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Alveolate (ciliates and dinoflagellates) grazers are integral components of the marine food web and must therefore be able to sense a range of mechanical and chemical signals produced by prey and predators, integrating them via signal transduction mechanisms to respond with effective prey capture and predator evasion behaviors. However, the sensory biology of alveolate grazers is poorly understood. Using novel techniques that combine electrophysiological measurements and high-speed videomicroscopy, we investigated the sensory biology of Favella sp., a model alveolate grazer, in the context of its trophic ecology. Favella sp. produced frequent rhythmic depolarizations (~500 ms long) that caused backward swimming and are responsible for endogenous swimming patterns relevant to foraging. Contact of both prey cells and non-prey polystyrene microspheres at the cilia produced immediate mechanostimulated depolarizations (~500 ms long) that caused backward swimming, and likely underlie aggregative swimming patterns of Favella sp. in response to patches of prey. Contact of particles at the peristomal cavity that were not suitable for ingestion resulted in depolarizations after a lag of ~600 ms, allowing time for particles to be processed before rejection. Ingestion of preferred prey particles was accompanied by transient hyperpolarizations (~1 s) that likely regulate this step of the feeding process. Predation attempts by the copepod Acartia tonsa elicited fast (~20 ms) animal-like action potentials accompanied by rapid contraction of the cell to avoid predation. We have shown that the sensory mechanisms of Favella sp. are finely tuned to the type, location, and intensity of stimuli from prey and predators. [ABSTRACT FROM AUTHOR]

Coccolithophores are the most significant producers of marine biogenic calcite, although the intracellular calcification process is poorly understood. In the case of Scyphosphaera apsteinii Lohmann 1902, flat ovoid muroliths and bulky, vase-shaped lopadoliths with a range of intermediate morphologies may be produced by a single cell. This polymorphic species is within the Zygodiscales, a group that remains understudied with respect to ultrastructure and coccolith ontogeny. We therefore undertook an analysis of cell ultrastructure, morphology, and coccolithogenesis. The cell ultrastructure showed many typical haptophyte features, with calcification following a similar pattern to that described for other heterococcolith bearing species including Emiliania huxleyi. Of particular significance was the reticular body role in governing fine-scale morphology, specifically the central pore formation of the coccolith. Our observations also highlighted the essential role of the inter- and intracrystalline organic matrix in growth and arrangement of the coccolith calcite. S. apsteinii secreted mature coccoliths that attached to the plasma membrane via fibrillar material. Time-lapse light microscopy demonstrated secretion of lopadoliths occurred base first before being actively repositioned at the cell surface. Significantly, growth irradiance influenced the coccosphere composition with fewer lopadoliths being formed relative to muroliths at higher light intensities. Overall, our observations support dynamic metabolic (i.e., in response to growth irradiance), sensory and cytoskeletal control over the morphology and secretion of polymorphic heterococcoliths. With a basic understanding of calcification established, S. apsteinii could be a valuable model to further study coccolithophore calcification and cell physiological responses to ocean acidification. [ABSTRACT FROM AUTHOR]

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 [Ca2+] 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 [Ca2+] at the cell tip corresponded to the time of direction change rather than the onset of the light stimulus, it would appear that Ca2+ constitutes a component of the switching mechanism that leads to reversal of the locomotion machinery. Our current evidence suggests that the source of this Ca2+ is intracellular. [ABSTRACT FROM AUTHOR]

Marine coccolithophorid phytoplankton are major producers of biogenic calcite, playing a significant role in the global carbon cycle. Predicting the impacts of ocean acidification on coccolithophore calcification has received much recent attention and requires improved knowledge of cellular calcification mechanisms. Uniquely amongst calcifying organisms, coccolithophores produce calcified scales (coccoliths) in an intracellular compartment and secrete them to the cell surface, requiring large transcellular ionic fluxes to support calcification. In particular, intracellular calcite precipitation using HCO3 - as the substrate generates equimolar quantities of H+ that must be rapidly removed to prevent cytoplasmic acidification. We have used electrophysiological approaches to identify a plasma membrane voltage-gated H+ conductance in Coccolithus pelagicus ssp braarudii with remarkably similar biophysical and functional properties to those found in metazoans. We show that both C. pelagicus and Emiliania huxleyi possess homologues of metazoan Hv1 H+ channels, which function as voltagegated H+ channels when expressed in heterologous systems. Homologues of the coccolithophore H+ channels were also identified in a diversity of eukaryotes, suggesting a wide range of cellular roles for the Hv1 class of proteins. Using single cell imaging, we demonstrate that the coccolithophore H+ conductance mediates rapid H+ efflux and plays an important role in pH homeostasis in calcifying cells. The results demonstrate a novel cellular role for voltage gated H+ channels and provide mechanistic insight into biomineralisation by establishing a direct link between pH homeostasis and calcification. As the coccolithophore H+ conductance is dependent on the trans-membrane H+ electrochemical gradient, this mechanism will be directly impacted by, and may underlie adaptation to, ocean acidification. The presence of this H+ efflux pathway suggests that there is no obligate use of H+ derived from calcification for intracellular CO2 generation. Furthermore, the presence of Hv1 class ion channels in a wide range of extant eukaryote groups indicates they evolved in an early common ancestor. [ABSTRACT FROM AUTHOR]

Motility of estuarine epipelic (mud-inhabiting) diatoms is an important adaptation to living in biofilms present within fine sediments. Motility allows cells to migrate within the photic zone in response to a wide range of environmental stimuli. The motile responses of two species of benthic diatoms to photon fluence rates and spectral quality were investigated. Cultures of Navicula perminuta (Grunow) in van Heurck and Cylindrotheca closterium (Ehrenb.) J. C. Lewin et Reimann both exhibited photoaccumulation at ∼200 μmol · m−2 · s−1 and photodispersal from photon flux densities (PFDs) of ∼15 μmol · m−2 · s−1. Photokinesis (changing cell speed) contributed toward photodispersal for both species, and red light (λ = 681–691 nm) was most effective at inducing this process. N. perminuta showed a phototactic (directional) response, with active movement in response to a light gradient. Although this response was exhibited in white light, these directional responses were only elicited by wavelengths from 430 to 510 nm. In contrast, C. closterium did not exhibit phototaxis under any light conditions used in this study. Motile benthic diatoms thus exhibit complex and sophisticated responses to light quantity and quality, involving combinations of photokinesis and phototaxis, which can contribute toward explaining the patterns of large-scale cell movements observed in natural estuarine biofilms. [ABSTRACT FROM AUTHOR]

Background: Electrical impulses in animals play essential roles in co-ordinating an array of physiological functions including movement, secretion, environmental sensing and development. Underpinning many of these electrical signals is a fast Na+- based action potential that has been fully characterised only in cells associated with the neuromuscular systems of multicellular animals. Such rapid action potentials are thought to have evolved with the first metazoans, with cnidarians being the earliest representatives. The present study demonstrates that a unicellular protist, the marine diatom Odontella sinensis, can also generate a fast Na+/Ca2+ based action potential that has remarkably similar biophysical and pharmacological properties to invertebrates and vertebrate cardiac and skeletal muscle cells. Methodology/Principal Findings: The kinetic, ionic and pharmacological properties of the rapid diatom action potential were examined using single electrode current and voltage clamp techniques. Overall, the characteristics of the fast diatom currents most closely resemble those of vertebrate and invertebrate muscle Na+/Ca2+ currents. Conclusions/Significance: This is the first demonstration of voltage-activated Na+ channels and the capacity to generate fast Na+-based action potentials in a unicellular photosynthetic organism. The biophysical and pharmacological characteristics together with the presence of a voltage activated Na+/Ca2+ channel homologue in the recently sequenced genome of the diatom Thalassiosira pseudonana, provides direct evidence supporting the hypothesis that this rapid signalling mechanism arose in ancestral unicellular eukaryotes and has been retained in at least two phylogenetically distant lineages of eukaryotes; opisthokonts and the stramenopiles. The functional role of the fast animal-like action potential in diatoms remains to be elucidated but is likely involved in rapid environmental sensing of these widespread and successful marine protists. [ABSTRACT FROM AUTHOR]

The formation and secretion of heterococcoliths by the non-motile life phase of the coccolithophore Coccolithus pelagicus was investigated using electron microscopy and time-lapse bright field imaging. Coccolithogenesis in C. pelagicus exhibited sequential mineralization of single coccoliths in Golgi-derived and nuclear-associated vesicles, a pattern similar to the formation of heterococcoliths in Emiliania huxleyi. Our TEM data show that only on maturation does the single coccolith vesicle migrate away from the nucleus before secretion. A reticular body, distinct from the Golgi body, was also clearly visible at the distal surface of the developing coccolith vesicle, suggesting this is a common structural feature in placolith cells that mineralize and secrete coccoliths one at a time. Time-lapse imaging revealed that the coccolith secretion process is rapid, taking 60-190 seconds, and involves considerable contractile activity to eject and position the coccolith on the surface of the cell. An intact flagellar root apparatus was discovered at the anterior pole of this non-motile cell from which polarized secretion of coccoliths occurs, which may indicate a novel role for such cytoskeletal structures. Freeze-fracture preparations revealed columnar deposits and adhesions linking the scales and coccolith baseplates to the cell, across the periplasmic space providing points of attachment for cellular movement. Rotatory movements of the cell relative to external coccoliths were exhibited by all actively calcifying cells. These movements enable the cell, while exhibiting morphologically polarized secretion, to locate and secrete a mature coccolith in a spatially well-defined manner. Finally, the time-lapse imaging approach described here provides an opportunity to quantify the regulation of coccolith production in single cells with high temporal resolution allowing responses of calcification to rapidly fluctuating environmental conditions such as light-dark transitions to be examined in detail, which has not been possible with bulk calcification studies. [ABSTRACT FROM AUTHOR]

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 [Ca2+] 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 [Ca2+] at the cell tip corresponded to the time of direction change rather than the onset of the light stimulus, it would appear that Ca2+ constitutes a component of the switching mechanism that leads to reversal of the locomotion machinery. Our current evidence suggests that the source of this Ca2+ is intracellular. [ABSTRACT FROM AUTHOR]

Odontocete cetaceans have undergone profound modifications to their integument and sensory systems and are generally thought to lack specialized exocrine glands that in terrestrial mammals function to produce chemical signals (Thewissen & Nummela, 2008). Keenan-Bateman et al. (2016, 2018), though, introduced an enigmatic exocrine gland, associated with the false gill slit pigmentation pattern in Kogia breviceps. These authors provided a preliminary description of this cervical gill slit gland in their helminthological studies of the parasitic nematode, Crassicauda magna. This study offers the first detailed gross and histological description of this gland and reports upon key differences between immature and mature individuals. Investigation reveals it is a complex, compound tubuloalveolar gland with a well-defined duct that leads to a large, and expandable central chamber, which in turn leads to two caudally projecting diverticula. All regions of the gland contain branched tubular and alveolar secretory regions, although most are found in the caudal diverticula, where the secretory process is holocrine. The gland lies between slips of cutaneous muscle, and is innervated by lamellar corpuscles, resembling Pacinian's corpuscles, suggesting that its secretory product may be actively expressed into the environment. Mature K. breviceps display larger gland size, and increased functional activity in glandular tissues, as compared to immature individuals. These results demonstrate that the cervical gill slit gland of K. breviceps shares morphological features of the specialized, chemical signaling, exocrine glands of terrestrial members of the Cetartiodactyla., (© 2021 American Association for Anatomy.)

The evolution of Na + -selective four-domain voltage-gated channels (4D-Na v s) in animals allowed rapid Na + -dependent electrical excitability, and enabled the development of sophisticated systems for rapid and long-range signaling. While bacteria encode single-domain Na + -selective voltage-gated channels (BacNa v ), they typically exhibit much slower kinetics than 4D-Na v s, and are not thought to have crossed the prokaryote-eukaryote boundary. As such, the capacity for rapid Na + -selective signaling is considered to be confined to certain animal taxa, and absent from photosynthetic eukaryotes. Certainly, in land plants, such as the Venus flytrap ( Dionaea muscipula ) 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, Emiliania huxleyi and Scyphosphaera apsteinii The EukCatB channels exhibit very rapid voltage-dependent activation and inactivation kinetics, and isoform-specific sensitivity to the highly selective 4D-Na v blocker tetrodotoxin. The results demonstrate that the capacity for rapid Na + -based signaling in eukaryotes is not restricted to animals or to the presence of 4D-Na v s. The EukCatB channels therefore represent an independent evolution of fast Na + -based electrical signaling in eukaryotes that likely contribute to sophisticated cellular control mechanisms operating on very short time scales in unicellular algae., (© 2020 American Society of Plant Biologists. All Rights Reserved.)

Rapid Na + /Ca 2+ -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 + /Ca 2+ channels (4D-Ca v s/Na v s). 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 v s [5-7]. Because these lineages appear to lack rapid Na + /Ca 2+ -based action potentials, 4D-Ca v /Na v s are generally considered necessary for fast Na + /Ca 2+ -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 [8-10]. We previously demonstrated that certain diatom genomes encode 4D-Ca v /Na v s [4] but also proteins of unknown function, resembling prokaryote single-domain, voltage-gated Na + channels (BacNa v s) [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 Ca 2+ -permeable channels, with rapid kinetics resembling metazoan 4D-Ca v s/Na v s. In Phaeodactylum tricornutum, which lacks 4D-Ca v /Na v s, EukCatAs underpin voltage-activated Ca 2+ signaling important for membrane excitability, and mutants exhibit impaired motility. EukCatAs therefore provide alternative mechanisms for rapid Na + /Ca 2+ signaling in eukaryotes and may functionally replace 4D-Ca v s/Na v s in pennate diatoms. Marine phytoplankton thus possess unique signaling mechanisms that may be key to environmental sensing in the oceans., (Copyright © 2019 The Authors. Published by Elsevier Ltd.. All rights reserved.)