Where did they come from, where will they go? Understanding changes in the structure of marine eukaryotic microalgal communities.

Marine eukaryotic microalgae are key organisms in our oceans and contribute substantially to primary productivity and ecosystem health. An understanding of current community structure and the impact of climate change on these communities is vital for assessing ecosystem resilience in a rapidly changing world. This thesis investigates the eukaryotic microalgal community (EMCs) structure across diverse habitats, with a focus on global patterns of diversity, and ecological responses of species from polar regions.

Chapter One provides an overview of existing literature that highlights the importance of microalgae as the foundation of the food web in marine ecosystems and their critical role in global biogeochemical cycles. It emphasises the impact of climate change on EMCs, including shifts in diversity and community structure and the flow-on effects for ecosystem function and higher trophic levels. Chapter Two investigates methodological considerations for characterising EMC diversity using metabarcoding, focusing on the effectiveness of two gene regions commonly used for metabarcoding studies, 18S ribosomal DNA V4 and V9. This chapter provides insights into optimising sequencing approaches, recommending the use of the 18S V9 region for high level assessments of coastal EMC.

Using the findings from the previous chapter, in Chapter Three I assessed sites spanning five ecoregions from the South Pacific to the Ross Sea, Antarctica using the 18S V9 region to characterise the diversity of EMCs. Trends in community composition and assessment of community formation demonstrated clear differences across ecoregions, emphasising the importance of baseline surveys to monitor EMC dynamics across spatial scales. Mechanisms driving these gradients, including habitat preferences and dispersal limitation also showed distinct patterns related to distance. Although there are inevitable limitations with large spatial scales and site selection the results aid in our understanding of the factors shaping microalgal community structure in marine ecosystems. Baseline surveys across large spatial scales are vital to develop an understanding of the current state of diversity in these important primary producers. Such surveys will allow a better understanding of future shifts because of climate change.

I leveraged a unique natural experiment in McMurdo Sound, Antarctica in Chapter Four. A series of storms during the autumn/winter of 2022 caused most of the consolidated fast ice in this region to form around five months later than normal. This situation has never been recorded before. Samples were taken from three sites, each with different ice conditions: typical consolidated fast ice (~2 m) with an associated sub ice platelet layer (SIPL; 2 - 3 m); late-forming fast ice (~1 m) with a SIPL (0.5 - 1 m); and late-forming fast ice without a SIPL. Significant shifts in community composition, biomass distribution and dominant species were observed among different ice conditions. Polar regions are experiencing amplified environmental stressors due to climate change.

Significant shifts in the observed ice associated EMC were driven by variability in environmental conditions, including light availability. These shifts in community composition may be compounded by shifts within microalgal cells themselves. As light has a substantial impact on growth rates and health of eukaryotic microalgae, Chapter Five used a controlled laboratory-based experiment to investigate the response of Antarctic ice-associated microalgae to changes in light levels, mimicking different ice conditions. Fatty acid profiles and transcriptomics were used to assess changes in relative nutritional value and the underlying molecular drivers of these changes, with a focus on the implications for ecosystem health. This chapter highlights the adaptive mechanisms of microalgae in response to changing environmental stressors and underscores the importance of understanding these responses for ecosystem management and conservation.

Chapter Six synthesises the findings from the previous chapters, highlighting the interconnectedness of microalgal community dynamics, environmental drivers, and ecosystem functioning in the context of climate change. The results of this thesis suggest that connectivity to other ecoregions and localised community shifts within SIPL EMCs could significantly influence the future state of Antarctic ice-associated EMCs. Specifically, the poleward movement of microalgae due to warming waters and the reduction in sea ice thickness will lead to less diverse, diatom-dominated communities. Significant reductions in EMC diversity and shifts in dominant taxa were observed, emphasising the vulnerability of these ecosystems to climate change. These changes at multiple levels of biological organisation, from community composition to intraspecies variations, will impact the availability of key biomolecules produced by EMCs, with potential to affect the Antarctic food web. The research highlights the importance of understanding how shifts in EMC diversity and composition alter the availability of essential biomolecules and their implications for primary consumers and higher trophic levels. The findings underscore the need for ongoing research to understand the potential impacts of climate change on Antarctic marine ecosystems. By addressing these future directions, we can better protect the intricate web of life in these fragile environments.