Polar dawn: how do dark-light transitions impact population growth and silica uptake in Antarctic diatoms?

This project addresses a key knowledge gap in ocean science: How do diatoms grow and take up nutrients in the dynamic light regimes of the Southern Ocean? The Southern Ocean (SO) is important because its high biological production supports large biodiverse food webs that disproportionately impact other global ecosystems. Diatoms are a type of phytoplankton – microscopic primary producers – that are unique in making their cell walls (frustules) out of silicon (Si). In the SO, diatoms dominate phytoplankton assemblages, and are the main contributors to primary production by photosynthesis.
A fundamental problem in marine ecology and biogeochemistry is representing diatom primary production and nutrient uptake in ecosystem and biogeochemical models. Such models, including those that feed into larger-scale climate models, link diatom trait values to light and nutrient levels. However, parameterisations of these relationships lack two crucial data types. First, we lack data on variation in light-trait relationships among species and strains, which drives changes to diatom communities, affecting food webs and biogeochemical cycles. Second, we lack data on how previous light experience affects growth and nutrient uptake in diatoms. Phytoplankton traits in models depend only on instantaneous light levels, even though diatoms grow both immediately in the spring, after prolonged darkness and later in the summer, after prolonged light. Thus, the relationships between light levels and diatom traits may be season specific, and will likely be impacted by how light availability to phytoplankton changes as sea ice coverage and ocean mixing are affected by global warming. This lack of basic biological knowledge limits the accuracy of models of primary production and nutrient cycling in the SO. Our proposed study will establish causal relationships between variation in light/dark regimes and variation in trait values associated with diatom growth and silica uptake for several common SO diatoms.
Understanding how both light and genetic variation affect diatom traits thus far has been limited by the capacity to carry out experiments with multiple strains of SO diatoms. This is mainly due to the lack of culture collections suitable for exploring trait variability within and between species, which requires multiple strains of multiple species. Our experimental expertise and culture collection are exceptionally well poised to investigate how variation in light regimes, and in diatom species/strains, affect ecologically and biogeochemically important traits in SO diatoms. We will do this by measuring growth rates and cellular processes across 28 diatom strains from seven species, under eight different light conditions, some with periods of darkness. By using an experimental setup with sufficient variation in both diatom identity and light regime, we can determine how much of the difference in diatom growth and physiology is caused by environmental (light) vs. genetic (species and strains) factors. Because diatoms must take up Si from their surroundings during growth to build their frustules, they play a key role in global ocean Si cycling. We will measure variation in diatom Si uptake in different light regimes to understand the effect of light and genetic variation on this crucial biogeochemical process. Understanding how light regimes, including those with periods of darkness, affect diatom production and Si uptake will provide information that ecological and Earth System models need to provide robust projections of changes to primary production and cycling of key nutrients in the SO over seasons and as climate change proceeds.