Diatom Cell Division Research Articles

Flavobacterial exudates disrupt cell cycle progression and metabolism of the diatom Thalassiosira pseudonana

Phytoplankton and bacteria form the base of marine ecosystems and their interactions drive global biogeochemical cycles. The effects of bacteria and bacteria-produced compounds on diatoms range from synergistic to pathogenic and can affect the physiology and transcriptional patterns of the interacting diatom. Here, we investigate physiological and transcriptional changes in the marine diatom Thalassiosira pseudonana induced by extracellular metabolites of a known antagonistic bacterium Croceibacter atlanticus. Mono-cultures of C. atlanticus released compounds that inhibited diatom cell division and elicited a distinctive morphology of enlarged cells with increased chloroplast content and enlarged nuclei, similar to what was previously observed when the diatom was co-cultured with live bacteria. The extracellular C. atlanticus metabolites induced transcriptional changes in diatom pathways that include recognition and signaling pathways, cell cycle regulation, carbohydrate and amino acid production, as well as cell wall stability. Phenotypic analysis showed a disruption in the diatom cell cycle progression and an increase in both intra- and extracellular carbohydrates in diatom cultures after bacterial exudate treatment. The transcriptional changes and corresponding phenotypes suggest that extracellular bacterial metabolites, produced independently of direct bacterial-diatom interaction, may modulate diatom metabolism in ways that support bacterial growth.

Testing the use of the silica deposition fluorescent probe PDMPO to estimate in situ growth rates of diatoms

AbstractThe fluorophore [2‐(4‐pyridyl)‐5{[4‐dimethylaminoethyl‐aminocarbamoyl‐methoxy]phenyl}oxazole], in short PDMPO, is incorporated in newly polymerized silica in diatom frustules and thereby provides a tool to estimate Si uptake, study diatom cell cycles but also determine mortality‐independent abundance‐based species specific‐growth rates in cultures and natural assemblages. In this study, the theoretical framework and applicability of the PDMPO staining technique to estimate diatom species specific‐growth rates were investigated. Three common polar diatom species, Pseudo‐nitzschia subcurvata, Chaetoceros simplex, and Thalassiosira sp., chosen in order to cover a broad range of species specific frustule and life‐cycle characteristics, were incubated over 24 h in control (no PDMPO) and with 0.125 and 0.6 μM PDMPO addition, respectively. Results indicate that specific‐growth rates of the species tested were not affected in both treatments with PDMPO addition. The specific‐growth rate estimates based on the PDMPO staining patterns (μPDMPO) were comparable and more robust than growth rates estimated from the changes in cell concentrations (μcc). This technique also allowed to investigate and highlight the importance of the illumination cycle (light and dark phases) on cell division in diatoms.

Temporal changes in diatom valve diameter indicate shifts in lake trophic status

When diatoms undergo vegetative cell division the new siliceous wall components are slightly smaller than those of the parent because they are produced within the confines of the parent wall. Thus, with continued growth the mean size of cells in a population declines. Given this unique feature of diatom cell division, if the growth of a species in a lake increases (decreases) under more (less) favorable conditions, then the mean size of the resulting population will decline (increase). Numerous paleolimnological investigations rely on shifts in the relative abundances of diatom species over time to infer lake conditions. Although relative abundance data yield information about the dominance of species in the community, they do not necessarily provide evidence about growth of a given species. For instance, a species could have increased in growth, but simply to a lesser extent than other taxa, resulting in a decline in relative abundance. In a similar fashion, relative abundance values can be misleading when used to infer environmental change, such as trophic status change in lakes. We propose that including data on mean size of diatom valves can yield greater insight into changes in growth and improve observations and conclusions based on relative abundance data. To test this concept, we examined changes in the mean diameter of Aulacoseira ambigua (Grunow) Simonsen valves relative to known shifts in lake trophic status in a core from Bantam Lake, Connecticut, representing ~ 130 years of sediment accumulation. The mean valve diameter of A. ambigua declined from 9.7 to 7.6 µm, with the largest declines clearly tracking significant increases in trophic status. We conclude that changes in the mean size of diatom frustules over time can provide valuable information for understanding long-term environmental changes.

Do diatoms dominate benthic production in shallow systems? A case study from a mixed seagrass bed

AbstractWe report an assessment for determining the contribution by diatoms to community productivity and respiration within a coastal benthic ecosystem with multiple autotrophs. During summer, cores of open sediment and seagrass habitat were collected from a lagoon within the Northern Gulf of Mexico. Cores were maintained in an outdoor mesocosm. Germanic acid, an inhibitor of diatom cell division, was added to half the cores and quantification of production and respiration was done. Inhibition of diatoms reduced benthic productivity within the seagrass habitat. 71–83% production was attributable to diatoms and this contribution moved the benthic system into net autotrophy. Diatom contribution to production in other habitat‐community components was more variable (varied from 0% to 86%). Findings underscore the ecological importance of diatoms as producers in seagrass beds, the role of seagrasses in maintaining productivity, and infer that diatoms may have similar contributions in other aquatic vegetated habitats.

Ubiquitous marine bacterium inhibits diatom cell division.

Intricate relationships between microorganisms structure the exchange of molecules between taxa, driving their physiology and evolution. On a global scale, this molecular trade is an integral component of biogeochemical cycling. As important microorganisms in the world's oceans, diatoms and bacteria have a large impact on marine biogeochemistry. Here, we describe antagonistic effects of the globally distributed flavobacterium Croceibacter atlanticus on a phylogenetically diverse group of diatoms. We used the model diatom Thalassiosira pseudonana to study the antagonistic impact in more detail. In co-culture, C. atlanticus attaches to T. pseudonana and inhibits cell division, inducing diatom cells to become larger and increase in chlorophyll a fluorescence. These changes could be explained by an absence of cytokinesis that causes individual T. pseudonana cells to elongate, accumulate more plastids and become polyploid. These morphological changes could benefit C. atlanticus by augmenting the colonizable surface area of the diatom, its photosynthetic capabilities and possibly its metabolic secretions.

Interaction and signalling between a cosmopolitan phytoplankton and associated bacteria.

Interactions between primary producers and bacteria impact the physiology of both partners, alter the chemistry of their environment, and shape ecosystem diversity. In marine ecosystems, these interactions are difficult to study partly because the major photosynthetic organisms are microscopic, unicellular phytoplankton. Coastal phytoplankton communities are dominated by diatoms, which generate approximately 40% of marine primary production and form the base of many marine food webs. Diatoms co-occur with specific bacterial taxa, but the mechanisms of potential interactions are mostly unknown. Here we tease apart a bacterial consortium associated with a globally distributed diatom and find that a Sulfitobacter species promotes diatom cell division via secretion of the hormone indole-3-acetic acid, synthesized by the bacterium using both diatom-secreted and endogenous tryptophan. Indole-3-acetic acid and tryptophan serve as signalling molecules that are part of a complex exchange of nutrients, including diatom-excreted organosulfur molecules and bacterial-excreted ammonia. The potential prevalence of this mode of signalling in the oceans is corroborated by metabolite and metatranscriptome analyses that show widespread indole-3-acetic acid production by Sulfitobacter-related bacteria, particularly in coastal environments. Our study expands on the emerging recognition that marine microbial communities are part of tightly connected networks by providing evidence that these interactions are mediated through production and exchange of infochemicals.

Functional characterization of the diatom cyclin-dependent kinase A2 as a mitotic regulator reveals plant-like properties in a non-green lineage.

BackgroundCyclin-dependent kinases (CDKs) are crucial regulators of cell cycle progression in eukaryotes. The diatom CDKA2 was originally assigned to the classical A-type CDKs, but its cell cycle phase-specific transcription at the G2-to-M phase transition is typical for plant-specific B-type CDKs.ResultsHere, we report the functional characterization of CDKA2 from the diatom Phaeodactylum tricornutum. Through a yeast two-hybrid library screen, CDKA2 was found to interact with the G2/M-specific CDK scaffolding factor CKS1. Localization of CDKA2 was found to be nuclear in interphase cells, while in cells undergoing cytokinesis, the signal extended to the cell division plane. In addition, overexpression of CDKA2 induced an overall reduction in the cell growth rate. Expression analysis of cell cycle marker genes in the overexpression lines indicates that this growth reduction is primarily due to a prolongation of the mitotic phase.ConclusionsOur study indicates a role for CDKA2 during cell division in diatoms. The functional characterization of a CDK with clear CDKB properties in a non-green organism questions whether the current definition of B-type CDKs being plant-specific might need revision.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0469-6) contains supplementary material, which is available to authorized users.

Influence of bacteria on cell size development and morphology of cultivated diatoms

SummaryVegetative cell division in diatoms often results in a decreased cell size of one of the daughter cells, which during long‐term cultivation may lead to a gradual decrease of the mean cell size of the culture. To restore the initial cell size, sexual reproduction is required, however, in many diatom cultures sexual reproduction does not occur. Such diatom cultures may lose their viability once the average size of the cells falls below a critical size. Cell size reduction therefore seriously restrains the long‐term stability of many diatom cultures. In order to study the bacterial influence on the size diminution process, we observed cell morphology and size distribution of the diatoms Achnanthidium minutissimum, Cymbella affiniformis and Nitzschia palea for more than two years in bacteria‐free conditions (axenic cultures) and in cultures that contain bacteria (xenic cultures). We found considerable morphological aberrations of frustule microstructures in A. minutissimum and C. affiniformis when cultivated under axenic conditions compared to the xenic cultures. These variations comprise significant cell length reduction, simplification and rounding of the frustule contour and deformation of the siliceous cell walls, features that are normally found in older cultures shortly before they die off. In contrast, the xenic cultures were well preserved and showed less cell length diminution. Our results show that bacteria may have a fundamental influence on the stability of long‐term cultures of diatoms.

Settlement and cell division of diatomNaviculacan be influenced by light of various qualities and intensities

Diatom settlement and cell division is important in two major aspects. Firstly, biofouling is a costly problem in the shipping industry that necessitates the effective inhibition of diatom settlement and proliferation. Secondly, biological coatings on the basis of ordered and densely packed cell lawns of diatom are useful for nano- and biotechnology. This study demonstrated that the settlement and cell division of the marine unicellular diatom Navicula sp. can be influenced by light-emitting diodes of various light qualities and intensities. Except for blue light, the settlement of diatoms was reduced by weak (approx. 0.14-6 µE m(-2) s(-1)) green, yellow or red light. When the irradiance intensity, however, was higher than 8-9 µE m(-2) s(-1), the settlement was stimulated. This phenomenon could be explained by the hypothesis of spatial interference between a chloroplast and a holdfast-like structure. Densely packed lawn of diatoms with uniform distribution can be fabricated for nanotechnologies, using blue light that stimulates diatom cell division.

AUREOCHROME1a-Mediated Induction of the Diatom-Specific Cyclin dsCYC2 Controls the Onset of Cell Division in Diatoms (Phaeodactylum tricornutum)

Cell division in photosynthetic organisms is tightly regulated by light. Although the light dependency of the onset of the cell cycle has been well characterized in various phototrophs, little is known about the cellular signaling cascades connecting light perception to cell cycle activation and progression. Here, we demonstrate that diatom-specific cyclin 2 (dsCYC2) in Phaeodactylum tricornutum displays a transcriptional peak within 15 min after light exposure, long before the onset of cell division. The product of dsCYC2 binds to the cyclin-dependent kinase CDKA1 and can complement G1 cyclin-deficient yeast. Consistent with the role of dsCYC2 in controlling a G1-to-S light-dependent cell cycle checkpoint, dsCYC2 silencing decreases the rate of cell division in diatoms exposed to light-dark cycles but not to constant light. Transcriptional induction of dsCYC2 is triggered by blue light in a fluence rate-dependent manner. Consistent with this, dsCYC2 is a transcriptional target of the blue light sensor AUREOCHROME1a, which functions synergistically with the basic leucine zipper (bZIP) transcription factor bZIP10 to induce dsCYC2 transcription. The functional characterization of a cyclin whose transcription is controlled by light and whose activity connects light signaling to cell cycle progression contributes significantly to our understanding of the molecular mechanisms underlying light-dependent cell cycle onset in diatoms.

Blue Light Checkpoint: How Blue Light Controls the Onset of Cell Division in Diatoms

Diatoms are tiny algae (see [figure][1]) that are ubiquitous to aquatic environments, from backyard ponds to the open ocean. There are over 200,000 diatom species, which together carry out roughly one-fifth of the world’s photosynthesis ([Armbrust, 2009][2]). Diatoms often live under variable

Co-limitation of diatoms by iron and silicic acid in the equatorial Pacific

The relative roles of silicon (Si) and iron (Fe) as limiting nutrients in the eastern equatorial Pacific (EEP) were examined in a series of nine microcosm experiments conducted over two years between 110°W and 140°W longitude. Si and Fe additions had consistently different but synergistic effects on macronutrient use, phytoplankton biomass and phytoplankton community structure. Silicon addition increased silicic acid use and biogenic silica production, but had no significant effect on the use of inorganic nitrogen or orthophosphate, chlorophyll accumulation, particulate inorganic (PIC) carbon accumulation, or plankton community composition relative to controls. That result, together with observations that Si addition increased the cellular Si content of the numerically dominant diatom by ∼50%, indicates that the main effect of Si was to regulate diatom silicification. Like the effect of Si, Fe addition increased the rate of silicic acid use and biogenic silica production and had no effect on PIC production. Unlike the effect of Si, Fe addition also enhanced rates of organic matter production, had no effect on cellular Si content of diatoms, and resulted in the growth of initially rare, large (>40 μm) diatoms relative to controls, indicating that Fe limitation acts mainly through its effects on growth rate and phytoplankton community composition. A pennate diatom of the genus Pseudo-nitzschia dominated the diatom assemblage in situ, grew readily in the controls and did not show a strong growth response to either Fe or Si addition suggesting that its growth was regulated by other factors such as grazing or light. Addition of germanium, an inhibitor of diatom cell division, eliminated the effects of Fe on macronutrient use, biogenic silica production and chlorophyll accumulation and phytoplankton community composition, consistent with a predominantly diatom response to Fe addition. The lack of a response of PIC production to Fe suggests that coccolithophores were not Fe limited. Addition of Fe and Si together resulted in the greatest levels of nutrient drawdown and biomass accumulation through the effect of Fe in promoting the growth of large diatoms. The results suggest a form of co-limitation with Si regulating diatom silicification and the rate of biogenic silica production while Fe regulates the production of organic matter through limitation of phytoplankton growth rates, in particular those of large diatoms. The results argue against Si regulation of new production in the EEP under average upwelling conditions. Iron addition was necessary and sufficient to stimulate complete removal of nitrate within the equatorial upwelling zone suggesting that new production was restricted by low ambient dissolved Fe consistent with results from in situ Fe fertilization experiments conducted to the south of the equator outside of the equatorial upwelling zone.

Genome-wide analysis of the diatom cell cycle unveils a novel type of cyclins involved in environmental signaling

Genes controlling the cell cycle in two diatoms have been identified and functionally characterized, revealing environmental regulation of the cell cycle.

Mitosis in diatoms: rediscovering an old model for cell division

Diatoms are important protists that generate one fifth of the oxygen produced annually on earth. These aquatic organisms likely derived from a secondary endosymbiosis event, and they display peculiar genomic and structural features that reflect their chimeric origin. Diatoms were one of the first models of cell division and these early studies revealed a range of interesting features including a unique acentriolar microtubule-organising centre. Unfortunately, almost nothing is known at the molecular level, in contrast to the advances in other experimental organisms. Recently the full genome sequences of two diatoms have been annotated and molecular tools have been developed. These resources offer new possibilities to re-investigate the mechanisms of cell division in diatoms by recruiting information from more intensively studied organisms. A renaissance of the topic is further justified by the current interest in diatoms as a source of biofuels and for understanding massive diatom proliferation events in response to environmental stimuli.

Study of the mechanism of diatom cell division by means of 29Si isotope tracing

Diatoms are delicate unicellular organisms enclosed in a silica frustule, that is made up of two valves. Multiplication of the diatoms occurs by ordinary mitotic cell division. During cell division each cell produces two daughter cells, each of them keeping one of the two valves of the mother cell and producing a new valve by absorbing the silicon present in the environment. The NanoSIMS 50 allows ion imaging to be performed on diatoms in order to determine the site of fixation of silicon. The aim of this study was to observe and compare the mechanism of the construction of the new valve after cell division. To this end, different types of diatoms have been transferred in a culture medium enriched with 29Si and after several days, the distribution of the different isotopes of silicon has been determined by NanoSIMS50 imaging. The construction of new valves has been observed and the isotopic ratio has been determined.

Infection of the single‐celled diatom Stephanodiscus alpinus by the chytrid Zygorhizidium: Parasite distribution within host population, changes in host cell size, and host–parasite size relationship

An epidemic caused by a Zygorhizidium species infecting the single‐celled planktonic centric diatom Stephanodiscus alpinus was analyzed for parasite distribution within the host population, final parasite size relative to host cell size, and size changes of infected and uninfected S. alpinus cells. Infections in the lake occurred at random within the whole host population. There was no evidence for aggregated or even distribution of the parasite individuals, indicating that the infections occur independently of each other. In enclosures in which light was enhanced compared to the lake, there tended to be an even parasite distribution within the host population, irrespective of whether plant nutrients were added. This suggests that infected host cells were negatively selected by the parasite zoospores under these conditions. Final parasite sporangium size and host cell size were positively correlated. Thus, parasite fecundity was limited by host cell size. Infected S. alpinus cells tended to be larger than uninfected cells, and the mean size of host cells within the population decreased during the epidemic. This might be due to selective infection of larger host cells or to the peculiar mode of cell division in diatoms.

Modelling Si-N-limited growth of diatoms

A mechanistic model for silicon (Si) physiology is developed, interfaced with a model of nitrogen IN) physiology, which is capable of simulating the major documented facets of Si-N physiology in diatoms. The model contains a cell cycle component that is involved in regulating the timing ol the synthesis of valves, girdles and setae. In addition to reproducing the timing of diatom cell division within a light- dark cycle, the model simulates the following features seen in real diatoms. Synthesis of valves only occurs during G 2 interphase and M, while the girdles and (if appropriate) setae are synthesized during G 1 Si stress alone results in a loss of setae, followed by a thinning of the valves in successive generations until a minimum Si cell quota is attained. After this point, the duration of G 2 increases and growth is Si limited. Concurrently, the carbon (C) cell quota increases, offering the capability to simulate the documented increase in sinking rates with Si stress. N stress alone results in an increase in the duration of G 1 and G 2 interphases, and high Si cell quotas, From this complex model which must be run for arrays of subpopulations to simulate non-synchronous growth, a simpler model is developed. This is capable of reproducing similar growth dynamics, although with no reference to component parts of the frustule. When allied to a photoacclimative submodel, a prediction of the model is that diatoms starved of Si will release increased amounts of dissolved organic C because cell growth is halted more rapidly than the photosystems can be degraded.

The species concept in diatoms

Diatoms are the most species-rich group of algae. They are ecologically widespread and have global significance in the carbon and silicon cycles, and are used increasingly in ecological monitoring, paleoecological reconstruction, and stratigraphic correlation. Despite this, the species taxonomy of diatoms is messy and lacks a satisfactory practical or conceptual basis, hindering further advances in all aspects of diatom biology. Several model systems have provided valuable insights into the nature of diatom species. A consilience of evidence (the ‘Waltonian species concept’) from morphology, genetic data, mating systems, physiology, ecology, and crossing behavior suggests that species boundaries have traditionally been drawn too broadly; many species probably contain several reproductively isolated entities that are worth taxonomic recognition at species level. Phenotypic plasticity, although present, is not a serious problem for diatom taxonomy. However, although good data are now available for demes living in sympatry, we have barely begun to extend studies to take into account variation between allopatric demes, which is necessary if a global taxonomy is to be built. Endemism has been seriously underestimated among diatoms, but biogeographical and stratigraphic patterns are difficult to discern, because of a lack of trustworthy data and because the taxonomic concepts of many authors are undocumented. Morphological diversity may often be a largely accidental consequence of physiological differentiation, as a result of the peculiarities of diatom cell division and the life cycle.

Cell division in diatoms

Cell division in diatoms

PHYTOPLANKTON GROWTH AND COMPOSITION IN SHIPBOARD CULTURES SUPPLIED WITH NITRATE, AMMONIUM, OR UREA AS THE NITROGEN SOURCE1

Coastal surface seawater off southern California was enriched with phosphate, silicate, vitamins B12, B1, and biotin, a chelated trace metal mixture, and with nitrogen as nitrate, ammonium, or urea, and the cultures were incubated on deck in daylight. Natural phytoplankton present in the seawater served as inoculum. Samples were taken every 6 hr, once the cultures were established. Diel periodicity was noted in diatom cell division, in nitrate and ammonium assimilation rate, in phosphate assimilation rate, and in photosynthetic rate measured at intervals under constant, artificial irradiance, but not in the rate of chlorophyll a synthesis. The chemical composition of the crops was influenced by the nitrogen source used for growth and by diel periodicity in assimilation rates. Most of the vitamin Bi content of the crop was synthesized by the organisms, only a small proportion being supplied initially from the medium. Some species known to require vitamins continued to grow after vitamin depletion from the medium, their requirements apparently satisfied by vitamins released by other species.