Degradation Of High Molecular Weight Polycyclic Aromatic Hydrocarbons By Selenastrum Capricornutum

Polycyclic aromatic hydrocarbons (PAHs) are a class of ubiquitous environmentalpersistent organic pollutants. They are demonstrated to be mutagenic, teratogenic andcarcinogenic. PAHs with high molecular weights are chemically stable and theirtoxicity increases with their molecular weights. Microbial degradation is the mainway to get rid of PAHs in the environment, including bacteria, fungi and microalgae.Studies on the degradation of PAHs have mainly focused on bacteria and fungi, withless attention on microalgae. As an effective PAHs degradation microorganism,microalgae attract increasing attentions. Selenastrum capricornutum, a freshwatergreen microalga, was selected as the research object because of its wide distribution inthe environment, easy cultivation and has PAHs degradation ability. The aim of theresearch was to study the degradation of PAHs by S. capricornutum, including theeffect of heavy metals and different light irradiations. According to the activeingredients, reactive oxygen species and metabolites of benzo[a]pyrene (BaP) by liveand dead algal cells, the PAHs degradation mechanism by S. capricornutum wereinvestigated. The degradation of high molecular weight PAHs by S. capricornutumwas thoroughly studied, it provided an effective complement to microbiological degradation of PAHs, and had the potential to bioremediation of PAHs-pollutedaquatic environment. The results of this study were summarized as follows:1. For low molecular weight PAHs (fluorene and phenanthrene), heavy metaldosage posed a significant, positive effect on their removal, in the treatment of thehighest dose of heavy metals (HM3, Cd2+:0.10mg L-1, Cu2+:1.0mg L-1, Ni2+:2.5mgL-1, Zn2+:1.0mg L-1), with up to99%of fluorene and89%of phenanthrene wereremoved from the medium in7days, which was mainly due to the cellulardegradation induced by heavy metal stress. For high molecular weight PAHs(fluoranthene, pyrene and benzo[a]pyrene), the presence of heavy metals did notaffect the removal efficiency.2. The addition of heavy metals had significant effect on the degradation offluorene. The metabolites of9-fluorenone and9-hydroxy fluorene were increasedwith the concentration of heavy metals. Exposure of the algal cells to the highest doseof heavy metals (HM3) resulted in the highest amounts of fluorene metabolites:9-hydroxy fluorene was the main metabolite, accounted for92.4%of total fluorenemetabolites. It was suggested that cellular degradation induced by heavy metals maybe the most important contributor to the degradation of fluorene. For phenanthrene,the low does of heavy metals (Cd2+:0.05mg L-1, Cu2+:0.05mg L-1, Ni2+:0.5mg L-1,Zn2+:0.05mg L-1) accelerated the degradation with highest level ofmonohydroxylated phenanthrene (8.78±0.87μg). Its removal was significantlyenhanced in HM3treatment but there was no corresponding increase in phenanthreneintermediates, implying that phenanthrene might have been further degraded to otherintermediates. One possible intermediate might be1-hydroxynaphthalene, which wasdetected in much higher quantity in HM3samples (3.33μg)) than in the other samples(0.13-0.47μg). The stress of heavy metals changed the degradation pathway ofphenanthrene. The metabolism of fluoranthene, pyrene and benzo[a]pyrene were notaffected by heavy metals.3. Gold light irradiation was more effective on the biodegradation of theselected PAHs in live algal cells than white light irradiation, but white light was moreeffective on PAHs photodegradation in dead cells. The degradation efficiency of seven PAHs, as well as the difference between live and dead microalgal cells, wasPAH compound-dependent. benz[a]anthracene (BaA) and benzo[a]pyrene (BaP) werehighly transformed in both cells and dead cells, benzo[b]fluoranthene (BbF),benzo[k]fluoranthene (BkF) and indeno[1,2,3-c,d]pyrene (IP) were transformed onlyin live cells, while dibenzo[a,h]anthracene (DA) and benzo[g,h,i]perylene (BghiP)were the most stable and recalcitrant compounds with least degradation in either liveor dead cells. Besides of BbF, BkF and IP, the degradation of the other4PAHs in deadalgal cells were more effective than live cells.4. Live cells of Chlorella sp. was less effective in degradation of BaP underwhite light irradiation, with only27.6%of BaP being degraded at Day7, than S.capricornutum which had70.8%of BaP degradation. The dead cells of both specieshad higher capabilities to transform BaP than live cells, and the degradationpercentages of BaP by S. capricornutum and Chlorella sp. were100%and98.1%,respectively at the end of7days of exposure. It suggested that the degradation of BaPin dead cells was photodegradation, which was algal species independent. BaPdegradation efficiency was correlative with the damage of algal cells. Hightemperature killed and freeze-thawed dead algal cells can promote thephotodegradation of BaP. The leakage of cellular contents from broken cells causedBaP degradation under irradiation. Chlorophyll extracted from algal cells canpromoted the degradation of BaP,98%of BaP was degraded on Day4, suggestingthat chlorophyll was the photodegradation active substance. Both the chlorophylldestroyed by heating in121oC for10min and irradiating under white light for4days,could also accelerate the BaP degradation, illustrating that chlorophyll with structuredestruction could still acted as photosensitizer.5. Hydroxyl radical and singlet oxygen were generated in live and dead algalcells of S. capricornutum. The production of hydroxyl radical in dead algal cells was83.6%more than live cells after4days exposure. The production of singlet oxygenwas higher than hydroxyl radical, live algal cells were more effective to generatesinglet oxygen. The spiked of BaP would consume some reactive oxygen species,leading to the reduction of hydroxyl radical and singlet oxygen. Reactive oxygen species have strong oxidizability, reactived with BaP in aquatic environment.6. For3-ring to4-ring PAHs (fluorene, phenanthren, fluoranthene and pyrene),most of the PAH metabolites by S. capricornutum were monohydroxylated PAHs. For5-ring PAH (BaP), dihydroxylated PAHs were the main metabolites. It was suggestedthat both monooxygenase and dioxygenase were existed in S. capricornutum. Themetabolism of low molecular weight PAHs was mianly by monooxygenase system,while via a dioxygenase pathway to metabolize high molecular weight PAHs. PAHshad biodegradation in live algal cell via oxygenase system, with hydroxylated PAHsas the main metabolites. PAHs had photodegradation in dead algal cells, the processwas that chlorophyll had photocatalytic generation of reactive oxygen species andthen had oxidation reaction with PAHs. The photodegradation products of PAHs wereketone and quinone compounds.