Study On The Diet Change And The Pollution Degree Of Polycyclic Aromatic Hydrocarbons In The Hadal Amphipods

The hadal zone（from 6 to 11 km water depth）is one of the least explored habitats on Earth,and the knowledge about trophic ecology of hadal organisms is very limited.Amphipods is an important species in the hadal ecosystem and plays a key role in the transfer of material and energy.This study analyzed fatty acids and carbon and nitrogen stable isotopes(δ¹³C,δ¹⁵N)of the hadal amphipods collected from three Pacific trenches,namely New Britain Trench（NBT;8.2–8.9km water depth）,Mariana Trench（MT;11km water depth）and Massau Trench（MS;6.9km water depth）.Amphipods from three hadal trenches were identified as Alicella gigantean（NBT）,Hirondellea gigas（MT）and Scopelocheirus schellenbergi（MS）,respectively.The goal of this dissertation is to determine the diet of amphipods in different hadal trenches and thereby assess its relationship to upper ocean biogeochemical property such as productivity（or food availability）.The predominance of C_18:1ω9,high C_18:1ω9/C_18:1ω7 and highδ¹⁵N values suggest that hadal amphipods are necrophagous in all three trenches.The inter-trench comparisons based on multiple biomarker proxies such as C_18:1ω9/C_18:1ω7,C_22:6ω3/C_20:5ω3,∑polyunsaturated/∑saturated fatty acids（∑PUFAs/∑SFAs）,andΣbranched fatty acids,as well as principal component analysis（PCA）show that the amphipods in the NBT are more dependent on high-quality organic matter（i.e.,carrion）,whereas those in the MT and MS utilize more detritus and bacterial organic matter as supplementary food.This inter-trench difference has been attributed to the effect of food availability that the NBT has higher net primary productivity（NPP）and a stronger terrestrial influence,whereas the MT and MS have the lower NPP and insignificant terrestrial influences.This study demonstrates that the diet of hadal animals is closely related to surface ocean biogeochemical property.In order to further explore how the diet changes from the juvenile to adult amphipods in the hadal environment and if there is any diet difference between sexes of hadal amphipods,this study has analyzed fatty acid biomarker and stable isotope ratios of the amphipod species（A.gigantean）that were collected from the same site in the NBT（8225 m depth）.Since all hadal amphipods were from the same site,the difference in food supply from upper ocean is minimal.A total of 50 samples were divided into five developmental stages according to body length（1–129 cm）and sex traits:Stages I（juvenile）,II（sub-adult）and III/IV/V（adult）.Adult amphipods could be further separated into male and female individuals according to sexual organs.Mean total fatty acid content for stage I(237.7±105.6 mg g^-1 dry weight)was significantly lower than that for stages II to V(476.4±49.0 mg g^-1dry weight).Theδ¹⁵N（14.7–18.4‰）positively correlated with the body length（R=0.33;P=0.03）,whereas the EPA/DHA(C_20:5ω3/C_22:6ω3)ratio（0.09–0.51）negatively correlated with the body length（R=–0.48;P<0.001）.These results suggested an increase of trophic level with growth.The PCA result revealed that stage I-individuals consumed more bacteria/zooplankton debris,stages II to IV-individuals primarily preyed on carrion,while stage V-individuals had more diverse diet（such as benthic fauna/algae and carrion）.Although the TFAs content was similar between the male and female amphipods（Stages III,IV and V）,their FA compositions were significantly different.Female amphipods had lower proportions of PUFAs,higher proportions of monounsaturated fatty acids（MUFAs）,and higherδ¹⁵N values compared to male amphipods,suggesting that female individuals rely more on carrion from higher trophic-level organisms.This sex-related difference,however,diminished from stage III to V.The fatty acid and stable isotopic data suggest an ontogenetic diet shift for hadal amphipods that may contribute to their success in the deepest ocean.This study also conducted compound-specific nitrogen isotope analysis of amino acids(CSIA-AAs;orδ¹⁵N_AA)and polycyclic aromatic hydrocarbons（PAHs）in order to test:（1）If there is relationship between feeding types and trophic levels（TL）of hadal amphipods;（2）if there is enrichment of pollutants in the hadal trench with increasing trophic level;and（3）if there is different PAHs sources with growth of hadal amphipods.Theδ¹⁵N_AAdata revealed that the average TL of 19 hadal amphipods was3.8±0.5.The TL for stage I（3.2±0.8）individuals was significantly lower than that for stagesⅢto V（3.9±0.2）individuals,which is consistent with the results of fatty acid biomarkers.A total of 7 PAHs were detected with the total PAHs content（∑PAHs）ranging from 305.0 to 5744.4 ng g^-1(wet weight and a mean of 1049.3 ng g^-1（wet weight）.The PAHs in hadal amphipods included the 2-ring（Nap）,3-ring（Ace,Fle,Phe,Ant）and 4-ring（Fla,Pyr）PAHs,which were dominated by Pyr and Phe.Overall,the content of high-molecular-weight PAHs was low,while the pollution of low-molecular-weight PAHs was relatively high.There was a significant negative correlation between∑PAHs and their TL（R²=0.4,P<0.01）,but no biomagnification phenomenon was found.The results of source analysis based on molecular indicators of PAHs showed that the PAHs in hadal amphipods were mainly from petroleum,coal combustion,coking and other mixed sources.The comparison between different stages of hadal amphipods suggested that the PAHs of juvenile amphipods were mainly derived from the incomplete combustion of fossil fuels,whereas the sources of PAHs of sub-adult and adult individuals were more diverse,including petroleum pollution sources,and to less extent,high-temperature combustion sources of fossil fuels.In conclusion,this dissertation has investigated the diet changes of hadal amphipods in different hadal trenches and among different ontogenetic stages.The results have demonstrated that the feeding habits of hadal amphipods was closely related to surface ocean biogeochemical property,and there is a significant diet change of the amphipods during the ontogenetic process,which may be a strategy for hadal amphipods to better adapt to the extreme and food limited environment.This dissertation also examined the pollution degree of PAHs for the first time,showing no significant biomagnification of PAHs likely due to low assimilation efficiencies and efficient metabolic transformation for hadal amphipods with higher trophic levels.In sum,the diversity and plasticity of diet and food feeding of amphipods in hadal trenches are an important contributor to their success in the deepest ocean reflected by a large number of individuals.