Rubisco in high Arctic tidewater glacier‐marine systems: A new window into phytoplankton dynamics

Abstract

The hundreds of tidewater glaciers found in the Canadian Arctic Archipelago have the potential to enhance delivery of nutrients and other material to the surface ocean. Despite this, their influence on marine ecosystems, specifically phytoplankton, is poorly characterized. Here we developed and applied a quantitative mass spectrometry‐based approach to measure phytoplankton ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) concentrations to examine differences in productivity in glacierized and non‐glacierized marine systems in Jones Sound, Nunavut, within Inuit Nunangat. Comparisons to chloroplast 16S rRNA gene amplicon sequencing data suggested that these measurements detect the majority of Rubisco produced in Jones Sound. Because Rubisco catalyzes carbon fixation, we used these measurements to estimate total and group‐specific primary production potential, which were within the range of historical primary production measurements made using classical methods in this region. Our measurements also revealed that up to 2% of total protein in the water column is Rubisco, and that Rubisco concentrations are correlated with chlorophyll fluorescence, with maxima near the nitracline. Rubisco produced by diatom genera Chaetoceros and Thalassiosira were higher in marine regions influenced by glaciers, while Rubisco from Micromonas (Chlorophyta) was greater in non‐glacierized regions. This suggests that future climate scenarios may favor smaller phytoplankton groups, like Micromonas, with consequences for food webs and carbon cycling. This study broadens our understanding of how tidewater glaciers will impact phytoplankton communities, now and in a warmer future, and lays the foundation for using this mass spectrometry‐based approach to quantify phytoplankton group‐specific carbon fixation potential in other marine regions.

Photosynthetic processes in Antarctic sea ice during the spring melt

“…Measured 14 C fixation rates were compared to potential carbon fixation rates calculated from Rubisco concentrations and an assumed Rubisco carboxylation rate (k c cat ; Young et al 2015;Roberts et al 2024). To quantify Rubisco concentrations, subsamples ($ 0.5 liter) of 1 : 1 melt volume were filtered onto 0.2-μm Sterivex™ filters for proteomic analysis, except for samples from Sta.…”

“…Protein extraction and preparation for mass spectrometry was performed as in Wu et al (2019). Measurements for the peptide of the large subunit of Rubisco (RbcL) were made via liquid chromatography-mass spectrometry using a Dionex Ultimate 3000 UPLC system interfaced with a TSQ Quantiva mass spectrometer run in selected reaction monitoring mode, as described (for different peptides) in Roberts et al (2024; see Supplementary Text for more details).…”

“…Rubisco concentrations provided a reasonable estimate of measured NBP L in surface seawater, a sample of rotten/brash ice and interior layers of sea ice, but overestimated rates for bottom-ice algae. Previous studies have suggested that Rubisco concentrations could be used to estimate potential maximum carbon fixation rates in phytoplankton (Losh et al 2013;Young et al 2015;Roberts et al 2024). This approach would require that all Rubisco be fully active and not substratelimited and that accurate estimates of its carboxylation rate be available.…”

“…Losh et al (2013) and Young et al (2015) found Rubisco to be fully active and CO 2saturated in phytoplankton. While carboxylation rates of Rubisco vary between taxa, diatoms and haptophytes have similar Rubisco carboxylation rates (Young et al 2016) and any differences are reduced at cold temperatures (Galmés et al 2016;Roberts et al 2024). There is still debate on whether cellular Rubisco content is regulated with growth rate.…”

“…Although more work is needed, we suggest that Rubisco concentrations could be used to calculate NBP L in polar oceans when light is saturating, such as in surface seawater and possibly sea ice in an advanced stage of melt. In addition, mass spectrometry-based proteomics, which enables quantification of RbcL peptides belonging to different photosynthetic groups, could lead to taxon-specific measurements of primary production in polar oceans (Roberts et al 2024).…”