Methylmercury in aquatic invertebrates : toxicological interaction with environmental factors and pollutants

국문 초록 (Abstract)

메틸수은(MeHg)은 수생 생태계에 지속적으로 잔존하는 독성 물질이지만, 실제 독성의 발현은 단일 요인보다 환경 속 여러 스트레스가 맞물릴 때 크게 달라진다. 본 논문은 무척추 수생모델을 대상으로 미세플라스틱과 수온 상승이 MeHg의 생물농축과 독성 발현을 어떻게 바꾸는지 살펴보고, 개체 수준의 생리·행동 지표를 산화스트레스, 신경계, 생식 관련 분자 반응과 통합해 해석하였다.
담수종 Daphnia magna를 이용해 생물유래 고분자인 PLA과 석유유래 저밀도 LDPE을 MeHg 단독 및 공노출 조건에서 비교하였다. PLA는 생애사 지표를 저하시켰고 MeHg 단독 대비 체내 MeHg 축적을 높여, 재질에 따라 운반 및 표면 상호작용 양상이 다름을 보여주었다. 반면 LDPE는 동일 조건에서 축적 변화가 제한적이었다. 또한 PLA는 산화적 부담을 높이고 항산화 방어계를 재조정한 반면, LDPE와 MeHg의 공노출에서는 AChE 활성이 더 크게 억제되는 등 신경계 기능 저하가 두드러졌다. Vitellogenin을 포함한 생식 관련 전사 반응 역시 두 재질에서 상이한 패턴을 보였으며, 이는 내분비 관련 작용 경로가 재질 특성에 따라 달라질 수 있음을 시사한다.
다은 생물군에서 온도–오염물 상호작용을 조사하였다. 해양 로티퍼에서는 수온 상승이 MeHg의 독성 표현을 강화하고 기능 저하를 앞당겼으며, 발달 단계에 따라 항산화 반응과 수은 분포가 달라졌다. 담수 D. magna에 적용한 결과, 수온 상승은 MeHg에 대한 급성 내성을 낮추고, 생존·번식·부화·유영 등 만성 지표의 손상을 심화시켰다. 수온 상승만으로는 항산화 체계가 활성화되었으나, MeHg와의 공노출에서는 이러한 방어가 약화되었고, 신경·운동 기능 억제는 보다 뚜렷해졌다. Vitellogenin과 ABC 수송체 반응은 온도 의존적으로 교란되어 생식 및 해독 경로가 함께 영향을 받음을 확인하였다. 한편 MeHg의 체내 축적은 온도와 무관하게 노출 농도에 비례해 증가하였으며, 이는 열 환경의 영향이 주로 흡수 증가보다는 독성 표현의 증폭으로 나타남을 분명히 한다.
종합하면, 재질 특성(PLA/LDPE)과 온도 상승은 MeHg 취약성을 결정짓는 핵심 조절자이다. 생물기반과 석유기반 미세플라스틱은 MeHg의 흡수와 경로별 독성에 서로 다르게 관여하고, 수온 상승은 산화환원 항상성 붕괴, 신경·운동 기능 저하, 생식 신호의 변화를 통해 위험을 일관되게 키운다. 본 연구는 미세플라스틱의 물성, 기후 관련 열 환경, 그리고 MeHg의 상호작용을 함께 고려하는 다중 스트레스 기반 생태학적 위해성 평가 틀의 필요성을 강조하며, 변화하는 환경에서 취약한 수생 무척추동물에 대한 MeHg 영향 예측을 위한 기초 자료를 제공한다.

다국어 초록 (Multilingual Abstract)

Methylmercury (MeHg) is a persistent risk to aquatic ecosystems, but its toxicity is related to various stresses. This dissertation explores how microplastics and increased temperatures affect MeHg bioaccumulation and toxic effects in aquatic invertebrate models, integrating organismal performance with oxidative, neuromotor, and reproduction-linked molecular endpoints.
Polylactic acid (PLA) and low-density polyethylene (LDPE) were compared alone and with MeHg in freshwater Daphnia magna. PLA exhibited worse life-history traits and increased MeHg body burden compared to MeHg alone, indicating polymer-specific vectoring and surface interactions. LDPE had little effect on accumulation under similar conditions. PLA increased oxidative pressure and modified antioxidant defenses, while LDPE with MeHg diminished cholinergic activity more. Vitellogenin along with reproduction-linked transcriptional genes varied between polymers, suggesting different endocrine effects.
Temperature–contaminant interactions were examined across taxa. In a marine rotifer, warming enhanced apparent MeHg potency, accelerated functional decline, and activated stress signaling, along with stage-dependent antioxidant responses and mercury distribution. Warming reduced D. magna's acute tolerance to MeHg and harmed chronic survival, reproduction, hatching, and swimming performance. Warming alone promoted antioxidant systems, but MeHg co-exposure weakened them. Neuromotor inhibition increased, and vitellogenin and ABC transporter responses showed temperature-dependent disruption of reproductive and detoxification pathways. Notably, MeHg accumulation increased with exposure concentration regardless of temperature, indicating that thermal effects are predominantly due to changes in toxicity rather than intake.
Both freshwater and marine systems reflect both polymer identity and temperature-modulated MeHg susceptibility. Bio-based and petroleum-derived particles affect MeHg consumption and pathway-specific toxicity differently, but warming enhances risk through disrupting redox homeostasis, lowering neuromotor function, and changing reproduction-linked signaling. These findings point out that multistressor ecological risk frameworks should explicitly consider microplastic properties and thermal regimes to forecast MeHg impacts on vulnerable aquatic invertebrates during environmental change.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1. General introduction 1
• 1.2. Methylmercury toxicity in aquatic organisms 6
• 1.2.1. Bioaccumulation and biomagnification 6
• 1.2.2. Physiological toxicity 8
• 1.2.3. Subcellular toxicity 10
• 1.2.4. Oxidative stress and neurotoxicity 11
• 1.2.5. Metabolism alternation, genotoxicity, and immunotoxicity 15
• 1.3. Environmental factors influencing the toxicity of MeHg 17
• 1.3.1. Temperature 21
• 1.3.2. pH 24
• 1.3.3. Dissolved organic compounds 26
• 1.3.4. Salinity 29
• 1.4. Interaction of methylmercury with co-existing pollutants and potential toxicity in aquatic organisms 32
• 1.4.1. Microplastics and nanoplastics 36
• 1.4.2. Organic pollutants 39
• 1.5. Conclusions and limitations 42
• Chapter 2. Toxicity comparison of polylactic acid and polyethylene microplastics co-exposed with methylmercury on Daphnia magna 46
• 2.1. Introduction 46
• 2.2. Materials and Methods 52
• 2.2.1. Chemicals and reagents 53
• 2.2.2. Model organism 54
• 2.2.3. In vivo chronic toxicity experiments 55
• 2.2.4. Accumulation of methyl mercury 56
• 2.2.5. Swimming behavior 57
• 2.2.6. Oxidative Stress and enzymatic activity assay 57
• 2.2.7. Total protein concentration 58
• 2.2.8. RNA extraction, cDNA synthesis, and quantitative real-time polymerase chain reaction 58
• 2.2.9. Statistical analysis 59
• 2.3. Results 60
• 2.3.1. Characterization of MPs 60
• 2.3.2. In Vivo Chronic Effects on Survival and Reproduction 62
• 2.3.3. Mercury concentration 66
• 2.3.4. Oxidative stress, antioxidant enzymatic activity, and gene expression profiles 66
• 2.3.5. Swimming speed and distance 70
• 2.3.6. AChE activity and gene expression 72
• 2.3.7. Reproduction related genes expression 74
• 2.4. Discussion 74
• 2.5. Conclusions 87
• Chapter 3. Single/combined effects of increased temperature and methylmercury on different stages of the marine rotifer Brachionus plicatilis 89
• 3.1. Introduction 89
• 3.2. Materials and methods 92
• 3.2.1. Animal culture and chemicals 92
• 3.2.2. Acute toxicity test 93
• 3.2.3. Physiological experiments 94
• 3.2.3.1. Fecundity, lifespan, and population 94
• 3.2.3.2. Spawning time, breeding time, and body growth 95
• 3.2.4. Biochemical test 95
• 3.2.5. Bioaccumulation analysis of Hg 95
• 3.2.6. Statistical analysis 96
• 3.3. Results 97
• 3.3.1. Survival rate and physiological effects 97
• 3.3.2. Total mercury concentration 99
• 3.3.3. Oxidative stress, enzymatic activity, and gene expression of antioxidants 100
• 3.3.4. Protein levels of the MAPK signaling pathway 105
• 3.4. Discussion 107
• 3.5. Conclusion 114
• Chapter 4. Temperature-dependent enhancement of methylmercury toxicity in Daphnia magna: Linking physiological and molecular stress responses 117
• 4.1. Introduction 117
• 4.2. Materials and methods 121
• 4.2.1. Experimental model and chemical 121
• 4.2.2. Acute and chronic toxicity 122
• 4.2.3. Mercury accumulation 123
• 4.2.4. Biochemical tests 123
• 4.2.5. Swimming behavior 124
• 4.2.6. Statistical analysis 124
• 4.3. Results 124
• 4.3.1. Acute toxicity and mercury accumulation 125
• 4.3.2. In vivo chronic effects 127
• 4.3.3. Oxidative stress, enzymatic activity and gene expression profile of antioxidant, and vitellogenin gene expression 127
• 4.3.4. Swimming behavior, and activity and gene expression of acetylcholinesterase 130
• 4.3.5. Glutathione-mediated system and ABC transporter 132
• 4.4. Discussion 134
• 4.5. Conclusion 143
• Chapter 5. Conclusion 145
• References 148