and adapt to complex environment were the main bacterial genuses.At four-week exposure,the relative abundances of some potential pathogens increased.The amount of photoautotrophic and chemoautotrophic bacteria increased during the six-week exposure.Although the bacteria could settle on different types of PD,the marker genera indicated that the bacteria had selectivity towards the polymer type of PD and tended to colonize their preferred substrate.The marker genera on each type of PD(ANOVA,p < 0.05)were Bacillus and Shewanella for PVC,Bdellovibrio and Lewinella for PP,Faecalibacterium and Veillonella for PE,Pseudoalteromonas and Alteromonas for PS and Alcanivorax for PU.The relative abundance of major genera among seawater samples,PD and sediments was significantly different.Meanwhile,Vibrio,as a potentially pathogenic bacteria could also colonize on PD and had a higher relative abundance compared with those on SW and SD in S4,S5,S6,S8 and S9 after the four-week exposure.Furthermore,the results showed a positive correlation between salinity and the enrichment of Vibrio colonizing PD in the Haihe River to the Bohai Bay.Briefly,in the estuarine environment(S4,S5,S6,S8 and S9)with salinity ? 26(± 2‰),the relative abundance of Vibrio on PD(at 4 weeks and 6 weeks)was 2-10 times higher than that of SW and SD.While in the mixed system of freshwater and seawater(S1 and S2)with salinity ? 18(± 2‰),the enrichment of Vibrio colonizing PD was not obvious.In addition,potentially pathogenic indexes of PD were higher than that of SW and SD(ANOVA,p < 0.05),indicating that the potentially pathogenic risk of PD in the marine environments. |
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