| During environmental migration,large plastic fragments degrade into tiny plastic particles（e.g.,microplastics（MPs）,5 mm）by natural forces such as ultraviolet radiation and wind,bringing a higher hazard to ecosystems.Existing research indicates that MPs are widely spread in the ocean,but their distribution in the coastal waters of Hainan Island is still poorly understood.Meanwhile,the rising demand and use of petroleum products have resulted in a major increase in the extent of oil pollution in the marine environment,resulting in a broad range of ecological and environmental issues.Specifically,the interaction between petroleum and MPs,which are abundant in the marine environment,and their environmental impacts are unknown.MPs degrade in seawater,causing changes in their physicochemical properties,and these irreversible alterations may have further consequences on their environmental behavior and biotoxicity.Large specific surface area and strong hydrophobicity make it simple for MPs to absorb other pollutants and form new complex pollutants,which may increase their biotoxicity.Currently,the majority of research on the aging of MPs and the biological and toxicological effects of their coexistence with pollutants is conducted in simulated indoor environments or a sole environmental medium.Nevertheless,there is a lack of research on the aging of MPs in complex environments such as real seawater,especially the aging time gradient and the coexistence of pollutants（e.g.,petroleum）.Therefore,it is essential to determine the aging characteristics and mechanisms of MPs,as well as the interaction between petroleum and MPs and their toxicological effects on microalgae.This research mainly found a certain level of MPs pollution in the coastal waters of Hainan Island.MPs would modify their environmental behavior and inhibit marine microalgae as their aging period increased,providing a reference for increasing the monitoring,prevention,and control of MPs as a new pollutant and determining their ecological risk in coastal waters.The specific research contents and conclusions of this dissertation are listed as follows:（1）In this study,seawater and sediment samples from six functional areas（i.e.,harbor,industrial district,sparsely populated area,tourist area,residential area,and aquaculture area）in the coastal waters of Hainan Island were collected,and the distribution,abundance,morphology,and composition of MPs were systematically investigated.The abundance of MPs in seawater ranged from 0.46 to 19.32 items/L.The largest concentration was in Qinglan Bay Estuary,while the lowest was on Sanya West Island.The abundance of MPs in sediment ranged from 41.18 to 750.63 items/kg,with an average of 372.47±62.10items/kg;the highest concentration was found at Tanmen Port,while the lowest was found in the Lingao Sea.It was detected that the MPs with smaller sizes exhibited a higher concentration.MPs were commonly black and white and predominantly linear and fragmented in shape.The dominating polymer was polyethylene terephthalate（PET）,which could be derived from laundry effluent.In the studied area,the petroleum concentration was 0.02-0.21 mg/L,with harbors being the most severely polluted.Additionally,this study found a substantial correlation between MPs pollution and petroleum in seawater,suggesting MPs and petroleum-based pollution shared similar sources.This study evaluates the characteristics of MPs pollution and their relationship with petroleum in Hainan Island,giving critical data for future research on marine ecosystem protection.（2）Based on the current situation and occurrence of MPs pollution in the coastal waters of Hainan Island in the previous chapter,the common polymers（polyethylene microparticles（mPE）,polypropylene microparticles（m PP）,polypropylene microparticles（m PS）,polyamide microparticles（m PA）and polyvinyl chloride microparticles（m PVC））were selected as the research objects.MPs with different particle sizes（13,165,and 550μm）were exposed to ultraviolet rays（UVA）for 15,30,90,and 180 days to simulate natural light-aging in a real seawater environment.The results show cracks,oxidation particles,wrinkles,and other surface changes emerge on MPs as they age.The amount of surface weathering was directly related to the aging time and was substantially connected with it.The surface roughness of MPs was the highest after 180 days of aging,and the aging phenomenon of mPE and m PP was obvious.The absorption peak intensity of the functional groups on the surface of MPs increased,and new peaks were formed.The crystallinity of MPs increased but decreased with aging time.（3）Following the aforementioned indoor aging simulation experimental results,mPE with evident physical and chemical property changes under ultraviolet aging settings was selected for further investigation.Herein the adsorption of petroleum on mPE（165,550μm）undergoing aging（Day 0,15,30,90,and 180）.The adsorption behavior of mPE to petroleum was further evaluated at varying p H（2,5,7.32,10,and 12）,temperature（4,15,25,45,and 65℃）,and in the presence of coexisting pollutants（Cu,bisphenol A（BPA）and petroleum）.The results showed that the adsorption capacity of mPE raised with a longer aging period and smaller-sized particles,while the adsorption capacity of 550 and 165μm size aged MPs increased by 12.7%-50.9%and 22.1%-63.9%,respectively.The adsorption kinetics and isotherm model of mPE on petroleum were well fitted by pseudo-second order,intraparticle diffusion,Freundlich and Langmuir models,showing the sorption behavior was controlled by the diffusion of pores,liquid film diffusion,and surface adsorption.The petroleum adsorption capacity of mPE was predominantly affected by surface roughness,specific surface area,hydrophobicity,oxidation functional groups,adsorption sites,and hydrogen bonds.At the same time,Zeta potential and crystallinity may not be the crucial factors.The maximum adsorption capacity of mPE to petroleum was achieved at p H 7.32and 25°C.The coexistence of multiple contaminants,such as Cu,BPA,and petroleum,will compete for adsorption sites on the mPE surface.These findings could provide theoretical data for the interaction and corresponding environmental behaviors when MPs coexist with other pollutants,especially for the environmental risk assessment in some specific areas（e.g.,ports）where the combined pollution of MPs and petroleum is relatively serious.（4）This chapter extends the previous chapter’s investigation of the interaction between mPE and petroleum pollutants.Specifically,it delves into the biotoxicity of individual mPE and petroleum and their combined effects on Chlorella vulgaris.Additionally,this chapter conducts a synthetic analysis of the variations in physical and chemical attributes of MPs and the interaction of petroleum pollutants with algal cell growth.This study aims to investigate the physiological responses of algal cells to both single and combined exposure to microplastics（MPs）and petroleum pollution.The single MPs at various particle sizes（i.e.,13,165,and 550μm）,concentrations（i.e.,0.01,0.1,and1 g/L）,and aging degree（i.e.,aged for 0 d and 90 d under UVA）,single petroleum（5%water accommodated fraction,WAF）,and their combinations（i.e.,5%WAF+165μm–0.1 g/L–aged 0 d mPE,5%WAF+165μm–0.1 g/L–aged 90 d mPE）,were found to pose toxicities to C.vulgaris,which inhibited the cell density,reduced chlorophyll,promoted the secretion of extracellular polymers（EPS）and increased oxidative stress response to resist pollutants.Furthermore,compared with the toxicity of petroleum on microalgal transcriptional function,mPE caused less toxicity to C.vulgaris and only impacted the posttranslational modification,protein turnover,and signal transduction processes.Most importantly,mPE reduced petroleum toxicity in C.vulgaris by regulating the ABC transporter,eukaryotic ribosome synthesis,and the metabolic pathways of the citrate cycle.Overall,this study elucidated the biotoxicity of MPs and petroleum compounds to C.vulgaris based on cell growth,physiological and biochemical responses,and transcriptomics and provided a warning for the combined biotoxicity and potential ecological risks of environmental MPs coexisting with other pollutants.（5）Based on the complex toxicity of C.vulgaris caused by the coexistence and interaction of mPE and petroleum pollutants found in the previous chapter,this chapter further explored the related mechanism of the above complex pollution on C.vulgaris biotoxicity in terms of microbial diversity in the habitat and nutrients utilization.The results revealed that the microbial diversity of the microcosm in the biotopes of C.vulgaris changed upon exposure to mPE and petroleum.The external factors,including the size of MPs particles,their concentration,aging time,and their combination with petroleum,were found to affect microbial diversity.The cellular utilization of elements such as Fe,Si,Ca,and Mg was inhibited,whereas the uptake of Mn,NO3-–N,and PO43-–P increased as compared to the control experiments.Furthermore,a structural equation model analyzed the relationship between nutrients and growth indicators.The results indicated that Fe and Mn directly affected the indirect NO3-–N absorption by C.vulgaris,which indirectly affected the dry cell weight（DCW）of the microalgae.The path coefficient of Fe and Mn affecting nitrate was 0.399 and 0.388,respectively.The absorption of N was the key step for C.vulgaris to resist stress.This study provides a novel analysis of the effects of MPs on microalgae growth from the perspective of microbial diversity in the habitat and nutrient elements for exploring the toxicity mechanisms.（6）The leaching of MPs additives and their negative effects on aquatic organisms remain to be systematically elucidated.This study investigated the toxicological effects of MPs leachate（mPE and m PVC after being accelerated leached by UVA for 15,90,and 180days in seawater）on microalga C.vulgaris in terms of cell growth inhibition,oxidative stress,and transcriptomes.Subsequently,the constituents of leachate from MPs that had undergone a 90-day aging process were analyzed in order to gain insight into the potential toxicity mechanisms of MPs on microalgal cells.The results revealed that both the leachates of mPE and m PVC resulted in an inhibited cell growth and increased oxidative stress of C.vulgaris,accompanied by a growth inhibition rate to microalgal cells of4.0%-36.2%and 7.1%-48.2%,respectively.Meanwhile,under identical mass concentration,the toxicological impacts on C.vulgaris exhibited a sequence of m PVC leachate>mPE>mPE leachate>m PVC.In contrast,MPs leaching time indicated no change in MPs leached toxicity.Moreover,the gene functions of"translation,ribosomal structure,and biogenesis"are mostly affected by MPs leachate.The inhibitory effects of m PVC leachate on microalgal cells are comparatively stronger than those of mPE leachate and pure MPs.This can be attributed to the leaching of a greater amount of substances from the polymer of m PVC,which primarily include Zn,farnesol isomer a,2,6-di-tert-butyl-4-methylphenol,and acetyl castor oil methyl ester.To summarize,this work improved our understanding of the ecotoxicological effects of MPs and MPs leachate.Meanwhile,it warned about the ecological risk posed by plastic additives. |
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