Taxonomically resolved iron uptake and carbon fixation by phytoplankton within a ferruginous, meromictic lake
Date
2022-08
Authors
Block, Kaleigh
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Swanner, Elizabeth
Wanamaker, Alan
Raich, James
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Abstract
Brownie Lake (BL) is ferruginous and meromictic, meaning it is both permanently stratified with abundant dissolved iron (>1 mM) within the monimolimnion. The chemocline of Brownie Lake has a deep chlorophyll maximum (DCM) at approximately five meters, or an abundance of chlorophyll a (chla) containing phytoplankton. Within the chemocline, dissolved iron increases with depth and dissolved oxygen and light decrease with depth. The chemocline and DCM in BL provide a unique opportunity to study photosynthetic primary productivity (PP) and iron uptake across a gradient of light, dissolved oxygen, and dissolved iron. In situ stable carbon isotope bottle experiments were used to quantify net PP (NPP) with depth in May 2021, and rapid light curves with a PHYTO-PAM-II instrument were used to estimate gross PP (GPP) in August 2021 from four taxonomic groups. Results from the bottle experiments show that with increasing iron concentrations and decreasing light and oxygen concentrations, NPP rates increased. These initial results show the DCM is a zone of increased carbon fixation. The DCM could be a zone where iron limitation is ameliorated, and as chlorophyll a concentrations decrease with increasing depth, iron is more abundant therefore resulting in higher PP rates. GPP estimates from the PHYTO-PAM-II revealed much lower carbon fixation rates than that of the bottle experiments. These discrepancies could have resulted from taking measurements during different months with different methods and sample types.
To explore the variation in iron uptake with depth, samples from the epilimnion and chemocline were sorted by flow cytometry based on differences in size and pigment fluorescence. Intracellular iron abundance was quantified using single particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-ToFMS). The results of these measurements were inconclusive due to cell loss after flow cytometry and before sample introduction to the sp-ICP-ToFMS. This could have occurred due to cell lysis during or after cell sorting with flow cytometry, and more work is necessary to refine this method and optimize cell recovery after sorting with flow cytometry to obtain more cells for intracellular iron quantification. This novel methodology has vast applications for both freshwater and marine systems and environmental samples and could be a powerful tool for the intracellular determinations of phytoplankton elemental composition. Additionally, the results from this work broaden our understanding of PP under varying aquatic conditions and can inform where the greatest PP occurred within past ferruginous oceans (> 540 million years ago).
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