The influence of light on copper-limited growth of an oceanic diatom, Thalassiosira oceanica (Coscinodiscophyceae)

Cu transport and complexation by the marine diatom Phaeodactylum tricornutum: Implications for trace metal complexation kinetics in the surface ocean

Elucidating whether dissolved Cu uptake is kinetically or thermodynamically controlled, and the effects of speciation on Cu transport by phytoplankton will allow better modeling of the fate and impact of dissolved Cu in the ocean. To address these questions, we performed Cu physiological and physicochemical experiments using the model diatom, Phaeodactylum tricornutum, grown in natural North Atlantic seawater (0.44 nM Cu). Using competitive ligand equilibration-cathodic stripping voltammetry (CLE-CSV), we measured two organic ligand types released by P. tricornutum to bind Cu (L1 and L2) at concentrations of ~0.35 nM L1 and 1.3 nM L2. We also established the presence of two putative Cu-binding sites at the cell surface of P. tricornutum (S1 and S2) with log K differing by ~5 orders of magnitude (i.e., 12.9 vs. 8.1) and cell surface densities by 9-fold. Only the high-affinity binding sites, S1, exhibit reductase activity. Using voltammetric kinetic measurements and a theoretical kinetic model, we calculated the forward and dissociation rate constants of L1 and S1. Complementary 67Cu uptake experiments identified a high- and a low-affinity Cu uptake system in P. tricornutum, with half-saturation constant (Km) of 154 nM and 2.63 μM dissolved Cu, respectively. In the P. tricornutum genome, we identified a putative high-affinity Cu transporter (PtCTR49224) and a putative ZIP-like, low-affinity Cu transporter (PtZIP49400). PtCTR49224 has high homology to Homo sapiens hCTR1, which depending on the accessibility to extracellular reducing agents, the hCTR1 itself is involved in the reduction of Cu2+ to Cu+ before internalization. We combined these physiological and physicochemical data to calculate the rate constants for the internalization of Cu, and established that while the high-affinity Cu uptake system (S1) is borderline between a kinetically or thermodynamically controlled system, the low-affinity Cu transporters, S2, is thermodynamically-controlled. We revised the inverse relationship between the concentrations of inorganic complexes of essential metals (i.e., Ni, Fe, Co, Zn, Cd, Mn and Cu) in the mixed layer and the formation rate constant of metal transporters in phytoplankton, highlighting the link between the chemical properties of phytoplankton metal transporters and the availability and speciation of trace metals in the surface ocean.

Copper boosts the biostimulant activity of a vegetal-derived protein hydrolysate in basil: morpho-physiological and metabolomics insights

In addition to be used as a plant protection agent, copper (Cu) is also an essential micronutrient for plant growth and development. The bioavailability of Cu in agricultural systems can be limited due to its specific physical-chemical characteristics, leading to imbalances in plant production. To address this issue, an experimental trial was conducted on Genovese basil (Ocimum basilicum L.) in protected conditions to comparatively evaluate the effects of a vegetable protein hydrolysate (VPH), free Cu and Cu complexed with peptides and amino acids of vegetal origin (Cu and Cu-VPH, respectively), and a combination of VPH and Cu-VPH (VPH+Cu-VPH). The study showed that the combined application of VPH+Cu-VPH led to a significant average increase of 16.3% in fresh yield compared to the untreated Control and Cu treatment. This finding was supported by an improved photosynthetic performance in ACO2 (+29%) and Fv/Fm (+7%). Furthermore, mineral analysis using ICP OES demonstrated that Cu and Cu-VPH treatments determined, on average, a 15.1-, 16.9-, and 1.9-fold increase in Cu in plant tissues compared to control, VPH, and VPH+Cu-VPH treatments, respectively. However, the VPH+Cu-VPH treatment induced the highest contents of the other analyzed ions, except for P. In particular, Mg, Mn, Ca, and Fe, which take part in the constitution of chlorophylls, water splitting system, and photosynthetic electron transport chain, increased by 23%, 21%, 25%, and 32% compared to respective controls. Indeed, this improved the photosynthetic efficiency and the carboxylation capacity of the plants, and consequently, the physiological and productive performance of Genovese basil, compared to all other treatments and control. Consistently, the untargeted metabolomics also pointed out a distinctive modulation of phytochemical signatures as a function of the treatment. An accumulation of alkaloids, terpenoids, and phenylpropanoids was observed following Cu treatment, suggesting an oxidative imbalance upon metal exposure. In contrast, a mitigation of oxidative stress was highlighted in Cu-VPH and VPH+Cu-VPH, where the treatments reduced stress-related metabolites. Overall, these results highlight an interaction between Cu and VPH, hence paving the way towards the combined use of Cu and biostimulants to optimize agronomic interventions.

Copper Requirement and Acquisition by Marine Microalgae

Copper is a critical metal nutrient required by marine microalgae but may be toxic when supplied in excess. Maintaining an optimal intracellular Cu content is thus fundamentally necessary for microalgae and relies on cellular regulatory metabolisms and the process of Cu uptake that buffers the variation in environmental Cu availability. In this article the current progress in understanding the Cu requirements and acquisition mechanisms of marine microalgae is reviewed. Cu requirement by microalgae is primarily determined by the amount of Cu-dependent enzymes involved in cellular metabolisms and can be adjusted by Cu-sparing pathways. Decrease in metabolic Cu quotas caused a decline in the abundance of cuproenzymes and the dependent cellular metabolisms, and an induction of Cu acquisition pathways. Conventional models of Cu uptake describe the dependence of Cu uptake rate on free Cu²⁺ ions or kinetically labile species. A reductive, high-affinity Cu uptake system in marine microalgae is identified which enables cells to directly utilize organically complexed Cu, highlighting the importance of cell surface Cu reduction in the marine Cu cycle. This review provides new insights into Cu uptake models that may update the existing knowledge of Cu availability in the ocean.

Light Stimulates Copper-Limited Growth of an Oceanic Diatom by Increasing Cellular Copper(II) Reduction─A Rate-Determining Step in Copper Uptake

Uptake of Cu by Thalassiosira oceanica requires that Cu(II) is reduced to Cu(I) prior to transport across the cell membrane. The reduction step is mediated biochemically by cellular reductases active with a broad range of Cu chemical species. Here, we report on the cellular Cu(II) reduction and Cu(I) uptake of a diatom under saturating and subsaturating irradiance. An increase in growth irradiance, from 50 to 400 μmol photons m^-2 s^-1, increased the rate of extracellular Cu(II) reduction and steady-state Cu uptake. Under these conditions, Cu-limited cells acquired Cu more efficiently and maintained faster rates of growth than Cu-limited cells in low light. Pseudo-first-order reaction rate constants were about 70-fold faster for Cu(I) uptake than for Cu(II) reduction so that reduction was the rate-determining step in Cu acquisition. Accordingly, steady-state Cu uptake rates predicted from the reduction rate constants agreed well with measured rates of Cu uptake obtained from cultures growing at low nanomolar Cu concentrations. Transcript abundance of putative Cu(II) reductases followed a similar pattern to cupric reductase activity, increasing in Cu-limited cells and with increasing growth irradiance. The results are significant in showing Cu(II) reduction as the rate-determining step in Cu uptake: they suggest that biologically mediated Cu(II) reduction may be an important part of the Cu cycle in surface waters of the open sea.

Transcriptomes of an oceanic diatom reveal the initial and final stages of acclimation to copper deficiency

Copper (Cu) concentration is greatly reduced in the open sea so that phytoplankton must adjust their uptake systems and acclimate to sustain growth. Acclimation to low Cu involves changes to the photosynthetic apparatus and specific biochemical reactions that use Cu, but little is known how Cu affects cellular metabolic networks. Here we report results of whole transcriptome analysis of a plastocyanin-containing diatom, Thalassiosira oceanica 1005, during its initial stages of acclimation and after long-term adaptation in Cu-deficient seawater. Gene expression profiles, used to identify Cu-regulated metabolic pathways, show downregulation of anabolic and energy-yielding reactions in Cu-limited cells. These include the light reactions of photosynthesis, carbon fixation, nitrogen assimilation and glycolysis. Reduction of these pathways is consistent with reduced growth requirements for C and N caused by slower rates of photosynthetic electron transport. Upregulation of oxidative stress defence systems persists in adapted cells, suggesting cellular damage by increased reactive oxygen species (ROS) occurs even after acclimation. Copper deficiency also alters fatty acid metabolism, possibly in response to an increase in lipid peroxidation and membrane damage driven by ROS. During the initial stages of Cu-limitation the majority of differentially regulated genes are associated with photosynthetic metabolism, highlighting the chloroplast as the primary target of low Cu availability. The results provide insights into the mechanisms of acclimation and adaptation of T. oceanica to Cu deficiency.

Marine ammonia-oxidising archaea and bacteria occupy distinct iron and copper niches

Ammonia oxidation by archaea and bacteria (AOA and AOB), is the first step of nitrification in the oceans. As AOA have an ammonium affinity 200-fold higher than AOB isolates, the chemical niche allowing AOB to persist in the oligotrophic ocean remains unclear. Here we show that marine isolates, Nitrosopumilus maritimus strain SCM1 (AOA) and Nitrosococcus oceani strain C-107 (AOB) have contrasting physiologies in response to the trace metals iron (Fe) and copper (Cu), holding potential implications for their niche separation in the oceans. A greater affinity for unchelated Fe may allow AOB to inhabit shallower, euphotic waters where ammonium supply is high, but competition for Fe is rife. In contrast to AOB, AOA isolates have a greater affinity and toxicity threshold for unchelated Cu providing additional explanation to the greater success of AOA in the marine environment where Cu availability can be highly variable. Using comparative genomics, we predict that the proteomic and metal transport basis giving rise to contrasting physiologies in isolates is widespread across phylogenetically diverse marine AOA and AOB that are not yet available in pure culture. Our results develop the testable hypothesis that ammonia oxidation may be limited by Cu in large tracts of the open ocean and suggest a relatively earlier emergence of AOB than AOA when considered in the context of evolving trace metal availabilities over geologic time.

Identification of copper-regulated proteins in an oceanic diatom,Thalassiosira oceanica1005

Plastocyanin-dependent diatoms adjust cellular metabolism to cope with chronic Cu deficiency.

NADPH-dependent extracellular superoxide production is vital to photophysiology in the marine diatom Thalassiosira oceanica

Superoxide and other reactive oxygen species (ROS) are commonly regarded as harmful progenitors of biological stress and death, but this view has been changing. Indeed, many phytoplankton actively generate extracellular superoxide under ideal growth conditions for reasons that are mysterious. Results from this study suggest that extracellular superoxide production by the marine diatom Thalassiosira oceanica may promote photosynthetic health by modulating the oxidation state of the cellular NADP⁺/NADPH pool. The key enzyme implicated in this process is present in other representative marine phytoplankton and global ocean metagenomes. Overall, these findings transform the perceived role of superoxide in the health and functioning of phytoplankton and present implications for redox balance, biogeochemistry, and ecology in the future ocean.

Reactive oxygen species (ROS) like superoxide drive rapid transformations of carbon and metals in aquatic systems and play dynamic roles in biological health, signaling, and defense across a diversity of cell types. In phytoplankton, however, the ecophysiological role(s) of extracellular superoxide production has remained elusive. Here, the mechanism and function of extracellular superoxide production by the marine diatom Thalassiosira oceanica are described. Extracellular superoxide production in T. oceanica exudates was coupled to the oxidation of NADPH. A putative NADPH-oxidizing flavoenzyme with predicted transmembrane domains and high sequence similarity to glutathione reductase (GR) was implicated in this process. GR was also linked to extracellular superoxide production by whole cells via quenching by the flavoenzyme inhibitor diphenylene iodonium (DPI) and oxidized glutathione, the preferred electron acceptor of GR. Extracellular superoxide production followed a typical photosynthesis-irradiance curve and increased by 30% above the saturation irradiance of photosynthesis, while DPI significantly impaired the efficiency of photosystem II under a wide range of light levels. Together, these results suggest that extracellular superoxide production is a byproduct of a transplasma membrane electron transport system that serves to balance the cellular redox state through the recycling of photosynthetic NADPH. This photoprotective function may be widespread, consistent with the presence of putative homologs to T. oceanica GR in other representative marine phytoplankton and ocean metagenomes. Given predicted climate-driven shifts in global surface ocean light regimes and phytoplankton community-level photoacclimation, these results provide implications for future ocean redox balance, ecological functioning, and coupled biogeochemical transformations of carbon and metals.