Pyrenoidal sequestration of cadmium impairs carbon dioxide fixation in a microalga

Abstract

Mixotrophic microorganisms are able to use organic carbon as well as inorganic carbon sources and thus, play an essential role in the biogeochemical carbon cycle. In aquatic ecosystems, the alteration of carbon dioxide (CO2) fixation by toxic metals such as cadmium – classified as a priority pollutant – could contribute to the unbalance of the carbon cycle. In consequence, the investigation of cadmium impact on carbon assimilation in mixotrophic microorganisms is of high interest. We exposed the mixotrophic microalga Chlamydomonas reinhardtii to cadmium in a growth medium containing both CO2 and labelled 13C‐[1,2] acetate as carbon sources. We showed that the accumulation of cadmium in the pyrenoid, where it was predominantly bound to sulphur ligands, impaired CO2 fixation to the benefit of acetate assimilation. Transmission electron microscopy (TEM)/X‐ray energy dispersive spectroscopy (X‐EDS) and micro X‐ray fluorescence (μXRF)/micro X‐ray absorption near‐edge structure (μXANES) at Cd LIII‐edge indicated the localization and the speciation of cadmium in the cellular structure. In addition, nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of the 13C/12C ratio in pyrenoid and starch granules revealed the origin of carbon sources. The fraction of carbon in starch originating from CO2 decreased from 73 to 39% during cadmium stress. For the first time, the complementary use of high‐resolution elemental and isotopic imaging techniques allowed relating the impact of cadmium at the subcellular level with carbon assimilation in a mixotrophic microalga.

Short-term effect of cadmium on the motility of three flagellated algal species

“…The intracellular sequestration of heavy metals can occur in pyrenoids, vacuoles, and lipid droplets as a highly efficient protective mechanism (Lage et al 1996;Monteiro et al 2011). Proteins, as the principal components of pyrenoids, could provide a binding spot for Cd on their thiol groups, as well as the starch grains around the pyrenoid which can bind Cd on the carboxyl groups (Bräutigam et al 2011;Penen et al 2020). Metal stress exerted on cells can stimulate a multifold increase in starch storage, thus imitating nutrient starvation conditions (Nishikawa et al 2003).…”

“…Since the exposure to Cd disrupts photosynthetic pathways (Faller et al 2005) and impairs inorganic carbon intake, i.e. CO 2 fixation, both pyrenoidal Cd sequestration and accumulation of carbon from acetate in the pyrenoid are initiated (Penen et al 2020). This accumulation of organic carbon, as an evolutionary mixotrophic trait, can thus grant advantage in surviving toxic metal pollution events (Penen et al 2020).…”

“…CO 2 fixation, both pyrenoidal Cd sequestration and accumulation of carbon from acetate in the pyrenoid are initiated (Penen et al 2020). This accumulation of organic carbon, as an evolutionary mixotrophic trait, can thus grant advantage in surviving toxic metal pollution events (Penen et al 2020). Secondly, the proteinaceous periplast, a sandwich-layered structure consisting of inner and surface periplast components that embrace the plasma membrane (Brett et al 1994), represents a strong adsorbent of heavy metals with numerous functional groups which include carboxyl, hydroxyl, amino, mercapto, and phosphate groups (Das et al 2008;Gupta and Rastogi 2009;Zhang et al 2020).…”

Quantitative elemental imaging in eukaryotic algae

“…Proteins like ferritin, a soluble 24-subunit oligomer, can sequester trace elements, ∼4500 Fe ions in ferritins case, 199 , 200 and other organelles like the starch-separated pyrenoid in the chloroplast have been found to contribute to Cd sequestration. 196 , 197 , 201 …”

Comparison of cell size, chlorophyll fluorescence and cadmium (Cd2+) bioaccumulation between wild-type and mutant strains of Chlamydomonas reinhardtii upon exposure to Cd2

“…Thiol ligands were also shown to increase with cells' exposure time to Cd, confirming thiol ligands' important role in Cd stress regulation. There is also pyrenoidal sequestration of Cd, leading to limited CO 2 fixation during Cd exposure (Penen et al, 2019).…”