Channels of Evolution: Unveiling Evolutionary Patterns in Diatom Ca2+ Signalling.

Diatoms are important primary producers in marine and freshwater environments, but little is known about the signalling mechanisms they use to detect changes in their environment. All eukaryotic organisms use Ca2+ signalling to perceive and respond to environmental stimuli, employing a range of Ca2+-permeable ion channels to facilitate the movement of Ca2+ across cellular membranes. We investigated the distribution of different families of Ca2+ channels in diatom genomes, with comparison to other members of the stramenopile lineage. The four-domain voltage-gated Ca2+ channels (Cav) are present in some centric diatoms but almost completely absent in pennate diatoms, whereas single-domain voltage-gated EukCatA channels were found in all diatoms. Glutamate receptors (GLRs) and pentameric ligand-gated ion channels (pLGICs) also appear to have been lost in several pennate species. Transient receptor potential (TRP) channels are present in all diatoms, but have not undergone the significant expansion seen in brown algae. All diatom species analysed lacked the mitochondrial uniporter (MCU), a highly conserved channel type found in many eukaryotes, including several stramenopile lineages. These results highlight the unique Ca2+-signalling toolkit of diatoms and indicate that evolutionary gains or losses of different Ca2+ channels may contribute to differences in cellular-signalling mechanisms between species.

Challenges facing sustainable protein production: Opportunities for cereals.

Rising demands for protein worldwide are likely to drive increases in livestock production, as meat provides ∼40% of dietary protein. This will come at a significant environmental cost, and a shift toward plant-based protein sources would therefore provide major benefits. While legumes provide substantial amounts of plant-based protein, cereals are the major constituents of global foods, with wheat alone accounting for 15-20% of the required dietary protein intake. Improvement of protein content in wheat is limited by phenotyping challenges, lack of genetic potential of modern germplasms, negative yield trade-offs, and environmental costs of nitrogen fertilizers. Presenting wheat as a case study, we discuss how increasing protein content in cereals through a revised breeding strategy combined with robust phenotyping could ensure a sustainable protein supply while minimizing the environmental impact of nitrogen fertilizer.

Cold-induced [Ca2+]cyt elevations function to support osmoregulation in marine diatoms.

Diatoms are a group of microalgae that are important primary producers in a range of open ocean, freshwater, and intertidal environments. The latter can experience substantial long- and short-term variability in temperature, from seasonal variations to rapid temperature shifts caused by tidal immersion and emersion. As temperature is a major determinant in the distribution of diatom species, their temperature sensory and response mechanisms likely have important roles in their ecological success. We examined the mechanisms diatoms use to sense rapid changes in temperature, such as those experienced in the intertidal zone. We found that the diatoms Phaeodactylum tricornutum and Thalassiosira pseudonana exhibit a transient cytosolic Ca2+ ([Ca2+]cyt) elevation in response to rapid cooling, similar to those observed in plant and animal cells. However, [Ca2+]cyt elevations were not observed in response to rapid warming. The kinetics and magnitude of cold-induced [Ca2+]cyt elevations corresponded with the rate of temperature decrease. We did not find a role for the [Ca2+]cyt elevations in enhancing cold tolerance but showed that cold shock induces a Ca2+-dependent K+ efflux and reduces mortality of P. tricornutum during a simultaneous hypo-osmotic shock. As intertidal diatom species may routinely encounter simultaneous cold and hypo-osmotic shocks during tidal cycles, we propose that cold-induced Ca2+ signaling interacts with osmotic signaling pathways to aid in the regulation of cell volume. Our findings provide insight into the nature of temperature perception in diatoms and highlight that cross-talk between signaling pathways may play an important role in their cellular responses to multiple simultaneous stressors.

Spatiotemporal patterns of intracellular Ca2+ signalling govern hypo-osmotic stress resilience in marine diatoms.

Diatoms are globally important phytoplankton that dominate coastal and polar-ice assemblages. These environments exhibit substantial changes in salinity over dynamic spatiotemporal regimes. Rapid sensory systems are vital to mitigate the harmful consequences of osmotic stress. Population-based analyses have suggested that Ca2+ signalling is involved in diatom osmotic sensing. However, mechanistic insight of the role of osmotic Ca2+ signalling is limited. Here, we show that Phaeodactylum Ca2+ elevations are essential for surviving hypo-osmotic shock. Moreover, employing novel single-cell imaging techniques we have characterised real-time Ca2+ signalling responses in single diatom cells to environmental osmotic perturbations. We observe that intracellular spatiotemporal patterns of osmotic-induced Ca2+ elevations encode vital information regarding the nature of the osmotic stimulus. Localised Ca2+ signals evoked by mild or gradual hypo-osmotic shocks are propagated globally from the apical cell tips, enabling fine-tuned cell volume regulation across the whole cell. Finally, we demonstrate that diatoms adopt Ca2+ -independent and dependent mechanisms for osmoregulation. We find that efflux of organic osmolytes occurs in a Ca2+ -independent manner, but this response is insufficient to mitigate cell damage during hypo-osmotic shock. By comparison, Ca2+ -dependent signalling is necessary to prevent cell bursting via precise coordination of K+ transport, and therefore is likely to underpin survival in dynamic osmotic environments.

A Novel Ca2+ Signaling Pathway Coordinates Environmental Phosphorus Sensing and Nitrogen Metabolism in Marine Diatoms.

Diatoms are a diverse and globally important phytoplankton group, responsible for an estimated 20% of carbon fixation on Earth. They frequently form spatially extensive phytoplankton blooms, responding rapidly to increased availability of nutrients, including phosphorus (P) and nitrogen (N). Although it is well established that diatoms are common first responders to nutrient influxes in aquatic ecosystems, little is known of the sensory mechanisms that they employ for nutrient perception. Here, we show that P-limited diatoms use a Ca2+-dependent signaling pathway, not previously described in eukaryotes, to sense and respond to the critical macronutrient P. We demonstrate that P-Ca2+ signaling is conserved between a representative pennate (Phaeodactylum tricornutum) and centric (Thalassiosira pseudonana) diatom. Moreover, this pathway is ecologically relevant, being sensitive to sub-micromolar concentrations of inorganic phosphate and a range of environmentally abundant P forms. Notably, we show that diatom recovery from P limitation requires rapid and substantial increases in N assimilation and demonstrate that this process is dependent on P-Ca2+ signaling. P-Ca2+ signaling thus governs the capacity of diatoms to rapidly sense and respond to P resupply, mediating fundamental cross-talk between the vital nutrients P and N and maximizing diatom resource competition in regions of pulsed nutrient supply.

Salt-responsive gut commensal modulates TH17 axis and disease.

A Western lifestyle with high salt consumption can lead to hypertension and cardiovascular disease. High salt may additionally drive autoimmunity by inducing T helper 17 (TH17) cells, which can also contribute to hypertension. Induction of TH17 cells depends on gut microbiota; however, the effect of salt on the gut microbiome is unknown. Here we show that high salt intake affects the gut microbiome in mice, particularly by depleting Lactobacillus murinus. Consequently, treatment of mice with L. murinus prevented salt-induced aggravation of actively induced experimental autoimmune encephalomyelitis and salt-sensitive hypertension by modulating TH17 cells. In line with these findings, a moderate high-salt challenge in a pilot study in humans reduced intestinal survival of Lactobacillus spp., increased TH17 cells and increased blood pressure. Our results connect high salt intake to the gut-immune axis and highlight the gut microbiome as a potential therapeutic target to counteract salt-sensitive conditions.