Biology subjects

Helliwell, K. E.

Publications and source records attributed to Helliwell, K. E..

8 recordsLinked to original sources

bioRxiv2025

A plasma membrane Ca2+-dependent protein kinase PtCDPK2 promotes phosphorus starvation resilience in Phaeodactylum tricornutum

Phosphorus (P) is an essential element limiting algal growth and productivity in aquatic ecosystems. Diatoms are important microalgae that thrive in nutrient-variable environments. Determining how diatoms perceive and respond to P availability is therefore crucial for understanding their ecological success. P-limited diatoms use a calcium (Ca2+)-dependent signalling pathway to sense and coordinate cellular responses to phosphate resupply. Despite the importance of Ca2+ signalling for diatom environmental sensing, apparatus enabling Ca2+ signal decoding is poorly understood. Here, we characterise the repertoire of an important group of Ca2+ sensor proteins--Ca2+ dependent protein kinases (CDPKs), in Phaeodactylum tricornutum. Several PtCDPKs are transcriptionally upregulated under P starvation. To determine whether PtCDPKs can coordinate P-starvation responses or act to transduce Ca2+ signals induced by P resupply, we functionally characterised PtCDPK2. PtCDPK2 is highly expressed in P-limited cells and localises to the cell periphery, suggesting a role regulating plasma membrane processes. Further, PtCDPK2 is co-regulated with the transcriptional regulator of P-starvation responses, PtPSR1. PtCDPK2 expression is also coordinated with the induction of P-Ca2+ signalling, which is driven by depletion of cellular P rather than external P exhaustion, or growth limitation. Ptcdpk2 mutants have significantly reduced photosynthetic efficiency and alkaline phosphatase activity under P starvation, but we do not find evidence for a direct role coordinating downstream responses to P resupply. These findings suggest PtCDPK2 is essential for regulating P-starvation physiology and reveals a role for Ca2+-signalling apparatus in promoting diatom tolerance in low P environments.

cell biology↗

bioRxiv2024

Seasonal niche overlap of diverse facultative antagonistic bacteria of diatoms in a productive coastal ecosystem

Biotic interactions between microbes underpin marine ecosystem health, governing the flux of carbon and other nutrients in the ocean. However, studying aquatic microbial interactions is challenging. Model systems can provide in depth understanding of the mechanisms driving such associations. Yet, insights of the prevalence and co-occurrence dynamics of laboratory model systems in natural ecosystems remain limited. By leveraging 16S and 18S metabarcoding combined with phylogenetic analysis, we assessed the environmental presence of facultative bacterial pathogens of one of the most globally abundant phytoplankton groups, the diatoms. Sampling microbial assemblages in a productive coastal ecosystem over the course of an annual cycle, we detected multiple algicidal bacteria that frequently exhibited overlapping co-occurrences. Together, these bacteria positively correlated with members of the potentially toxic genus Pseudo-nitzchia, as well as temperature. Our study indicates that antagonistic bacteria occupy shared temporal niches and demonstrates the need to consider their cumulative impacts on diatom population health, including in future ocean conditions.

ecology↗

bioRxiv2023

Diatoms exhibit dynamic chloroplast calcium signals in response to high light and oxidative stress

Diatoms are a group of silicified algae that play a major role in marine and freshwater ecosystems. Diatom chloroplasts were acquired by secondary endosymbiosis and exhibit important structural and functional differences from the primary plastids of land plants and green algae. Many functions of primary plastids, including photoacclimation and inorganic carbon acquisition, are regulated by calcium-dependent signalling processes. Calcium signalling has also been implicated in the photoprotective responses of diatoms, although the nature of calcium elevations in diatom chloroplasts and their wider role in cell signalling remains unknown. Using genetically encoded calcium indicators, we find that the diatom Phaeodactylum tricornutum exhibits dynamic chloroplast calcium elevations that are distinct from those found in land plants. Chloroplast calcium ([Ca2+]chl) acts independently from the cytosol and is not elevated by stimuli that induce large cytosolic calcium ([Ca2+]cyt) elevations. In contrast, high light and exogenous hydrogen peroxide (H2O2) induce large, sustained calcium elevations in the chloroplast stroma that are not replicated in the cytosol. Measurements using the fluorescent H2O2 sensor roGFP2-Orp1 indicate that [Ca2+]chl elevations induced by these stimuli correspond to the accumulation of H2O2 in the chloroplast. [Ca2+]chl elevations were also induced by the addition of methyl viologen, which acts to generate superoxide within the chloroplast, and by treatments that disrupt non-photochemical quenching (NPQ). The findings indicate that diatoms generate specific calcium elevations in the chloroplast in response to high light and oxidative stress that likely modulate the activity of calcium-sensitive components in photoprotection and other regulatory pathways.

plant biology↗

bioRxiv2023

Cryptic bacterial pathogens of diatoms peak during senescence of a winter diatom bloom

Diatoms are globally abundant algae that form extensive blooms in aquatic ecosystems. Certain bacteria behave antagonistically towards diatoms, killing or inhibiting their growth. Despite their crucial implications to diatom health and bloom control, insight of the prevalence and dynamics of antagonistic bacteria in nature is lacking. We report an ecosystem assessment of the diversity and seasonal patterns of bacterial antagonists of diatoms via regular plaque-assay sampling in the Western English Channel (WEC), where diatoms frequently bloom. Unexpectedly, peaks in antagonist detection did not occur during characteristic spring blooms, but coincided with a winter bloom of Coscinodiscus, suggesting bacterial pathogens likely influence distinct diatom host populations. We isolated multiple antagonists, spanning 4 classes and 10 bacterial orders. Many species had no prior reports of pathogenicity towards diatoms, and we verified diatom growth inhibitory effects of 8 isolates. In all cases tested, pathogenicity was activated by pre-exposure to diatom organic matter. Discovery of widespread cryptic antagonistic activity evident under specific conditions, indicates that bacterial pathogenicity towards diatoms is more prevalent than previously recognised. Finally, mining Tara Oceans data revealed the global biogeography of WEC antagonists and co-occurrence patterns with diatom hosts. Our study indicates that multiple, diverse antagonistic bacteria have potential to impact diatom growth and bloom dynamics in marine waters globally.

microbiology↗

bioRxiv2022

Cold-induced 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 significant 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 have examined the mechanisms diatoms use to sense rapid changes in temperature, such as those experienced in the intertidal zone. We find 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 correlate with the rate of temperature decrease. We do not find a role for the [Ca2+]cyt elevations in enhancing cold tolerance, but show that cold shock induces a Ca2+-dependent K+ efflux and reduces mortality of P. tricornutum during a simultaneous hypo-osmotic shock. As inter-tidal diatom species may routinely encounter simultaneous cold and hypo-osmotic shocks during tidal cycles, we propose that cold-induced Ca2+ signalling interacts with osmotic signalling 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 signalling pathways may play an important role in their cellular responses to multiple simultaneous stressors.

plant biology↗

bioRxiv2020

A novel Ca2+ signalling pathway co-ordinates 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 and nitrogen. 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 diatoms use a novel and highly-sensitive Ca2+-dependent signalling pathway, not previously described in eukaryotes, to sense and respond to the critical macronutrient phosphorus. We demonstrate that phosphorus-Ca2+ signalling is essential for regulating diatom recovery from phosphorus limitation, by controlling rapid and substantial increases in nitrogen assimilation. Phosphorus-Ca2+ signalling thus mediates fundamental cross-talk between the vital nutrients P and N to maximise resource competition, and likely governs the success of diatoms as major bloom formers in regions of pulsed nutrient supply. Importantly, our study demonstrates that distinct mechanisms for nutrient sensing have evolved in photosynthetic eukaryotes.

microbiology↗

bioRxiv2020

A novel single-domain Na+-selective voltage-gated channel in photosynthetic eukaryotes

The evolution of Na+-selective four-domain voltage-gated channels (4D-Navs) in animals allowed rapid Na+-dependent electrical excitability, and enabled the development of sophisticated systems for rapid and long-range signalling. Whilst bacteria encode single-domain Na+-selective voltage-gated channels (BacNav), they typically exhibit much slower kinetics than 4D-Navs, and are not thought to have crossed the prokaryote-eukaryote boundary. As such, the capacity for rapid Na+-selective signalling is considered to be confined to certain animal taxa, and absent from photosynthetic eukaryotes. Certainly, in land plants, such as the Venus Flytrap where fast electrical excitability has been described, this is most likely based on fast anion channels. Here, we report a unique class of eukaryotic Na+-selective single-domain channels (EukCatBs) that are present primarily in haptophyte algae, including the ecologically important calcifying coccolithophores. The EukCatB channels exhibit very rapid voltage-dependent activation and inactivation kinetics, and sensitivity to the highly selective 4D-Nav blocker tetrodotoxin. The results demonstrate that the capacity for rapid Na+-based signalling in eukaryotes is not restricted to animals or to the presence of 4D-Navs. The EukCatB channels therefore represent an independent evolution of fast Na+-based electrical signalling in eukaryotes that likely contribute to sophisticated cellular control mechanisms operating on very short time scales in unicellular algae. One Sentence SummaryThe capacity for rapid Na+-based signalling has evolved in ecologically important coccolithophore species via a novel class of voltage-gated Na+ channels, EukCatBs.

microbiology↗

bioRxiv2019

Physiological and molecular responses of a newly evolved auxotroph of Chlamydomonas to B12 deprivation

The corrinoid B12 is synthesised only by prokaryotes yet is widely required by eukaryotes as an enzyme cofactor. Microalgae have evolved B12 dependence on multiple occasions and we previously demonstrated that experimental evolution of the non-requiring alga Chlamydomonas reinhardtii in media supplemented with B12 generated a B12-dependent mutant (hereafter metE7). This clone provides a unique opportunity to study the physiology of a nascent B12 auxotroph. Our analyses demonstrate that B12 deprivation of metE7 disrupted C1 metabolism, caused an accumulation of starch and triacylglycerides and a decrease in photosynthetic pigments, proteins and free amino acids. B12 deprivation also caused a substantial increase in reactive oxygen species (ROS), which preceded rapid cell death. Surprisingly, survival could be improved without compromising growth by simultaneously depriving the cells of nitrogen, suggesting a type of cross protection. Significantly, we found further improvements in survival under B12 limitation and an increase in B12 use-efficiency after metE7 underwent a further period of experimental evolution, this time in coculture with a B12-producing bacterium. Therefore, although an early B12-dependent alga would likely be poorly adapted to B12 deprivation, association with B12-producers can ensure long-term survival whilst also providing the environment to evolve mechanisms to better tolerate B12 limitation.

biochemistry↗