Cooperative motility, force generation and mechanosensing in a foraging non-photosynthetic diatom

Diatoms are ancestrally photosynthetic microalgae. However, some underwent a major evolutionary transition, losing photosynthesis to become obligate heterotrophs. The molecular and physiological basis for this transition is unclear. Here, we isolate and characterize new strains of non-photosynthetic diatoms from the coastal waters of Singapore. These diatoms occupy diverse ecological niches and display glucose-mediated catabolite repression, a classical feature of bacterial and fungal heterotrophs. Live-cell imaging reveals deposition of secreted extracellular polymeric substance (EPS). Diatoms moving on pre-existing EPS trails (runners) move faster than those laying new trails (blazers). This leads to cell-to-cell coupling where runners can push blazers to make them move faster. Calibrated micropipettes measure substantial single-cell pushing forces, which are consistent with high-order myosin motor cooperativity. Collisions that impede forward motion induce reversal, revealing navigation-related force sensing. Together, these data identify aspects of metabolism and motility that are likely to promote and underpin diatom heterotrophy.

On the role of cell surface associated, mucin-like glycoproteins in the pennate diatom Craspedostauros australis (Bacillariophyceae)

Diatoms are single-celled microalgae with silica-based cell walls (frustules) that are abundantly present in aquatic habitats, and form the basis of the food chain in many ecosystems. Many benthic diatoms have the remarkable ability to glide on all natural or man-made underwater surfaces using a carbohydrate- and protein-based adhesive to generate traction. Previously, three glycoproteins, termed FACs (Frustule Associated Components), have been identified from the common fouling diatom Craspedostauros australis and were implicated in surface adhesion through inhibition studies with a glycan-specific antibody. The polypeptide sequences of FACs remained unknown, and it was unresolved whether the FAC glycoproteins are indeed involved in adhesion, or whether this is achieved by different components sharing the same glycan epitope with FACs. Here we have determined the polypeptide sequences of FACs using peptide mapping by LC-MS/MS. Unexpectedly, FACs share the same polypeptide backbone (termed CaFAP1), which has a domain structure of alternating Cys-rich and Pro-Thr/Ser-rich regions reminiscent of the gel-forming mucins. By developing a genetic transformation system for C. australis, we were able to directly investigate the function of CaFAP1-based glycoproteins in vivo. GFP-tagging of CaFAP1 revealed that it constitutes a coat around all parts of the frustule and is not an integral component of the adhesive. CaFAP1-GFP producing transformants exhibited the same properties as wild type cells regarding surface adhesion and motility speed. Our results demonstrate that FAC glycoproteins are not involved in adhesion and motility, but might rather act as a lubricant to prevent fouling of the diatom surface.

Mechanical testing of particle streaming and intact extracellular mucilage nanofibers reveal a role of elastic force in diatom motility

Diatoms are unicellular microalgae with a rigid cell wall, able to glide on surfaces by releasing nanopolymeric fibers through central slits known as raphes. Here we consider the modelNitszchia communisto perform quantitative studies on two complementary aspects involved in diatom gliding. Using video microscopy and automated image analysis, we measure the motion of test beads as they are pulled by extracellular polymeric substances (EPS) fibers at the diatom raphe (particle streaming). A multimodal distribution of particle speed is found, evidencing the appearance of short-time events of high speed and acceleration (known as jerky motion) and suggesting that different mechanisms contribute to set diatom velocity during gliding. Furthermore, we use optical tweezers to obtain force-extension records for extracellular diatom nanofibers; records are well described by the worm-like chain model of polymer elasticity. In contrast to previous studies based on application of denaturing force (in the nN regime), application of low force (up to 6 pN) and using enable us to obtain the persistence length of intact fibers. From these measurements, mechanical parameters of EPS fibers such as radius and elastic constant are estimated. Furthermore, by modeling particle streaming as a spring in parallel with a dashpot, we show that the time involved in the release of mechanical energy after fiber detachment from beads (elastic snapping) agrees with our observations of jerky motion. We conclude that the smooth and jerky motions displayed by gliding diatoms correspond to molecular motors and elastic snapping, respectively, thus providing quantitative elements that incorporate to current models of the mechanics behind diatom locomotion.

Frustule morphogenesis of raphid pennate diatom Encyonema ventricosum (Agardh) Grunow

Diatoms stand out among other microalgae due to the high diversity of species-specific silica frustules whose components (valves and girdle bands) are formed within the cell in special organelles called silica deposition vesicles (SDVs). Research on cell structure and morphogenesis of frustule elements in diatoms of different taxonomic groups has been carried out since the 1950s but is still relevant today. Here, cytological features and valve morphogenesis in the freshwater raphid pennate diatom Encyonema ventricosum (Agardh) Grunow have been studied using light and transmission electron microscopy of cleaned frustules and ultrathin sections of cells, and scanning electron and atomic force microscopy of the frustule surface. Data have been obtained on chloroplast structure: the pyrenoid is spherical, penetrated by a lamella (a stack of two thylakoids); the girdle lamella consists of several short lamellae. The basic stages of frustule morphogenesis characteristic of raphid pennate diatoms have been traced, with the presence of cytoskeletal elements near SDVs being observed throughout this process. Degradation of the plasmalemma and silicalemma is shown to take place when the newly formed valve is released into the space between sister cells. The role of vesicular transport and exocytosis in the gliding of pennate diatoms is discussed.

Differences in dynamic behavior of single diatom cells caused by changing wavelengths

We investigate a motion of diatom cells stimulated by a halogen lamp irradiation. Diatom cells are single-celled organisms which have chloroplast. Chloroplast contains photosynthetic pigment which absorbs blue light (wave length of the light is 400 nm-450 nm) and red one (650 nm-700 nm). Light intensity of the halogen lamp is fixed about 500 Lx during the experiment. We used colored films to cut the blue or red light and observed motion of diatom cells by using the optical microscope. We found that the speed of diatom cells decreases when the colored film is inserted, and it increases after ejecting the film. It is noted that the light intensity is constant during the experiment, which means that we change wave length of the irradiated light. Our results show that the average speed of diatom cells is influenced by not the light intensity but the wave length of the light.

Inorganic carbon availability in benthic diatom communities: photosynthesis and migration

Diatom-dominated microphytobenthos (MPB) is the main primary producer of many intertidal and shallow subtidal environments, being therefore of critical importance to estuarine and coastal food webs. Owing to tidal cycles, intertidal MPB diatoms are subjected to environmental conditions far more variable than the ones experienced by pelagic diatoms (e.g. light, temperature, salinity, desiccation and nutrient availability). Nevertheless, benthic diatoms evolved adaptation mechanisms to these harsh conditions, including the capacity to move within steep physical and chemical gradients, allowing them to perform photosynthesis efficiently. In this contribution, we will review present knowledge on the effects of dissolved inorganic carbon (DIC) availability on photosynthesis and productivity of diatom-dominated MPB. We present evidence of carbon limitation of photosynthesis in benthic diatom mats and highly productive MPB natural communities. Furthermore, we hypothesize that active vertical migration of epipelic motile diatoms could overcome local depletion of DIC in the photic layer, providing the cells alternately with light and inorganic carbon supply. The few available longer-term experiments on the effects of inorganic carbon enrichment on the productivity of diatom-dominated MPB have yielded inconsistent results. Therefore, further studies are needed to properly assess the response of MPB communities to increased CO2 and ocean acidification related to climate change.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Surface sensing and stress-signalling in Ulva and fouling diatoms - potential targets for antifouling: a review

Understanding the underlying signalling pathways that enable fouling algae to sense and respond to surfaces is essential in the design of environmentally friendly coatings. Both the green alga Ulva and diverse diatoms are important ecologically and economically as they are persistent biofoulers. Ulva spores exhibit rapid secretion, allowing them to adhere quickly and permanently to a ship, whilst diatoms secrete an abundance of extracellular polymeric substances (EPS), which are highly adaptable to different environmental conditions. There is evidence, now supported by molecular data, for complex calcium and nitric oxide (NO) signalling pathways in both Ulva and diatoms being involved in surface sensing and/or adhesion. Moreover, adaptation to stress has profound effects on the biofouling capability of both types of organism. Targets for future antifouling coatings based on surface sensing are discussed, with an emphasis on pursuing NO-releasing coatings as a potentially universal antifouling strategy.

Motion analysis and ultrastructural study of a colonial diatom, Bacillaria paxillifer

The pennate diatom, Bacillaria paxillifer, forms a colony in which adjacent cells glide smoothly and almost continuously, yet no obvious apparatus driving the movement, such as flagella or cilia, is observed. Thus far, neither the mechanism nor physiological significance of this movement has been well understood. Here, we report quantitative analysis of the gliding motion of B. paxillifer and morphological analysis of this diatom with light and electron microscopes. The gliding of pairs of adjacent B. paxillifer cells in a colony was cyclic with rather constant periods while the average gliding period varied from a few seconds to multiples of 10 s among colonies. The gliding was compromised reversibly by inhibitors for actin and myosin, suggesting involvement of the actomyosin system. Indeed, we observed two closely apposed actin bundles near the raphe by fluorescence-labeled phalloidin staining. Using electron microscopy, we observed filamentous structures that resemble the actin bundles seen with fluorescence microscopy, and we also found novel electron-dense structures located between the plasma membrane and these actin-like filaments. From these and other observations, we suggest that B. paxillifer also uses actin bundles and propose a putative myosin as a molecular motor in the gliding of unicellular diatoms.

A tyrosine-rich cell surface protein in the diatom Amphora coffeaeformis identified through transcriptome analysis and genetic transformation

Diatoms are single-celled eukaryotic microalgae that are ubiquitously found in almost all aquatic ecosystems, and are characterized by their intricately structured SiO2 (silica)-based cell walls. Diatoms with a benthic life style are capable of attaching to any natural or man-made submerged surface, thus contributing substantially to both microbial biofilm communities and economic losses through biofouling. Surface attachment of diatoms is mediated by a carbohydrate- and protein- based glue, yet no protein involved in diatom underwater adhesion has been identified so far. In the present work, we have generated a normalized transcriptome database from the model adhesion diatom Amphora coffeaeformis. Using an unconventional bioinformatics analysis we have identified five proteins that exhibit unique amino acid sequences resembling the amino acid composition of the tyrosine-rich adhesion proteins from mussel footpads. Establishing the first method for the molecular genetic transformation of A. coffeaeformis has enabled investigations into the function of one of these proteins, AC3362, through expression as YFP fusion protein. Biochemical analysis and imaging by fluorescence microscopy revealed that AC3362 is not involved in adhesion, but rather plays a role in biosynthesis and/or structural stability of the cell wall. The methods established in the present study have paved the way for further molecular studies on the mechanisms of underwater adhesion and biological silica formation in the diatom A. coffeaeformis.

Isolation and biochemical characterization of underwater adhesives from diatoms

Many aquatic organisms are able to colonize surfaces through the secretion of underwater adhesives. Diatoms are unicellular algae that have the capability to colonize any natural and man-made submerged surfaces. There is great technological interest in both mimicking and preventing diatom adhesion, yet the biomolecules responsible have so far remained unidentified. A new method for the isolation of diatom adhesive material is described and its amino acid and carbohydrate composition determined. The adhesive materials from two model diatoms show differences in their amino acid and carbohydrate compositions, but also share characteristic features including a high content of uronic acids, the predominance of hydrophilic amino acid residues, and the presence of 3,4-dihydroxyproline, an extremely rare amino acid. Proteins containing dihydroxyphenylalanine, which mediate underwater adhesion of mussels, are absent. The data on the composition of diatom adhesives are consistent with an adhesion mechanism based on complex coacervation of polyelectrolyte-like biomolecules.

Underwater locomotion strategy by a benthic pennate diatom Navicula sp

The mechanism of diatom locomotion has been widely researched but still remains a hypothesis. There are several questionable points on the prevailing model proposed by Edgar, and some of the observed phenomena cannot be completely explained by this model. In this paper, we undertook detailed investigations of cell structures, locomotion, secreted mucilage, and bending deformation for a benthic pennate diatom Navicula species. According to these broad evidences, an updated locomotion model is proposed. For Navicula sp., locomotion is realized via two or more pseudopods or stalks protruded out of the frustules. The adhesion can be produced due to the pull-off of one pseudopod or stalk from the substratum through extracellular polymeric substances. And the positive pressure is generated to balance the adhesion because of the push-down of another pseudopod or stalk onto the substratum. Because of the positive pressure, friction is generated, acting as a driving force of locomotion, and the other pseudopod or stalk can detach from the substratum, resulting in the locomotion. Furthermore, this model is validated by the force evaluation and can better explain observed phenomena. This updated model would provide a novel aspect on underwater locomotion strategy, hence can be useful in terms of artificial underwater locomotion devices.

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.

Influence of illumination on settlement of diatom Navicula sp

Diatoms are responsible for biofouling, which causes many problems in various marine industries. This study examined the effects of different light conditions (intensity, incident direction, time of illumination) on the settling behavior of the marine diatom Navicula sp. on glass surfaces. The density of this diatom's settlement on glass was strongly influenced by light conditions. Moreover, very weak light emitted on the bottom of the culture dish could also rapidly inhibit diatom settlement. These phenomena were explained by spatial interference between chloroplast and holdfast-like structures inside the thecae. The holdfast-like structure is observed to be responsible for diatom locomotion and hence the settlement behavior. It was proposed that the interrelation of illumination and attachment of diatoms allowed them to better adapt to the habitat with higher efficiency of attachment and successive reproduction.

The stabilisation potential of individual and mixed assemblages of natural bacteria and microalgae

It is recognized that microorganisms inhabiting natural sediments significantly mediate the erosive response of the bed ("ecosystem engineers") through the secretion of naturally adhesive organic material (EPS: extracellular polymeric substances). However, little is known about the individual engineering capability of the main biofilm components (heterotrophic bacteria and autotrophic microalgae) in terms of their individual contribution to the EPS pool and their relative functional contribution to substratum stabilisation. This paper investigates the engineering effects on a non-cohesive test bed as the surface was colonised by natural benthic assemblages (prokaryotic, eukaryotic and mixed cultures) of bacteria and microalgae. MagPI (Magnetic Particle Induction) and CSM (Cohesive Strength Meter) respectively determined the adhesive capacity and the cohesive strength of the culture surface. Stabilisation was significantly higher for the bacterial assemblages (up to a factor of 2) than for axenic microalgal assemblages. The EPS concentration and the EPS composition (carbohydrates and proteins) were both important in determining stabilisation. The peak of engineering effect was significantly greater in the mixed assemblage as compared to the bacterial (x 1.2) and axenic diatom (x 1.7) cultures. The possibility of synergistic effects between the bacterial and algal cultures in terms of stability was examined and rejected although the concentration of EPS did show a synergistic elevation in mixed culture. The rapid development and overall stabilisation potential of the various assemblages was impressive (x 7.5 and ×9.5, for MagPI and CSM, respectively, as compared to controls). We confirmed the important role of heterotrophic bacteria in "biostabilisation" and highlighted the interactions between autotrophic and heterotrophic biofilm consortia. This information contributes to the conceptual understanding of the microbial sediment engineering that represents an important ecosystem function and service in aquatic habitats.

Cellular and Molecular Mechanics of Gliding Locomotion in Eukaryotes

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

The mechanism of diatom locomotion. II. Identification of actin

Nitrobenzooxadiazole-phallacidin binds specifically to the microfilamentous bundles associated with the raphe of a motile diatom, providing evidence for the actin-like nature of the filaments. Since the raphe is implicated in cell locomotion the locomotive force may be generated by the associated microfilaments. Alternatively, the microfilaments may be involved in the translocation of vesicles containing mucilage secreted during movement.