Research Scenario: Microplastic Transport and Biogeochemical Cycling in Arctic Permafrost Thaw Lakes

Introduction and Background: Arctic regions are experiencing unprecedented rates of warming, leading to widespread permafrost thaw. This thaw releases vast stores of ancient organic carbon, nutrients, and previously frozen contaminants into freshwater ecosystems, including the numerous thaw lakes that dot the Arctic landscape. While the biogeochemical implications of carbon and nutrient release are extensively studied, the fate and impact of anthropogenic contaminants, particularly microplastics (MPs), in these rapidly changing environments remain largely unknown. Global plastic production continues to rise, and MPs are now ubiquitous across all environments, including remote Arctic regions, transported by atmospheric currents, ocean currents, and riverine input. Recent studies have detected MPs in Arctic snow, sea ice, and even deep-sea sediments, indicating pervasive long-range transport.

The Emerging Problem: Freshwater thaw lakes are critical hotspots for biogeochemical activity and biodiversity in the Arctic. They are also terminal sinks for pollutants entering the terrestrial and aquatic systems. As permafrost thaws, previously trapped contaminants, including historical plastic debris buried within the permafrost active layer, may be mobilized. Furthermore, local sources such as research stations, indigenous communities, and tourism activities contribute to microplastic pollution. The unique characteristics of Arctic thaw lakes—such as their shallow depths, seasonal ice cover, extreme temperature fluctuations, and highly stratified water columns—could significantly alter the transport, degradation, and ecological interactions of MPs compared to temperate freshwater systems. Current literature provides a significant knowledge gap regarding the specific mechanisms of MP sequestration, remobilization, and biogeochemical interactions within these dynamic and vulnerable ecosystems. There is a critical need to understand how MPs interact with the unique microbial communities, organic matter, and sediment dynamics characteristic of permafrost thaw lakes.

Current Limitations and Rationale: Existing research on microplastics in freshwater systems primarily focuses on temperate lakes and rivers, often overlooking the distinct environmental parameters of the Arctic. Studies on MP fate in cold environments are scarce, and even fewer consider the complex interplay of permafrost thaw, unique hydrological regimes, and specialized microbial communities. The methods for extracting and quantifying MPs from high-organic-content sediments, like those found in thaw lakes, are also still developing, leading to inconsistencies and underestimations. Without a comprehensive understanding of MP behavior in these systems, accurate risk assessment for Arctic wildlife and human communities relying on these ecosystems for subsistence, as well as predicting long-term contaminant cycling in a rapidly warming Arctic, is impossible.

Preliminary Observations and Justification: Our preliminary field reconnaissance in the Alaskan North Slope revealed widespread visible plastic debris along the margins of several thaw lakes, particularly near historical encampments and recent disturbances. Sediment cores collected from the littoral zones of three representative thaw lakes (Lake Kivalina, Lake Ikpikpuk, and Lake Teshekpuk) revealed a consistent presence of microplastic particles (fibers and fragments, 0.1-5 mm in size) at depths correlating with historical permafrost thaw events identified through ${ }^{210}Pb$ dating. FTIR analysis confirmed common polymer types, including polyethylene, polypropylene, and PVC. Interestingly, samples from deeper, older sediment layers (pre-1950s) showed lower MP concentrations compared to surface and recently thawed layers, suggesting a recent increase in accumulation rates, possibly exacerbated by permafrost thaw and enhanced transport. Furthermore, incubation experiments in the lab with water and sediment slurries from these lakes showed that certain microbial consortia, isolated from the active layer of thawing permafrost, were capable of forming visible biofilms on MP surfaces within 4-6 weeks at $4^{\circ}C$, suggesting potential for biological alteration or degradation. Initial gene sequencing (16S rRNA) of these biofilms indicates the dominance of psychrophilic (cold-loving) bacteria and archaea previously implicated in hydrocarbon degradation. However, the extent to which these processes lead to actual breakdown or merely surface modification is unknown. We also observed preliminary evidence of MP uptake by key Arctic invertebrates (e.g., chironomid larvae) in laboratory feeding trials, leading to a noticeable decrease in feeding rates and growth in exposed individuals, although the mechanisms and population-level effects are unclear. These preliminary data strongly suggest that Arctic thaw lakes are active sites for microplastic accumulation and transformation, warranting an urgent and detailed investigation into their fate and ecological consequences.

Potential Avenues for Research: This emerging environmental challenge presents unique research opportunities to investigate novel biogeochemical pathways for contaminant cycling in rapidly changing cold regions. Understanding the physical and biological processes governing microplastic transport and transformation in these unique systems is critical for predicting their long-term impact on Arctic ecosystems and global carbon cycles. Research could explore the influence of freeze-thaw cycles on MP fragmentation, the role of cryo-concentration in MP accumulation, or the extent of microbial degradation of various polymer types under Arctic conditions. Furthermore, the potential for MPs to act as vectors for contaminants or pathogens, or to alter nutrient cycling within these carbon-rich environments, represents significant unexplored territory. The unique combination of permafrost thaw, extreme seasonality, and specialized microbial life offers a novel natural laboratory to study the fundamental principles of contaminant biogeochemistry in a rapidly warming world.

Evaluation Criteria/Rubric for NSF 'Specific Aims' Page (Total: 100 points)

This rubric outlines the key criteria and point allocations for evaluating a drafted 'Specific Aims' page, reflecting typical expectations for a compelling NSF grant proposal.

1. Problem Statement & Background (15 points)

• 5 points: Clear, concise, and effectively identifies a significant scientific problem or knowledge gap.
• 5 points: Provides sufficient, relevant background to contextualize the problem and establish its importance.
• 5 points: Compelling rationale for why the proposed research is needed now.

2. Central Hypothesis (15 points)

• 5 points: Hypothesis is clearly stated and original.
• 5 points: Hypothesis is testable and falsifiable.
• 5 points: Logically connects to the identified problem/knowledge gap.

3. Specificity of Aims (25 points)

• 5 points (per aim, up to 4 aims): Each aim is distinct, clearly stated, measurable, and achievable within the scope of the project.
• 5 points: Aims collectively address the central hypothesis and demonstrate a logical progression towards the overall research goal.

4. Approach & Outcomes (15 points)

• 5 points (per aim, up to 3 aims): For each aim, the general approach is briefly but clearly described and appears feasible.
• 5 points: Expected outcomes for each aim are articulated and appear significant.
• 5 points: Outcomes collectively advance understanding of the problem.

5. Intellectual Merit (15 points)

• 5 points: Clearly articulates the potential for the proposed research to advance fundamental knowledge and understanding.
• 5 points: Highlights the novelty, originality, and creativity of the proposed ideas.
• 5 points: Convinces the reviewer of the potential for transformative impact on the field.

6. Broader Impacts (10 points)

• 4 points: Clearly articulates how the research will benefit society or contribute to specific societal outcomes (e.g., education, economic development, public welfare).
• 3 points: Demonstrates creativity and feasibility in the proposed broader impacts activities.
• 3 points: Aligns with NSF's broader impacts criteria (e.g., diversity, scientific literacy, infrastructure, policy implications).

7. Overall Persuasiveness & Conciseness (5 points)

• 2 points: Writing is clear, concise, and easy to follow; adheres to length guidelines.
• 3 points: The 'Specific Aims' page is highly persuasive, compelling the reviewer of the significance and feasibility of the proposed research.