Abstract
In the Southern Ocean (SO), iron (Fe) limitation strongly inhibits phytoplankton growth and generally decreases their primary productivity. Diatoms are a key component in the carbon (C) cycle, by taking up large amounts of anthropogenic CO2 through the biological carbon pump. In this study, we investigated the effects of Fe availability (no Fe and 4 nM FeCl3 addition) on the physiology of Chaetoceros cf. simplex, an ecologically relevant SO diatom. Our results are the first combining oxygen evolution and uptake rates with particulate organic carbon (POC) build up, pigments, photophysiological parameters and intracellular trace metal (TM) quotas in an Fe-deficient Antarctic diatom. Decreases in both oxygen evolution (through photosynthesis, P) and uptake (respiration, R) coincided with a lowered growth rate of Fe-deficient cells. In addition, cells displayed reduced electron transport rates (ETR) and chlorophyll a (Chla) content, resulting in reduced cellular POC formation. Interestingly, no differences were observed in non-photochemical quenching (NPQ) or in the ratio of gross photosynthesis to respiration (GP:R). Furthermore, TM quotas were measured, which represent an important and rarely quantified parameter in previous studies. Cellular quotas of manganese, zinc, cobalt and copper remained unchanged while Fe quotas of Fe-deficient cells were reduced by 60% compared with High Fe cells. Based on our data, Fe-deficient Chaetoceros cf. simplex cells were able to efficiently acclimate to low Fe conditions, reducing their intracellular Fe concentrations, the number of functional reaction centers of photosystem II (RCII) and photosynthetic rates, thus avoiding light absorption rather than dissipating the energy through NPQ. Our results demonstrate how Chaetoceros cf. simplex can adapt their physiology to lowered assimilatory metabolism by decreasing respiratory losses.
Highlights
The Southern Ocean (SO) is the worlds largest high nutrients, low chlorophyll (HNLC) region
It has been shown that the Gross oxygen production (GP):R ratio is a critical parameter to assess the net primary production (NPP) depending on light and temperature conditions (Bozzato et al 2019)
This study presents a novel experimental approach, by combining both photosynthesis and respiration, in order to better understand the effects of Fe-limitation on carbon uptake in the SO
Summary
The Southern Ocean (SO) is the worlds largest high nutrients, low chlorophyll (HNLC) region In this region, despite the abundant supply of macronutrients such as N and P, phytoplankton growth and primary production are mainly limited by the trace element iron (Fe). Fe limitation controls the photoacclimation status by lowering pigment quotas like chlorophyll a (Chla) and fucoxanthin (van Oijen et al 2004; Pankowski and McMinn 2009; Feng et al 2010). Besides these general physiological adjustments, the acclimation to Fe limitation enhances taxon-specific photophysiological differences, for example between haptophytes and diatoms. These different photophysiological acclimation strategies of the two taxa were attributed to the habitats they occupy: diatoms usually thrive in waters with a shallow mixed layer depth, thereby experiencing higher irradiance levels, whereas haptophytes like Phaeocystis antarctica are better adapted to low irradiance levels found in a deeply mixed water column (Arrigo and van Dijken 2003)
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