Optical niche partitioning of phytoplankton and implications for carbon fixation

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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- Phytoplankton, the photosynthetic microbes of the oceans, are known for their diversity of photosynthetic mechanisms that ultimately fuel carbon fixation and growth. This diversity includes, but is not limited to, the pigments in the pigment antennae that capture or dissipate (energy of) different wavelengths of light, and the photoregulation strategies that optimise photosynthesis via photoprotection and energy dissipation. The light climate experienced by phytoplankton in the marine environment, particularly the coastal oceans, is complex and highly dynamic. Marine phytoplankton possess a number of photoadaptive strategies for optimising photosynthesis in different light environments. However not all strategies are available to all phytoplankton or are phenotypically expressed to the same degree across taxa and within taxa in response to changes in light. This results in niche partitioning in different optical environments (niches) based on the photosynthetic functional traits present and the associated photoregulation capacity, in response to the light. A number of knowledge gaps exist in our understanding of light-induced changes in phytoplankton biogeography, physiology and carbon fixation, specifically in Australian waters. Additionally, we need to improve bio-optical monitoring techniques that can be used to rapidly estimate carbon fixation or screen natural communities for the presence of photosynthetic functional traits and taxa within Australia’s vast coastline. This thesis combines bio-optical investigations at the cellular, community and regional level to characterise optical niches in Australian waters and better understand their implications for phytoplankton carbon fixation. It is shown that the inherent optical properties throughout Australia’s northern and eastern waters are highly variable and yet create distinct optical niches that in turn drive predictable changes in phytoplankton community composition, physiology and carbon fixation. The ambient light and nutrient conditions in Australian coastal waters impose resource constraints which select for specific photosynthetic traits such as the quantum efficiency for carbon fixation ϕ, maximum quantum yield of photosynthesis Fv/Fm and functional absorption cross section for photosynthesis at PSII σPSII. This thesis demonstrates the successful application of rapid bio-optical techniques, which specifically probe these photosynthetic traits, for estimating phytoplankton net primary productivity, including a chlorophyll-a fluorescence-based and phytoplankton absorption-based model. Investigations into the photoregulation strategies of coastal phytoplankton, including the specific study of marine cyanobacteria Synechococcus sp. demonstrate the variability in photoprotection mechanisms employed by different phytoplankton taxa and the role of the optical niche in regulating this response. This thesis provides new insights into how changes to the utilisation of light for photosynthesis at the cellular level impacts on the quantum efficiency for carbon fixation and propagates to community scale net primary productivity (NPP), driving uncertainty in bio-optical measurements estimating NPP at the regional scale.
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