Sea-ice bloom dynamics, putative parasitoids, and crude-oil impacts to microbiota modified by irradiance

Abstract

Arctic sea ice serves as a critical habitat for microbial communities, supporting, in part, the Arctic food web and influencing global biogeochemical cycles. However, climate change is rapidly altering this environment leading to a shorter ice-covered period and increased light transmittance. Greater accessibility due to less ice coverage is increasing industrial and commercial activities which raises the risk of crude oil spills that threaten sea-ice microbial communities. This dissertation explores the seasonal dynamics of prokaryotes and unicellular eukaryotes during the spring algal bloom, and the impacts of potential light stress in combination with crude oil exposure on Arctic sea-ice diatoms and microbial communities with time-series data and laboratory-based experiments. Chapter 2 examines microbial community succession during the 2021 vernal ice-algal bloom near Utqiagvik, Alaska. The bloom reached was larger than previously observed in this region. An unprecedented bloom of the oil-degrading bacterium Oleispira suggested potential environmental hydrocarbon contamination. Metabarcoding and co­ occurrence analyses revealed that diatoms, particularly Nitzschia spp., were primary hosts for parasitoid taxa such as chytrids Cryothecomonas and, highlighting the potential for top-down control of algal populations and the maintenance of diversity. Chapters 3 & 4 investigate the interactive effects of crude oil exposure and irradiance on Arctic sea-ice diatoms. These experimental results show species-specific responses to oil, with Fragilariopsis cylindrus being highly sensitive, while Synedropsis hyperborea exhibited stimulated growth at low oil concentrations. (Meta)transcriptomic analyses in Chapter 4 revealed that oil exposure induced a switch of metabolism in diatoms from autotrophic to catabolic, particularly in pathways related to lipid degradation. The findings suggest that oil spills may favor flagellates over diatoms, shifting microbial community composition with potential consequences to biogeochemical cycles. Together, these studies provide novel insights into the importance of internal lipid reserves, alternative metabolic pathways, and microbial interactions in supporting microalgal resilience within the Arctic sympagic ecosystem.