Quantifying key photosynthetic enzymes in natural phytoplankton communities in the North Atlantic

Abstract

Marine phytoplankton are central to biogeochemical cycling within the ocean, driving enzyme catalysed redox reactions using a set of evolutionary conserved metabolic proteins. Data are presented from four cruises in the North Atlantic in both the sub-tropics (D326, winter 2008 and FeAST, summer 2008) and the high latitudes (D350, spring 2010 and D354, summer 2010). Optimisation of quantitative immunoblotting enabled detection of target photosynthetic proteins and resulted in a method which provided extraction efficiencies averaging 71% 12% and good reproducibility. Quantification of target proteins within natural marine communities, are combined with photophysiological measurements and biochemical data, to determine protein distribution patterns and suggest potential acclimation patterns in response to environmental forcing.

Quantification of photosystem II (PSII) and Rubisco, was achieved from phytoplankton communities in the sub-tropical North Atlantic, in both surface waters and from the deep-chlorophyll maximum (DCM). Increased PSII abundance, relative to total protein, observed in samples from the DCM suggest that phytoplankton within the sub-tropical North Atlantic, photoacclimate to lower irradiance by increasing the abundance of PSII reaction centres.

Data collected from the high latitude North Atlantic (HLNA) during spring and summer 2010 suggest that, during summer, primary productivity in the Irminger Basin was limited by iron (Fe) availability while co-limitation by Fe and nitrate was constraining phytoplankton growth in the Iceland Basin. The most significant inter-season difference in protein abundance was a decrease in Rubisco in summer. This decline was potentially attributed to a decrease in nitrate availability between spring and summer. Low PSII:Chl a ratios, coupled with depressed photosynthetic efficiency (Fv/Fm), were observed in the Irminger basin during summer, potentially as a result of larger antenna sizes and/or excess uncoupled Chl-binding proteins, in response to Fe limitation. Nutrient addition incubation experiments in the HLNA, during summer 2010, further supported the in situ data. In particular, the molecular level response of these communities to Fe addition was characterised by a marked increase in PSII:Chl a.

Overall this thesis has enabled absolute quantification of key photosynthetic proteins from natural communities in the North Atlantic and has addressed potential environmental drivers for changes in protein abundance. Future quantification of key photosynthetic proteins, in natural communities, will provide applications in primary productivity estimates.

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Identifiers

ORCID for Mark Moore: orcid.org/0000-0002-9541-6046

ORCID for Duncan Purdie: orcid.org/0000-0001-6672-1722

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