A fast Na+/Ca2+-based action potential in a marine diatom

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.

Electrophysiological techniques in marine microalgae study: A new perspective for harmful algal bloom (HAB) research

Electrophysiological techniques, by measuring bioelectrical signals and ion channel activities in tissues and cells, are now widely utilized to study ion channel-related physiological functions and their underlying mechanisms. Electrophysiological techniques have been extensively employed in the investigation of animals, plants, and microorganisms; however, their application in marine algae lags behind that in other organisms. In this paper, we present an overview of current electrophysiological techniques applicable to algae while reviewing the historical usage of such techniques in this field. Furthermore, we explore the potential specific applications of electrophysiological technology in harmful algal bloom (HAB) research. The application prospects in the studies of stress tolerance, competitive advantage, nutrient absorption, toxin synthesis and secretion by HAB microalgae are discussed and anticipated herein with the aim of providing novel perspectives on HAB investigations.

Genome analysis of Parmales, the sister group of diatoms, reveals the evolutionary specialization of diatoms from phago-mixotrophs to photoautotrophs

The order Parmales (class Bolidophyceae) is a minor group of pico-sized eukaryotic marine phytoplankton that contains species with cells surrounded by silica plates. Previous studies revealed that Parmales is a member of ochrophytes and sister to diatoms (phylum Bacillariophyta), the most successful phytoplankton group in the modern ocean. Therefore, parmalean genomes can serve as a reference to elucidate both the evolutionary events that differentiated these two lineages and the genomic basis for the ecological success of diatoms vs. the more cryptic lifestyle of parmaleans. Here, we compare the genomes of eight parmaleans and five diatoms to explore their physiological and evolutionary differences. Parmaleans are predicted to be phago-mixotrophs. By contrast, diatoms have lost genes related to phagocytosis, indicating the ecological specialization from phago-mixotrophy to photoautotrophy in their early evolution. Furthermore, diatoms show significant enrichment in gene sets involved in nutrient uptake and metabolism, including iron and silica, in comparison with parmaleans. Overall, our results suggest a strong evolutionary link between the loss of phago-mixotrophy and specialization to a silicified photoautotrophic life stage early in diatom evolution after diverging from the Parmales lineage.

Temperature sensitivity of carbon concentrating mechanisms in the diatom Phaeodactylum tricornutum

Marine diatoms are key primary producers across diverse habitats in the global ocean. Diatoms rely on a biophysical carbon concentrating mechanism (CCM) to supply high concentrations of CO₂ around their carboxylating enzyme, RuBisCO. The necessity and energetic cost of the CCM are likely to be highly sensitive to temperature, as temperature impacts CO₂ concentration, diffusivity, and the kinetics of CCM components. Here, we used membrane inlet mass spectrometry (MIMS) and modeling to capture temperature regulation of the CCM in the diatom Phaeodactylum tricornutum (Pt). We found that enhanced carbon fixation rates by Pt at elevated temperatures were accompanied by increased CCM activity capable of maintaining RuBisCO close to CO₂ saturation but that the mechanism varied. At 10 and 18 °C, diffusion of CO₂ into the cell, driven by Pt's 'chloroplast pump' was the major inorganic carbon source. However, at 18 °C, upregulation of the chloroplast pump enhanced (while retaining the proportion of) both diffusive CO₂ and active HCO₃^- uptake into the cytosol, and significantly increased chloroplast HCO₃^- concentrations. In contrast, at 25 °C, compared to 18 °C, the chloroplast pump had only a slight increase in activity. While diffusive uptake of CO₂ into the cell remained constant, active HCO₃^- uptake across the cell membrane increased resulting in Pt depending equally on both CO₂ and HCO₃^- as inorganic carbon sources. Despite changes in the CCM, the overall rate of active carbon transport remained double that of carbon fixation across all temperatures tested. The implication of the energetic cost of the Pt CCM in response to increasing temperatures was discussed.

Regulation and integration of membrane transport in marine diatoms

Diatoms represent one of the most successful groups of marine phytoplankton and are major contributors to ocean biogeochemical cycling. They have colonized marine, freshwater and ice environments and inhabit all regions of the World's oceans, from poles to tropics. Their success is underpinned by a remarkable ability to regulate their growth and metabolism during nutrient limitation and to respond rapidly when nutrients are available. This requires precise regulation of membrane transport and nutrient acquisition mechanisms, integration of nutrient sensing mechanisms and coordination of different transport pathways. This review outlines transport mechanisms involved in acquisition of key nutrients (N, C, P, Si, Fe) by marine diatoms, illustrating their complexity, sophistication and multiple levels of control.

Electrical Signaling and Its Functions Under Conditions of Abiotic Stress: A Review of Methodological Approaches and Physiological Implications

In contrast to chemical messengers, electrical signals such as action potentials and variation potentials can transmit information much faster over long distances. Electrical signals can be triggered by various abiotic stress factors and are propagated via plasmodesmata over short distances and within the phloem over long distances. Thus, in addition to assimilate transport from sources to sinks, the phloem serves as a communication highway for various types of information. Key factors for systemic signaling in the phloem are peptides, RNAs, hormones, and electrical signals. In recent years, there has been increasing evidence that rapid communication by means of electrical signals is essential for various plant physiological processes. Thus, this chapter focuses on electrical signaling and various associated physiological effects, such as regulation of leaf movements, assimilate transport, photosynthesis, and gas exchange, as well as plant water status.

Evolution of long-distance signalling upon plant terrestrialization: comparison of action potentials in Characean algae and liverworts

BACKGROUND

In this review, we summarize data concerning action potentials (APs) - long-distance electrical signals in Characean algae and liverworts. These lineages are key in understanding the mechanisms of plant terrestrialization. Liverworts are postulated to be pioneer land plants, whereas aquatic charophytes are considered the closest relatives to land plants. The drastic change of the habitat was coupled with the adaptation of signalling systems to the new environment.

SCOPE

APs fulfil the 'all-or-nothing' law, exhibit refractory periods and propagate with a uniform velocity. Their ion mechanism in the algae and liverworts consists of a Ca2+ influx (from external and internal stores) followed by/coincident with a Cl- efflux, which both evoke the membrane potential depolarization, and a K+ efflux leading to repolarization. The molecular identity of ion channels responsible for these fluxes remains unknown. Publication of the Chara braunii and Marchantia polymorpha genomes opened up new possibilities for studying the molecular basis of APs. Here we present the list of genes which can participate in AP electrogenesis. We also point out the differences between these plant species, e.g. the absence of Ca2+-permeable glutamate receptors (GLRs) and Cl--permeable SLAC1 channel homologues in the Chara genome. Both these channels play a vital role in long-distance signalling in liverworts and vascular plants. Among the common properties of APs in liverworts and higher plants is their duration (dozens of seconds) and the speed of propagation (mm s-1), which are much slower than in the algae (seconds, and dozens of mm s-1, respectively).

CONCLUSIONS

Future studies with combined application of electrophysiological and molecular techniques should unravel the ion channel proteins responsible for AP generation, their regulation and transduction of those signals to physiological responses. This should also help to understand the adaptation of the signalling systems to the land environment and further evolution of APs in vascular plants.

Evolution of synapses and neurotransmitter systems: The divide-and-conquer model for early neural cell-type evolution

Nervous systems evolved around 560 million years ago to coordinate and empower animal bodies. Ctenophores - one of the earliest-branching lineages - are thought to share a few neuronal genes with bilaterians and may have evolved neurons convergently. Here we review our current understanding of the evolution of neuronal molecules in nonbilaterians. We also reanalyse single-cell sequencing data in light of new cell-cluster identities from a ctenophore and uncover evidence supporting the homology of one ctenophore neuron-type with neurons in Bilateria. The specific coexpression of the presynaptic proteins Unc13 and RIM with voltage-gated channels, neuropeptides and homeobox genes pinpoint a spiking sensory-peptidergic cell in the ctenophore mouth. Similar Unc13-RIM neurons may have been present in the first eumetazoans to rise to dominance only in stem Bilateria. We hypothesise that the Unc13-RIM lineage ancestrally innervated the mouth and conquered other parts of the body with the rise of macrophagy and predation during the Cambrian explosion.

Integrative Neuroscience of Paramecium, a "Swimming Neuron"

Paramecium is a unicellular organism that swims in fresh water by beating thousands of cilia. When it is stimulated (mechanically, chemically, optically, thermally…), it often swims backward then turns and swims forward again. This "avoiding reaction" is triggered by a calcium-based action potential. For this reason, some authors have called Paramecium a "swimming neuron." This review summarizes current knowledge about the physiological basis of behavior of Paramecium.

Origins of eukaryotic excitability

All living cells interact dynamically with a constantly changing world. Eukaryotes, in particular, evolved radically new ways to sense and react to their environment. These advances enabled new and more complex forms of cellular behaviour in eukaryotes, including directional movement, active feeding, mating, and responses to predation. But what are the key events and innovations during eukaryogenesis that made all of this possible? Here we describe the ancestral repertoire of eukaryotic excitability and discuss five major cellular innovations that enabled its evolutionary origin. The innovations include a vastly expanded repertoire of ion channels, the emergence of cilia and pseudopodia, endomembranes as intracellular capacitors, a flexible plasma membrane and the relocation of chemiosmotic ATP synthesis to mitochondria, which liberated the plasma membrane for more complex electrical signalling involved in sensing and reacting. We conjecture that together with an increase in cell size, these new forms of excitability greatly amplified the degrees of freedom associated with cellular responses, allowing eukaryotes to vastly outperform prokaryotes in terms of both speed and accuracy. This comprehensive new perspective on the evolution of excitability enriches our view of eukaryogenesis and emphasizes behaviour and sensing as major contributors to the success of eukaryotes. This article is part of the theme issue 'Basal cognition: conceptual tools and the view from the single cell'.

Brevetoxin and Conotoxin Interactions with Single-Domain Voltage-Gated Sodium Channels from a Diatom and Coccolithophore

The recently characterized single-domain voltage-gated ion channels from eukaryotic protists (EukCats) provide an array of novel channel proteins upon which to test the pharmacology of both clinically and environmentally relevant marine toxins. Here, we examined the effects of the hydrophilic µ-CTx PIIIA and the lipophilic brevetoxins PbTx-2 and PbTx-3 on heterologously expressed EukCat ion channels from a marine diatom and coccolithophore. Surprisingly, none of the toxins inhibited the peak currents evoked by the two EukCats tested. The lack of homology in the outer pore elements of the channel may disrupt the binding of µ-CTx PIIIA, while major structural differences between mammalian sodium channels and the C-terminal domains of the EukCats may diminish interactions with the brevetoxins. However, all three toxins produced significant negative shifts in the voltage dependence of activation and steady state inactivation, suggesting alternative and state-dependent binding conformations that potentially lead to changes in the excitability of the phytoplankton themselves.

Dynamics of microcystins and saxitoxin in the Indian River Lagoon, Florida

Harmful algal blooms that can produce toxins are common in the Indian River Lagoon (IRL), which covers ~250 km of Florida's east coast. The current study assessed the dynamics of microcystins and saxitoxin in six segments of the IRL: Banana River Lagoon (BRL), Mosquito Lagoon (ML), Northern IRL (NIRL), Central IRL (CIRL), Southern IRL (SIRL), and the St. Lucie Estuary (SLE). Surface water samples (n = 40) collected during the 2018 wet and 2019 dry season were analyzed to determine associations between toxins and temperature, salinity, pH, oxygen saturation, concentrations of dissolved nutrients and chlorophyll-a, presence of biosynthetic genes for toxins, relative abundance of planktonic species, and composition of the microbial community. The potential toxicity of samples was assessed using multiple mammalian cell lines. Enzyme-Linked Immunosorbent Assays were used to determine concentrations of microcystins and saxitoxin. Overall, the microcystins concentration ranged between 0.01-85.70 µg/L, and saxitoxin concentrations ranged between 0.01-2.43 µg/L across the IRL. Microcystins concentrations were 65% below the limit of quantification (0.05 µg/L), and saxitoxin concentrations were 85% below the limit of detection (0.02 µg/L). Microcystins concentrations were higher in the SLE, while saxitoxin was elevated in the NIRL and BRL. Cytotoxicity related to the presence of microcystins was seen in the SLE during the wet season. No significant patterns between cytotoxicity and saxitoxin were identified. Dissolved nutrients were identified as the most highly related parameters, explaining 53% of microcystin and 47% of saxitoxin variability. Multivariate models suggested cyanobacteria, flagellates, ciliates, and diatoms as the subset of microorganisms whose abundances were maximally correlated with saxitoxin and microcystins concentrations. Lastly, biosynthetic genes for microcystins were detected in the SLE and for saxitoxin in the BRL and NIRL. These results highlight the synergistic roles environmental and biological parameters play in influencing the dynamics of toxin production by harmful algae in the IRL.

Extremely Potent Block of Bacterial Voltage-Gated Sodium Channels by µ-Conotoxin PIIIA

µ-Conotoxin PIIIA, in the sub-picomolar, range inhibits the archetypal bacterial sodium channel NaChBac (NavBh) in a voltage- and use-dependent manner. Peptide µ-conotoxins were first recognized as potent components of the venoms of fish-hunting cone snails that selectively inhibit voltage-gated skeletal muscle sodium channels, thus preventing muscle contraction. Intriguingly, computer simulations predicted that PIIIA binds to prokaryotic channel NavAb with much higher affinity than to fish (and other vertebrates) skeletal muscle sodium channel (Nav 1.4). Here, using whole-cell voltage clamp, we demonstrate that PIIIA inhibits NavBac mediated currents even more potently than predicted. From concentration-response data, with [PIIIA] varying more than 6 orders of magnitude (10⁻¹² to 10⁻⁵ M), we estimated an IC₅₀ = ~5 pM, maximal block of 0.95 and a Hill coefficient of 0.81 for the inhibition of peak currents. Inhibition was stronger at depolarized holding potentials and was modulated by the frequency and duration of the stimulation pulses. An important feature of the PIIIA action was acceleration of macroscopic inactivation. Docking of PIIIA in a NaChBac (NavBh) model revealed two interconvertible binding modes. In one mode, PIIIA sterically and electrostatically blocks the permeation pathway. In a second mode, apparent stabilization of the inactivated state was achieved by PIIIA binding between P2 helices and trans-membrane S5s from adjacent channel subunits, partially occluding the outer pore. Together, our experimental and computational results suggest that, besides blocking the channel-mediated currents by directly occluding the conducting pathway, PIIIA may also change the relative populations of conducting (activated) and non-conducting (inactivated) states.

Plants Neither Possess nor Require Consciousness

In claiming that plants have consciousness, 'plant neurobiologists' have consistently glossed over the remarkable degree of structural and functional complexity that the brain had to evolve for consciousness to emerge. Here, we outline a new hypothesis proposed by Feinberg and Mallat for the evolution of consciousness in animals. Based on a survey of the brain anatomy, functional complexity, and behaviors of a broad spectrum of animals, criteria were established for the emergence of consciousness. The only animals that satisfied these criteria were the vertebrates (including fish), arthropods (e.g., insects, crabs), and cephalopods (e.g., octopuses, squids). In light of Feinberg and Mallat's analysis, we consider the likelihood that plants, with their relative organizational simplicity and lack of neurons and brains, have consciousness to be effectively nil.

Diatom Signaling: A Novel Channel Type Identified

Our nerve and muscle cells communicate using voltage-dependent four-domain sodium channels. A recent study in eukaryotic phytoplankton reveals that single-domain, prokaryote-like, voltage-gated Na⁺- and Ca²⁺-permeable channels are key to environmental sensing in the oceans.

Alternative Mechanisms for Fast Na+/Ca2+ Signaling in Eukaryotes via a Novel Class of Single-Domain Voltage-Gated Channels

Rapid Na⁺/Ca²⁺-based action potentials govern essential cellular functions in eukaryotes, from the motile responses of unicellular protists, such as Paramecium [1, 2], to complex animal neuromuscular activity [3]. A key innovation underpinning this fundamental signaling process has been the evolution of four-domain voltage-gated Na⁺/Ca²⁺ channels (4D-Ca_vs/Na_vs). These channels are widely distributed across eukaryote diversity [4], albeit several eukaryotes, including land plants and fungi, have lost voltage-sensitive 4D-Ca_v/Na_vs [5, 6, 7]. Because these lineages appear to lack rapid Na⁺/Ca²⁺-based action potentials, 4D-Ca_v/Na_vs are generally considered necessary for fast Na⁺/Ca²⁺-based signaling [7]. However, the cellular mechanisms underpinning the membrane physiology of many eukaryotes remain unexamined. Eukaryotic phytoplankton critically influence our climate as major primary producers. Several taxa, including the globally abundant diatoms, exhibit membrane excitability [8, 9, 10]. We previously demonstrated that certain diatom genomes encode 4D-Ca_v/Na_vs [4] but also proteins of unknown function, resembling prokaryote single-domain, voltage-gated Na⁺ channels (BacNa_vs) [4]. Here, we show that single-domain channels are actually broadly distributed across major eukaryote phytoplankton lineages and represent three novel classes of single-domain channels, which we refer collectively to as EukCats. Functional characterization of diatom EukCatAs indicates that they are voltage-gated Na⁺- and Ca²⁺-permeable channels, with rapid kinetics resembling metazoan 4D-Ca_vs/Na_vs. In Phaeodactylum tricornutum, which lacks 4D-Ca_v/Na_vs, EukCatAs underpin voltage-activated Ca²⁺ signaling important for membrane excitability, and mutants exhibit impaired motility. EukCatAs therefore provide alternative mechanisms for rapid Na⁺/Ca²⁺ signaling in eukaryotes and may functionally replace 4D-Ca_vs/Na_vs in pennate diatoms. Marine phytoplankton thus possess unique signaling mechanisms that may be key to environmental sensing in the oceans.

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Novel class of single-domain, voltage-gated channels (EukCatAs) identified in diatoms

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EukCatAs are fast voltage-gated Na⁺- and Ca²⁺-permeable channels

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EukCatAs underpin voltage-activated Ca²⁺ signaling and membrane excitability

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EukCatAs may have functionally replaced 4D-Ca_v/Na_v channels in pennate diatoms

Eukaryotic Voltage-Gated Sodium Channels: On Their Origins, Asymmetries, Losses, Diversification and Adaptations

The appearance of voltage-gated, sodium-selective channels with rapid gating kinetics was a limiting factor in the evolution of nervous systems. Two rounds of domain duplications generated a common 24 transmembrane segment (4 × 6 TM) template that is shared amongst voltage-gated sodium (Na_v1 and Na_v2) and calcium channels (Ca_v1, Ca_v2, and Ca_v3) and leak channel (NALCN) plus homologs from yeast, different single-cell protists (heterokont and unikont) and algae (green and brown). A shared architecture in 4 × 6 TM channels include an asymmetrical arrangement of extended extracellular L5/L6 turrets containing a 4-0-2-2 pattern of cysteines, glycosylated residues, a universally short III-IV cytoplasmic linker and often a recognizable, C-terminal PDZ binding motif. Six intron splice junctions are conserved in the first domain, including a rare U12-type of the minor spliceosome provides support for a shared heritage for sodium and calcium channels, and a separate lineage for NALCN. The asymmetrically arranged pores of 4x6 TM channels allows for a changeable ion selectivity by means of a single lysine residue change in the high field strength site of the ion selectivity filter in Domains II or III. Multicellularity and the appearance of systems was an impetus for Na_v1 channels to adapt to sodium ion selectivity and fast ion gating. A non-selective, and slowly gating Na_v2 channel homolog in single cell eukaryotes, predate the diversification of Na_v1 channels from a basal homolog in a common ancestor to extant cnidarians to the nine vertebrate Na_v1.x channel genes plus Nax. A close kinship between Na_v2 and Na_v1 homologs is evident in the sharing of most (twenty) intron splice junctions. Different metazoan groups have lost their Na_v1 channel genes altogether, while vertebrates rapidly expanded their gene numbers. The expansion in vertebrate Na_v1 channel genes fills unique functional niches and generates overlapping properties contributing to redundancies. Specific nervous system adaptations include cytoplasmic linkers with phosphorylation sites and tethered elements to protein assemblies in First Initial Segments and nodes of Ranvier. Analogous accessory beta subunit appeared alongside Na_v1 channels within different animal sub-phyla. Na_v1 channels contribute to pace-making as persistent or resurgent currents, the former which is widespread across animals, while the latter is a likely vertebrate adaptation.

Interaction of a dinoflagellate neurotoxin with voltage-activated ion channels in a marine diatom

Background

The potent neurotoxins produced by the harmful algal bloom species Karenia brevis are activators of sodium voltage-gated channels (VGC) in animals, resulting in altered channel kinetics and membrane hyperexcitability. Recent biophysical and genomic evidence supports widespread presence of homologous sodium (Na⁺) and calcium (Ca²⁺) permeable VGCs in unicellular algae, including marine phytoplankton. We therefore hypothesized that VGCs of these phytoplankton may be an allelopathic target for waterborne neurotoxins produced by K. brevis blooms that could lead to ion channel dysfunction and disruption of signaling in a similar manner to animal Na⁺ VGCs.

Methods

We examined the interaction of brevetoxin-3 (PbTx-3), a K. brevis neurotoxin, with the Na⁺/Ca²⁺ VGC of the non-toxic diatom Odontella sinensis using electrophysiology. Single electrode current- and voltage- clamp recordings from O. sinensis in the presence of PbTx-3 were used to examine the toxin’s effect on voltage gated Na⁺/Ca²⁺ currents. In silico analysis was used to identify the putative PbTx binding site in the diatoms. We identified Na⁺/Ca²⁺ VCG homologs from the transcriptomes and genomes of 12 diatoms, including three transcripts from O. sinensis and aligned them with site-5 of Na⁺ VGCs, previously identified as the PbTx binding site in animals.

Results

Up to 1 µM PbTx had no effect on diatom resting membrane potential or membrane excitability. The kinetics of fast inward Na⁺/Ca²⁺ currents that underlie diatom action potentials were also unaffected. However, the peak inward current was inhibited by 33%, delayed outward current was inhibited by 25%, and reversal potential of the currents shifted positive, indicating a change in permeability of the underlying channels. Sequence analysis showed a lack of conservation of the PbTx binding site in diatom VGC homologs, many of which share molecular features more similar to single-domain bacterial Na⁺/Ca²⁺ VGCs than the 4-domain eukaryote channels.

Discussion

Although membrane excitability and the kinetics of action potential currents were unaffected, the permeation of the channels underlying the diatom action potential was significantly altered in the presence of PbTx-3. However, at environmentally relevant concentrations the effects of PbTx- on diatom voltage activated currents and interference of cell signaling through this pathway may be limited. The relative insensitivity of phytoplankton VGCs may be due to divergence of site-5 (the putative PbTx binding site), and in some cases, such as O. sinensis, resistance to toxin effects may be because of evolutionary loss of the 4-domain eukaryote channel, while retaining a single domain bacterial-like VGC that can substitute in the generation of fast action potentials.

Collective electrical oscillations of a diatom population induced by dark stress

Diatoms are photosynthetic microalgae, a group with a major environmental role on the planet due to the biogeochemical cycling of silica and global fixation of carbon. However, they can evolve into harmful blooms through a resourceful communication mechanism, not yet fully understood. Here, we demonstrate that a population of diatoms under darkness show quasi-periodic electrical oscillations, or intercellular waves. The origin is paracrine signaling, which is a feedback, or survival, mechanism that counteracts changes in the physicochemical environment. The intracellular messenger is related to Ca²⁺ ions since spatiotemporal changes in their concentration match the characteristics of the intercellular waves. Our conclusion is supported by using a Ca²⁺ channel inhibitor. The transport of Ca²⁺ ions through the membrane to the extracellular medium is blocked and the intercellular waves disappear. The translation of microalgae cooperative signaling paves the way for early detection and prevention of harmful blooms and an extensive range of stress-induced alterations in the aquatic ecosystem.

Comparative studies of saxitoxin (STX) -induced cytotoxicity in Neuro-2a and RTG-2 cell lines: An explanation with respect to changes in ROS

Saxitoxin (STX), a paralytic shellfish toxin (PST) produced from toxic bloom-forming dinoflagellates, was selected to comparatively investigate the induction of cytotoxicity and apoptosis and a possible mechanism based on changes in the antioxidant defence system of two cellular strains: the mouse neuroblastoma cell line Neuro-2a and the rainbow trout fish cell line RTG-2. Increasing concentrations of STX (0-256 nM) presented little cytotoxic or apoptotic effects on the two cell lines. Measurements of cellular viability, lethal ratio and LDH leakage showed slight changes in Neuro-2a and RTG-2 cells (p > 0.05), and similar results were observed for cellular morphology and apoptotic rates. The contents of the main reactive oxygen species (ROS) components, superoxide anion (O₂^-) and hydrogen peroxide (H₂O₂), were markedly increased in Neuro-2a cell with STX exposure at middle (15 nM) and high (150 nM) concentrations (p < 0.05), and the simultaneous increase of the ratio of reduced/oxidized glutathione (GSH/GSSG) (p < 0.05) inferred the occurrence of oxidative stress. However, little difference was observed in all treated groups of RTG-2 cells. The activities of three antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), were significantly enhanced in Neuro-2a cells in the middle and high concentration groups (p < 0.05), while glutathione peroxidase (GPX) obviously decreased (p < 0.05) in all treated groups. Little change was found in RTG-2 cells with the same exposures. These results provided evidence that STX exposure altered the redox status of Neuro-2a cells and resulted in oxidative stress, but the same exposure exerted little effect on RTG-2 cells. Therefore, Neuro-2a cells are more sensitive than reproductive cells to STX exposure, and the antioxidant systems appears to be partly responsible for this differentiation response.

Dynamic sinking behaviour in marine phytoplankton: rapid changes in buoyancy may aid in nutrient uptake

Phytoplankton sinking is an important property that can determine community composition in the photic zone and material loss to the deep ocean. To date, studies of diatom suspension have relied on bulk measurements with assumptions that bulk rates adequately capture the essential characteristics of diatom sinking. However, recent work has illustrated that individual diatom sinking rates vary considerably from the mean bulk rate. In this study, we apply high-resolution optical techniques, individual-based observations of diatom sinking and a recently developed method of flow visualization around freely sinking cells. The results show that in both field samples and laboratory cultures, some large species of centric diatoms are capable of a novel behaviour, whereby cells undergo bursts of rapid sinking that alternate with near-zero sinking rates on the timescales of seconds. We also demonstrate that this behaviour is under direct metabolic control of the cell. We discuss these results in the context of implications for nutrient flux to the cell surface. While nutrient flux in large diatoms increases during fast sinking, current mass transport models cannot incorporate the unsteady sinking behaviour observed in this study. However, large diatoms appear capable of benefiting from the enhanced nutrient flux to their surface during rapid sinking even during brief intervening periods of near-zero sinking rates.

Evolutionary origin of synapses and neurons - Bridging the gap

The evolutionary origin of synapses and neurons is an enigmatic subject that inspires much debate. Non-bilaterian metazoans, both with and without neurons and their closest relatives already contain many components of the molecular toolkits for synapse functions. The origin of these components and their assembly into ancient synaptic signaling machineries are particularly important in light of recent findings on the phylogeny of non-bilaterian metazoans. The evolution of synapses and neurons are often discussed only from a metazoan perspective leaving a considerable gap in our understanding. By taking an integrative approach we highlight the need to consider different, but extremely relevant phyla and to include the closest unicellular relatives of metazoans, the ichthyosporeans, filastereans and choanoflagellates, to fully understand the evolutionary origin of synapses and neurons. This approach allows for a detailed understanding of when and how the first pre- and postsynaptic signaling machineries evolved.

The effects of pH and pCO2 on photosynthesis and respiration in the diatom Thalassiosira weissflogii

The response of marine phytoplankton to the ongoing increase in atmospheric pCO₂ reflects the consequences of both increased CO₂ concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO₂ and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H₂¹⁸O. In long-term experiments (~6-h) at saturating light intensity, we observed no effects of pH or pCO₂ on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of pCO₂ or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO₂-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H⁺ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO₂ predicted for the twenty-first century.

Modulation of host ROS metabolism is essential for viral infection of a bloom-forming coccolithophore in the ocean

The cosmopolitan coccolithophore Emiliania huxleyi is a unicellular eukaryotic alga responsible for vast blooms in the ocean. These blooms have immense impact on large biogeochemical cycles and are terminated by a specific large double-stranded DNA E. huxleyi virus (EhV, Phycodnaviridae). EhV infection is accompanied by induction of hallmarks of programmed cell death and production of reactive oxygen species (ROS). Here we characterized alterations in ROS metabolism and explored its role during infection. Transcriptomic analysis of ROS-related genes predicted an increase in glutathione (GSH) and H2O2 production during infection. In accordance, using biochemical assays and specific fluorescent probes we demonstrated the overproduction of GSH during lytic infection. We also showed that H2O2 production, rather than superoxide, is the predominant ROS during the onset of the lytic phase of infection. Using flow cytometry, confocal microscopy and multispectral imaging flow cytometry, we showed that the profound co-production of H2O2 and GSH occurred in the same subpopulation of cells but at different subcellular localization. Positively stained cells for GSH and H2O2 were highly infected compared with negatively stained cells. Inhibition of ROS production by application of a peroxidase inhibitor or an H2O2 scavenger inhibited host cell death and reduced viral production. We conclude that viral infection induced remodeling of the host antioxidant network that is essential for a successful viral replication cycle. This study provides insight into viral replication strategy and suggests the use of specific cellular markers to identify and quantify the extent of active viral infection during E. huxleyi blooms in the ocean.

Electrical Wiring and Long-Distance Plant Communication

Electrical signalling over long distances is an efficient way of achieving cell-to-cell communication in living organisms. In plants, the phloem can be considered as a 'green cable' that allows the transmission of action potentials (APs) induced by stimuli such as wounding and cold. Measuring phloem potential changes and separating them from secondary responses of surrounding tissues can be achieved using living aphids as bioelectrodes. Two glutamate receptor-like genes (GLR3.3 and 3.6) were identified as being involved in the propagation of electrical activity from the damaged to undamaged leaves. However, phloem APs are initiated and propagated independently of these glutamate receptors. Here, we propose new screening approaches to obtain further information on the components required for electrical signalling in phloem cables.

From damage response to action potentials: early evolution of neural and contractile modules in stem eukaryotes

Eukaryotic cells convert external stimuli into membrane depolarization, which in turn triggers effector responses such as secretion and contraction. Here, we put forward an evolutionary hypothesis for the origin of the depolarization-contraction-secretion (DCS) coupling, the functional core of animal neuromuscular circuits. We propose that DCS coupling evolved in unicellular stem eukaryotes as part of an 'emergency response' to calcium influx upon membrane rupture. We detail how this initial response was subsequently modified into an ancient mechanosensory-effector arc, present in the last eukaryotic common ancestor, which enabled contractile amoeboid movement that is widespread in extant eukaryotes. Elaborating on calcium-triggered membrane depolarization, we reason that the first action potentials evolved alongside the membrane of sensory-motile cilia, with the first voltage-sensitive sodium/calcium channels (Nav/Cav) enabling a fast and coordinated response of the entire cilium to mechanosensory stimuli. From the cilium, action potentials then spread across the entire cell, enabling global cellular responses such as concerted contraction in several independent eukaryote lineages. In animals, this process led to the invention of mechanosensory contractile cells. These gave rise to mechanosensory receptor cells, neurons and muscle cells by division of labour and can be regarded as the founder cell type of the nervous system.

Quantitative description of ion transport via plasma membrane of yeast and small cells

Modeling of ion transport via plasma membrane needs identification and quantitative understanding of the involved processes. Brief characterization of main ion transport systems of a yeast cell (Pma1, Ena1, TOK1, Nha1, Trk1, Trk2, non-selective cation conductance) and determining the exact number of molecules of each transporter per a typical cell allow us to predict the corresponding ion flows. In this review a comparison of ion transport in small yeast cell and several animal cell types is provided. The importance of cell volume to surface ratio is emphasized. The role of cell wall and lipid rafts is discussed in respect to required increase in spatial and temporary resolution of measurements. Conclusions are formulated to describe specific features of ion transport in a yeast cell. Potential directions of future research are outlined based on the assumptions.

Antifouling action of polyisoprene-based coatings by inhibition of photosynthesis in microalgae

Previous studies have demonstrated that ionic and non-ionic natural rubber-based coatings inhibit adhesion and growth of marine bacteria, fungi, microalgae, and spores of macroalgae. Nevertheless, the mechanism of action of these coatings on the different micro-organisms is not known. In the current study, antifouling activity of a series of these rubber-based coatings (one ionic and two non-ionic) was studied with respect to impacts on marine microalgal photosynthesis using pulse-amplitude-modulation (PAM) fluorescence. When grown in contact with the three different coatings, an inhibition of photosynthetic rate (relative electron transport rate, rETR) was observed in all of the four species of pennate diatoms involved in microfouling, Cocconeis scutellum, Amphora coffeaeformis, Cylindrotheca closterium, and Navicula jeffreyi. The percentage of inhibition ranged from 44% to 100% of the controls, depending on the species and the coating. The ionic coating was the most efficient antifouling (AF) treatment, and C. scutellum and A. coffeaeformis are the most sensitive and tolerant diatoms tested, respectively. Photosynthetic inhibition was reversible, as almost complete recovery of rETR was observed 48 h post exposure, after detachment of cells from the coatings. Thus, the antifouling activity seemed mostly due to an effect of contact with materials. It is hypothesized that photosynthetic activity was suppressed by coatings due to interference in calcium availability to the microalgal cells; Ca(2+) has been shown to be an essential micro/macro nutrient for photosynthesis, as well as being involved in cell adhesion and motility in pennate diatoms.

Proton channels in algae: reasons to be excited

A fundamental requirement of all eukaryotes is the ability to translocate protons across membranes. This is critical in bioenergetics, for compartmentalized metabolism, and to regulate intracellular pH (pH(i)) within a range that is compatible with cellular metabolism. Plants, animals, and algae utilize specialized transport machinery for membrane energization and pH homeostasis that reflects the prevailing ionic conditions in which they evolved. The recent characterization of H(+)-permeable channels in marine and freshwater algae has led to the discovery of novel functions for these transport proteins in both cellular pH homeostasis and sensory biology. Here we review the potential implications for understanding the origins and evolution of membrane excitability and the phytoplankton-based marine ecosystem responses to ocean acidification.

Induction to oxidative stress by saxitoxin investigated through lipid peroxidation in Neuro 2A cells and Chlamydomonas reinhardtii alga

Saxitoxin (STX) is a cyanotoxin, which can cause neurotoxic effects and induce ecological changes in aquatic environments, a potential risk to public and environmental health. Many studies of cytotoxicity on animal cells and algae have been performed, although few compare the toxic effects between the two models. In this sense, we investigated the oxidative stress induced by STX (0.4-3.0 nM) in two different cellular models: Neuro-2A (N2A) cells and Chlamydomonas reinhardtii alga by quantification of malondialdehyde (MDA) levels as indicative of lipid peroxidation (LPO). Also was evaluated the antioxidant defense of these cells systems after exposure to STX by the addition of antioxidants in N2A cells culture, and by the measure of antioxidants enzymes activity in C. reinhardtii cells. The MDA levels of N2A cells increased from 15% to 113% for 0.4 and 3.0 nM of STX, respectively, as compared to control. Superoxide-dismutase and catalase did not appear to protect the cell from STX effect while, in cells treated with vitamin E, the rates of MDA production decreased significantly, except for higher concentrations of STX. No MDA productions were observed in algal cells however some effects on antioxidant enzymes activity were observed when algae were exposed to 3.0 nM STX. Our results indicate that the concentrations of STX that may induce oxidative stress through LPO are different in animal and phytoplankton communities. A combination of algal and animal bioassays should be conducted for reliable assessment of oxidative stress induced by STX.

CALCIUM RELEASE FROM INTRACELLULAR STORES IS NECESSARY FOR THE PHOTOPHOBIC RESPONSE IN THE BENTHIC DIATOM NAVICULA PERMINUTA (BACILLARIOPHYCEAE)(1)

Complex photoreceptor pathways exist in algae to exploit light as a sensory stimulus. Previous studies have implicated calcium in blue-light signaling in plants and algae. A photophobic response to high-intensity blue light was characterized in the marine benthic diatom Navicula perminuta (Grunow) in van Heurck. Calcium modulators were used to determine the involvement of calcium in the signaling of this response, and the fluorescent calcium indicator Calcium Crimson was used to image changes in intracellular [Ca(2+) ] during a response. A localized, transient elevation of Calcium Crimson fluorescence was seen at the cell tip at the time of cell reversal. Intracellular calcium release inhibitors produced a significant decrease in the population photophobic response. Treatments known to decrease influx of extracellular calcium had no effect on the population photophobic response but did cause a significant decrease in average cell speed. As the increase in intracellular [Ca(2+) ] at the cell tip corresponded to the time of direction change rather than the onset of the light stimulus, it would appear that Ca(2+) constitutes a component of the switching mechanism that leads to reversal of the locomotion machinery. Our current evidence suggests that the source of this Ca(2+) is intracellular.

Osmotic stress triggers toxin production by the dinoflagellate Karenia brevis

With the increase in frequency of harmful algal blooms (HABs) worldwide, a better understanding of the mechanisms that influence toxin production is needed. Karenia brevis, the major HAB dinoflagellate in the Gulf of Mexico, produces potent neurotoxins, known as brevetoxins. Human health is directly impacted by blooms of K. brevis through consumption of shellfish contaminated by accumulated brevetoxins (neurotoxic shellfish poisoning) or from aerosolized brevetoxins in sea spray (reduced respiratory function); however, the reason for brevetoxin production has remained a mystery. Here we show that brevetoxin production increased dramatically in response to osmotic stress in three of the four K. brevis clones examined. By rapidly changing salinity to simulate a shift from oceanic conditions to a decreased salinity typical of coastal conditions, brevetoxin production was triggered. As a result, brevetoxin cell quota increased by >14-fold, while growth rate remained unchanged. Live images of K. brevis cells were also examined to assess changes in cell volume. In the K. brevis Wilson clone, cells responded quickly to hypoosmotic stress by increasing their brevetoxin cell quota from ∼10 to 160 pg of brevetoxin per cell, while cell volume remained stable. In contrast, the K. brevis SP1 clone, which has a consistently low brevetoxin cell quota (<1 pg per cell), was unable to balance the hypoosmotic stress, and although brevetoxin production remained low, average cell volume increased. Our findings close a critical gap in knowledge regarding mechanisms for toxin production in K. brevis by providing an explanation for toxin production in this harmful dinoflagellate.

Investigation of animal and algal bioassays for reliable saxitoxin ecotoxicity and cytotoxicity risk evaluation

Contamination of water bodies by saxitoxin can result in various toxic effects in aquatic organisms. Saxitoxin contamination has also been shown to be a threat to human health in several reported cases, even resulting in death. In this study, we evaluated the sensitivity of animal (Neuro-2A) and algal (Chlamydomonas reinhardtii) bioassays to saxitoxin effect. Neuro-2A cells were found to be sensitive to saxitoxin, as shown by a 24 h EC50 value of 1.5 nM, which was obtained using a cell viability assay. Conversely, no saxitoxin effect was found in any of the algal biomarkers evaluated, for the concentration range tested (2-128 nM). These results indicate that saxitoxin may induce toxic effects in animal and human populations at concentrations where phytoplankton communities are not affected. Therefore, when evaluating STX risk of toxicity, algal bioassays do not appear to be reliable indicators and should always be conducted in combination with animal bioassays.

Predicting what extra-terrestrials will be like: and preparing for the worst

It is difficult to imagine evolution in alien biospheres operating in any manner other than Darwinian. Yet, it is also widely assumed that alien life-forms will be just that: strange, un-nerving and probably repulsive. There are two reasons for this view. First, it is assumed that the range of habitable environments available to extra-terrestrial life is far wider than on Earth. I suggest, however, that terrestrial life is close to the physical and chemical limits of life anywhere. Second, it is a neo-Darwinian orthodoxy that evolution lacks predictability; imagining what extra-terrestrial life would look like in any detail is a futile exercise. To the contrary, I suggest that the outcomes of evolution are remarkably predictable. This, however, leads us to consider two opposites, both of which should make our blood run cold. The first, and actually extremely unlikely, is that alien biospheres will be strikingly similar to our terrestrial equivalent and that in such biospheres intelligence will inevitably emerge. The reasons for this revolve around the ubiquity of evolutionary convergence, the determinate structure of the Tree of Life and molecular inherency. But if something like a human is an inevitability, why do I also claim that the first possibility is 'extremely unlikely'? Simply because the other possibility is actually the correct answer. Paradoxically, we and our biosphere are completely alone. So which is worse? Meeting ourselves or meeting nobody?

Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling

Much of our current knowledge on the mechanisms by which Ca(2+) signals are generated in photosynthetic eukaryotes comes from studies of a relatively small number of model species, particularly green plants and algae, revealing some common features and notable differences between 'plant' and 'animal' systems. Physiological studies from a broad range of algal cell types have revealed the occurrence of animal-like signalling properties, including fast action potentials and fast propagating cytosolic Ca(2+) waves. Genomic studies are beginning to reveal the widespread occurrence of conserved channel types likely to be involved in Ca(2+) signalling. However, certain widespread 'ancient' channel types appear to have been lost by certain groups, such as the embryophytes. More recent channel gene loss is also evident from comparisons of more closely related algal species. The underlying processes that have given rise to the current distributions of Ca(2+) channel types include widespread retention of ancient Ca(2+) channel genes, horizontal gene transfer (including symbiotic gene transfer and acquisition of bacterial genes), gene loss and gene expansion within taxa. The assessment of the roles of Ca(2+) channel genes in diverse physiological, developmental and life history processes represents a major challenge for future studies.