Silicified cell walls as a defensive trait in diatoms

Organismal trade-offs and the pace of planktonic life

No one is perfect, and organisms that perform well in some habitat or with respect to some tasks, do so at the cost of performance in others: there are inescapable trade-offs. Organismal trade-offs govern the structure and function of ecosystems and attempts to demonstrate and quantify trade-offs have therefore been an important goal for ecologists. In addition, trade-offs are a key component in trait-based ecosystem models. Here, I synthesise evidence of trade-offs in plankton organisms, from bacteria to zooplankton, and show how a slow-fast gradient in life histories emerges. I focus on trade-offs related to the main components of an organism's Darwinian fitness, that is resource acquisition, survival, and propagation. All consumers need to balance the need to eat without being eaten, and diurnal vertical migration, where zooplankton hide at depth during the day to avoid visual predators but at the cost of missed feeding opportunities in the productive surface layer, is probably the best documented result of this trade-off. However, there are many other more subtle but equally important behaviours that similarly are the result of an optimisation of these trade-offs. Most plankton groups have also developed more explicit defence mechanisms, such as toxin production or evasive behaviours that are harnessed in the presence of their predators; the costs of these have often proved difficult to quantify or even demonstrate, partly because they only materialise under natural conditions. Finally, all multicellular organisms must allocate time and resources among growth, reproduction, and maintenance (e.g. protein turnover and DNA repair), and mate finding may compromise both survival and feeding. The combined effects of all these trade-offs is the emergence of a slow-fast gradient in the pace-of-life, likely the most fundamental principle for the organisation of organismal life histories. This crystallisation of trade-offs may offer a path to further simplification of trait-based models of marine ecosystems.

Widespread Production of Polyunsaturated Aldehydes by Benthic Diatoms of the North Pacific Ocean's Salish Sea

Diatoms are key primary producers across marine, freshwater, and terrestrial ecosystems. They are responsible for photosynthesis and secondary production that, in part, support complex food webs. Diatoms can produce phytochemicals that have transtrophic ecological effects which increase their competitive fitness. Polyunsaturated aldehydes (PUAs) are one class of diatom-derived phytochemicals that are known to have allelopathic and anti-herbivory properties. The anti-herbivory capability of PUAs results from their negative effect on grazer fecundity. Since their discovery, research has focused on their production by pelagic marine diatoms, and their effects on copepod egg production, hatching success, and juvenile survival and development. Few investigations have explored PUA production by the prolific suite of benthic marine diatoms, despite their importance to coastal trophic systems. In this study, we tested eight species of benthic diatoms for the production of the bioactive PUAs 2,4-heptadienal, 2,4-octadienal, and 2,4-decadienal. Benthic diatom species were isolated from the Salish Sea, an inland sea within the North Pacific ecosystem. All species were found to be producers of at least two PUAs in detectable concentrations, with five species producing all three PUAs in quantifiable concentrations. Our results indicate that production of PUAs from Salish Sea benthic diatoms may be widespread, and thus these compounds may contribute to benthic coastal food web dynamics through heretofore unrecognized pathways. Future studies should expand the geographic scope of investigations into benthic diatom PUA production and explore the effects of benthic diatoms on benthic consumer fecundity.

Exploring contribution of phytoplankton cell death to settleable particulate organic carbon in the East China Sea in spring

Phytoplankton's death contributes to marine settleable particulate organic matter (POM). In this study, we used laboratory cultivation of different algal species to identify a positive correlation between the cumulative number of dead algal cells and POC>75 (carbon content of the settleable POM). The contribution coefficient of cell death to POC>75 varied among different algal species. Additionally, the field survey and incubation experiment were conducted in the East China Sea (ECS) to explore the spatial-temporal correlation between phytoplankton death and POC>75. The results concluded that phytoplankton death was the main factor controlling POC>75. In the ECS, the relationship between the surface cumulative mass of POC>75 and the cumulative number of dead cells followed: Cumulative mass of POC>75(mg) = 0.487 × Cumulative number of dead cells (/104) + 0.069. This study provided a methodology to quantitatively explain the relationship between phytoplankton death and settleable POM.

Mechanism of branching morphogenesis inspired by diatom silica formation

The silica-based cell walls of diatoms are prime examples of genetically controlled, species-specific mineral architectures. The physical principles underlying morphogenesis of their hierarchically structured silica patterns are not understood, yet such insight could indicate novel routes toward synthesizing functional inorganic materials. Recent advances in imaging nascent diatom silica allow rationalizing possible mechanisms of their pattern formation. Here, we combine theory and experiments on the model diatom Thalassiosira pseudonana to put forward a minimal model of branched rib patterns-a fundamental feature of the silica cell wall. We quantitatively recapitulate the time course of rib pattern morphogenesis by accounting for silica biochemistry with autocatalytic formation of diffusible silica precursors followed by conversion into solid silica. We propose that silica deposition releases an inhibitor that slows down up-stream precursor conversion, thereby implementing a self-replicating reaction-diffusion system different from a classical Turing mechanism. The proposed mechanism highlights the role of geometrical cues for guided self-organization, rationalizing the instructive role for the single initial pattern seed known as the primary silicification site. The mechanism of branching morphogenesis that we characterize here is possibly generic and may apply also in other biological systems.

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

A Microfabricated, Flow-Driven Grinding Mill for Mechanical Cell Lysing

This paper presents the design, microfabrication, and demonstration of a novel microfluidic grinding mill for the lysis of the dinoflagellate, Alexandrium, a neurotoxin-producing genus of algae that is responsible for red tide and paralytic shellfish poisoning. The mill consists of a high-speed, hydrodynamically driven microrotor coupled to a micro grinding mill that lyses robust algal cells by mechanical abrasion with single-pass efficiencies as high as 97%. These efficiencies are comparable to, or better than, current mechanical and chemical lysing methods without adding complications associated with harsh chemical additives that can interfere with subsequent downstream bioanalysis. Release of cytoplasm from lysed algae was confirmed using polymerase chain reaction (PCR) amplification of Alexandrium DNA using dinoflagellate primers.

A review of the taxonomic diversity, host-parasite interactions, and experimental research on chytrids that parasitize diatoms

Diatoms (Bacillariophyta) are a major source of primary production on Earth, generating between 1/4 to 1/2 of all oxygen. They are found in almost all bodies of water, the ice of mountains, the arctic and the antarctic, and soils. Diatoms are also a major source of food in aquatic systems, a key component of the silica cycle, and are carbon capturers in oceans. Recently, diatoms have been examined as sources of biofuels, food, and other economic boons. Chytrids are members of the Kingdom fungi comprising, at a minimum, Chytridiomycota, Blastocladiomycota, and Neocallimastigales. Most chytrids are saprobes, plant pathogens, or parasites, and play an important role in aquatic ecosystems. Chytrid parasitism of diatoms has been reported to cause epidemics of over 90% fatality, though most of the information regarding these epidemics is limited to interactions between just a few hosts and parasites. Given the ubiquity of diatoms, their importance in natural and economic systems, and the massive impact epidemics can have on populations, the relative lack of knowledge regarding parasitism by chytrids is alarming. Here we present a list of the firsthand accounts of diatoms reported parasitized by chytrids. The list includes 162 named parasitic chytrid-diatom interactions, with 63 unique chytrid taxa from 11 genera, and 74 unique diatom taxa from 28 genera. Prior to this review, no list of all documented diatom-chytrid interactions existed. We also synthesize the currently known methods of infection, defense, and experiments examining diatoms and chytrids, and we document the great need for work examining both a greater breadth of taxonomic diversity of parasites and hosts, and a greater depth of experiments probing their interactions. This resource is intended to serve as a building block for future researchers studying diatom-parasite interactions and global planktonic communities in both fresh and marine systems.

Quantitative elemental imaging in eukaryotic algae

All organisms, fundamentally, are made from the same raw material, namely the elements of the periodic table. Biochemical diversity is achieved by how these elements are utilized, for what purpose, and in which physical location. Determining elemental distributions, especially those of trace elements that facilitate metabolism as cofactors in the active centers of essential enzymes, can determine the state of metabolism, the nutritional status, or the developmental stage of an organism. Photosynthetic eukaryotes, especially algae, are excellent subjects for quantitative analysis of elemental distribution. These microbes utilize unique metabolic pathways that require various trace nutrients at their core to enable their operation. Photosynthetic microbes also have important environmental roles as primary producers in habitats with limited nutrient supplies or toxin contaminations. Accordingly, photosynthetic eukaryotes are of great interest for biotechnological exploitation, carbon sequestration, and bioremediation, with many of the applications involving various trace elements and consequently affecting their quota and intracellular distribution. A number of diverse applications were developed for elemental imaging, allowing subcellular resolution, with X-ray fluorescence microscopy (XFM, XRF) being at the forefront, enabling quantitative descriptions of intact cells in a non-destructive method. This Tutorial Review summarizes the workflow of a quantitative, single-cell elemental distribution analysis of a eukaryotic alga using XFM.

Comparative assessment of algaecide performance on freshwater phytoplankton: Understanding differential sensitivities to frame cyanobacteria management

Cyanobacterial bloom represent a growing threat to global water security. With fast proliferation, they raise great concern due to potential health and socioeconomic concerns. Algaecides are commonly employed as a mitigative measure to suppress and manage cyanobacteria. However, recent research on algaecides has a limited phycological focus, concentrated predominately on cyanobacteria and chlorophytes. Without considering phycological diversity, generalizations crafted from these algaecide comparisons present a biased perpective. To limit the collateral impacts of algaecide interventions on phytoplankton communities it is critical to understand differential phycological sensitivities for establishing optimal dosage and tolerance thresholds. This research attempts to fill this knowledge gap and provide effective guidelines to frame cyanobacterial management. We investigate the effect of two common algaecides, copper sulfate (CuSO₄) and hydrogen peroxide (H₂O₂), on four major phycological divisions (chlorophytes, cyanobacteria, diatoms, and mixotrophs). All phycological divisions exhibited greater sensitivity to copper sulfate, except chlorophytes. Mixotrophs and cyanobacteria displayed the highest sensitivity to both algaecides with the highest to lowest sensitivity being observed as follows: mixotrophs, cyanobacteria, diatoms, and chlorophytes. Our results suggest that H₂O₂ represents a comparable alternative to CuSO₄ for cyanobacterial control. However, some eukaryotic divisions such as mixotrophs and diatoms mirrored cyanobacteria sensitivity, challenging the assumption that H₂O₂ is a selective cyanocide. Our findings suggest that optimizing algaecide treatments to suppress cyanobacteria while minimizing potential adverse effects on other phycological members is unattainable. An apparent trade-off between effective cyanobacterial management and conserving non-targeted phycological divisions is expected and should be a prime consideration of lake management.

Siliceous scales in the centrohelid heliozoan Raphidocystis contractilis facilitate settlement to the substratum

The centrohelid heliozoan Raphidocystis contractilis has hundreds of small scales on the surface of the cell body. To understand the biological functions of the scales, comparative examinations were conducted between wild-type and scale-deficient strains that has naturally lost scales after long-term cultivation. The scale-deficient strain exhibited decreased adhesion to the substratum and had a lower sedimentation rate in water than the wild-type strain, suggesting that the scale may have the ability to attach quickly and strongly to the substratum. Percoll density gradient centrifugation showed that the scale-deficient strain had a lower density than that of the wild-type strain. In the wild-type strain, more scaled cells were observed in the higher specific gravity fractions. During the long-term culture of cells, only the cells suspended in the upper area of the flask were transferred to fresh medium. By repeating this procedure, we may have selected only cells that did not possess normal scales. In the natural environment, centrohelid heliozoans are easily flushed away if they cannot adhere strongly to the bottom. These results suggest that they use scales to ensure effective adhesion to the substratum.

The Co-Evolution Aspects of the Biogeochemical Role of Phytoplankton in Aquatic Ecosystems: A Review

In freshwater and marine ecosystems, the phytoplankton community is based on microalgae and cyanobacteria, which include phylogenetically very diverse groups of oxygenic photoautotrophs. In the process of evolution, they developed a wide range of bio(geo)chemical adaptations that allow them to effectively use solar radiation, CO₂, and nutrients, as well as major and trace elements, to form O₂ and organic compounds with a high chemical bond energy. The inclusion of chemical elements in the key processes of energy and plastic metabolism in the cell is determined by redox conditions and the abundance and metabolic availability of elements in the paleoenvironment. Geochemical evolution, which proceeded simultaneously with the evolution of biosystems, contributed to an increase in the number of metals and trace elements acting as cofactors of enzymes involved in metabolism and maintaining homeostasis in the first photoautotrophs. The diversity of metal-containing enzymes and the adaptive ability to replace one element with another without losing the functional properties of enzymes ensured the high ecological plasticity of species and allowed microalgae and cyanobacteria to successfully colonize a wide variety of habitats. In this review, we consider the main aspects of the modern concepts of the biogeochemical evolution of aquatic ecosystems and the role of some metals in the main bioenergetic processes in photosynthetic prokaryotes and eukaryotes. We present generalized data on the efficiency of the assimilation of key nutrients by phytoplankton and their importance in the cycle of carbon, silicon, nitrogen, phosphorus, sulfur, and iron. This article presents modern views on the evolutionary prerequisites for the formation of elemental signatures in different systematic groups of microalgae, as well as the possibility of using the stoichiometric ratio in the study of biological and geochemical processes in aquatic ecosystems.

Morphological, physiological, and transcriptional responses of the freshwater diatom Fragilaria crotonensis to elevated pH conditions

Harmful algal blooms (HABs) caused by the toxin-producing cyanobacteria Microcystis spp., can increase water column pH. While the effect(s) of these basified conditions on the bloom formers are a high research priority, how these pH shifts affect other biota remains understudied. Recently, it was shown these high pH levels decrease growth and Si deposition rates in the freshwater diatom Fragilaria crotonensis and natural Lake Erie (Canada-US) diatom populations. However, the physiological mechanisms and transcriptional responses of diatoms associated with these observations remain to be documented. Here, we examined F. crotonensis with a set of morphological, physiological, and transcriptomic tools to identify cellular responses to high pH. We suggest 2 potential mechanisms that may contribute to morphological and physiological pH effects observed in F. crotonensis. Moreover, we identified a significant upregulation of mobile genetic elements in the F. crotonensis genome which appear to be an extreme transcriptional response to this abiotic stress to enhance cellular evolution rates-a process we have termed "genomic roulette." We discuss the ecological and biogeochemical effects high pH conditions impose on fresh waters and suggest a means by which freshwater diatoms such as F. crotonensis may evade high pH stress to survive in a "basified" future.

RNA-seq analysis reveals genes related to photosynthetic carbon partitioning and lipid production in Phaeodactylum tricornutum under alkaline conditions

Alkaline pH can induce triacylglyceride accumulation in microalgae, however its molecular mechanism remains elusive. Here, we investigated the effect of 2-[N-cyclohexylamino]-ethane-sulfonic acid (CHES) -induced intracellular alkalization on the lipid production in Phaeodactylum tricornutum. Intracellular pH was increased upon CHES treatment, displaying a high BCECF fluorescence ratio. CHES treatment significantly induced lipid accumulation but had no change in cell density and biomass. The expression of genes involved in photoreaction, carbon fixation, glycolysis, pentose phosphate pathway, amino acid catabolism, GS/GOGAT cycle, TCA cycle, triacylglyceride assembly, de novo fatty acid synthesis were up-regulated, while that in amino acid biosynthesis were down-regulated under CHES conditions. Correspondingly, the activity of phosphoribulokinase, carbonic anhydrase, pyruvate dehydrogenase and acetaldehyde dehydrogenase were enhanced by CHES treatment. Chloroplast-specific biological processes were activated by CHES treatment in P. tricornutum, which redirects the flux of carbon into lipid biosynthesis, meanwhile stimulates de novo fatty acid biosynthesis, leading to lipid accumulation under CHES conditions. These indicate an enhancement of intermediate metabolism, resulting in lipid accumulation by CHES.

Solids retention time dependent, tunable diatom hierarchical micro/nanostructures and their effect on nutrient removal

The hierarchical three-dimensional (3D) micro/nanostructures of diatoms make them a promising biomaterial for fabricating nanomaterials, producing bioactive pharmaceuticals or nutraceuticals, and removing micropollutants. For diatom production in a continuous flow system, little is known how bioreactor operating parameters, especially solids retention time (SRT), affect the 3D structures of diatoms. This study demonstrated that tunable diatom micro/nanostructures could be produced by varying the SRT of membrane bioreactors (MBRs). A diatom strain (Stephanodiscus hantzschii) was cultivated in two identical MBRs with a fixed hydraulic retention time (HRT) of 24 h and staged SRTs from 5, to 10, and to 20 d. As SRTs increased from 5 to 20 d, important characteristics of diatom micro/nanostructures showed linear decreases: the diameters of foramina on the areola layer decreased from 170 ± 10 to 130 ± 12 nm, the numbers of nanopores per cribrum layer decreased from 20 ± 3 to 12 ± 2, and the specific surface areas of the diatoms decreased from 36.01 ± 1.27 to 12.67 ± 2.45 m²·g^-1. However, the average diatom heights increased from 2.9 ± 0.3 to 3.9 ± 0.4 µm, while diatom cell diameter (5 µm) and nanopore size (20 nm) remained unchanged. The silicon content of diatoms also linearly increased with SRT. The decrease in diatom porosity and increase in silicon content were probably due to the reduced diatom growth rates (likely resulting in less pores) at increasing SRTs, which also facilitated silica deposition as the overall diatom population stayed longer in the MBRs. As the SRTs increased from 5 to 10, and to 20 d, the nitrate (NO₃^-) removal efficiency decreased from 75% to 70%, and to 60%, respectively, whereas phosphorus (P) removal efficiency increased from 74% to 80%, and to 90%, respectively. The opposite trends in efficiencies were because NO₃^--N was removed by cellular uptake and biomass waste whereas P was mainly removed through diatom-assisted chemical precipitation.

Shedding light on silica biomineralization by comparative analysis of the silica-associated proteomes from three diatom species

Morphogenesis of the intricate patterns of diatom silica cell walls is a protein-guided process, yet to date only very few such silica biomineralization proteins have been identified. Therefore, it is currently unknown whether all diatoms share conserved proteins of a basal silica forming machinery, and whether unique proteins are responsible for the morphogenesis of species-specific silica patterns. To answer these questions, we extracted proteins from the silica of three diatom species (Thalassiosira pseudonana, Thalassiosira oceanica, and Cyclotella cryptica) by complete demineralization of the cell walls. Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) analysis of the extracts identified 92 proteins that we name 'soluble silicome proteins' (SSPs). Surprisingly, no SSPs are common to all three species, and most SSPs showed very low similarity to one another in sequence alignments. In-depth bioinformatics analyses revealed that SSPs could be grouped into distinct classes based on short unconventional sequence motifs whose functions are yet unknown. The results from the in vivo localization of selected SSPs indicates that proteins, which lack sequence homology but share unconventional sequence motifs may exert similar functions in the morphogenesis of the diatom silica cell wall.

Investigating the Morphology and Mechanics of Biogenic Hierarchical Materials at and below Micrometer Scale

Investigating and understanding the intrinsic material properties of biogenic materials, which have evolved over millions of years into admirable structures with difficult to mimic hierarchical levels, holds the potential of replacing trial-and-error-based materials optimization in our efforts to make synthetic materials of similarly advanced complexity and properties. An excellent example is biogenic silica which is found in the exoskeleton of unicellular photosynthetic algae termed diatoms. Because of the complex micro- and nanostructures found in their exoskeleton, determining the intrinsic mechanical properties of biosilica in diatoms has only partly been accomplished. Here, a general method is presented in which a combination of in situ deformation tests inside an SEM with a realistic 3D model of the frustule of diatom Craspedostauros sp. (C. sp.) obtained by electron tomography, alongside finite element method (FEM) simulations, enables quantification of the Young’s modulus (E = 2.3 ± 0.1 GPa) of this biogenic hierarchical silica. The workflow presented can be readily extended to other diatom species, biominerals, or even synthetic hierarchical materials.

Multi-Element Composition of Diatom Chaetoceros spp. from Natural Phytoplankton Assemblages of the Russian Arctic Seas

Simple Summary

Despite the long history of studying the elemental composition of phytoplankton and its individual ecological and systematic groups or specific algae species, the global dataset is far from completed. Our original research aims to study the elemental composition of a certain taxonomic group of marine diatoms, whose representatives make a significant contribution to primary production in the Arctic Ocean. The data on the chemical composition of diatom microalgae are discussed concerning their role in the global biogeochemical circulation of elements in the ocean. In particular, the obtained data make a prominent input to the study of the multi-element composition of marine diatom species, namely Chaetoceros spp., inhabiting the shelf seas of the Arctic Ocean. These data may be used as a basis for the cultivation of marine diatom strains for obtaining commercially promising producers of biogenic silica or valuable biological products that can be used as raw materials in the production of feed and nutrition for agriculture and aquaculture.

Abstract

Data on the elemental composition of the diatom Chaetoceros spp. from natural phytoplankton communities of Arctic marine ecosystems are presented for the first time. Samples were collected during the 69th cruise (22 August–26 September 2017) of the R/V Akademik Mstislav Keldysh in the Kara, Laptev, and East Siberian Seas. The multi-element composition of the diatom microalgae was studied by ICP-AES and ICP-MS methods. The contents of major (Na, Mg, Al, Si, P, S, K and Ca), trace (Li, Be, B, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb, Bi, Th and U) and rare earth (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) elements varied greatly, which was probably associated with the peculiarities of the functional state and mineral nutrition of phytoplankton in the autumn period. Biogenic silicon was the dominant component of the chemical composition of Chaetoceros spp., averaging 19.10 ± 0.58% of dry weight (DW). Other significant macronutrients were alkaline (Na and K) and alkaline earth (Ca and Mg) metals as well as biogenic (S and P) and essential (Al and Fe) elements. Their total contents varied from 1.26 to 2.72% DW, averaging 2.07 ± 0.43% DW. The Al:Si ratio for natural assemblages of Chaetoceros spp. of the shelf seas of the Arctic Ocean was 5.8 × 10⁻³. The total concentrations of trace and rare earth elements on average were 654.42 ± 120.07 and 4.14 ± 1.37 μg g⁻¹ DW, respectively. We summarize the scarce data on the average chemical composition of marine and oceanic phytoplankton and discuss the limitations and approaches of such studies. We conclude on the lack of data and the need for further targeted studies on this issue.

Predator-induced defence in a dinoflagellate generates benefits without direct costs

Inducible defences in phytoplankton are often assumed to come at a cost to the organism, but trade-offs have proven hard to establish experimentally. A reason for this may be that some trade-off costs only become evident under resource-limiting conditions. To explore the effect of nutrient limitation on trade-offs in toxin-producing dinoflagellates, we induced toxin production in Alexandrium minutum by chemical cues from copepods under different levels of nitrogen limitation. The effects were both nitrogen- and grazer-concentration dependent. Induced cells had higher cellular toxin content and a larger fraction of the cells was rejected by a copepod, demonstrating the clear benefits of toxin production. Induced cells also had a higher carbon and nitrogen content, despite up to 25% reduction in cell size. Unexpectedly, induced cells seemed to grow faster than controls, likely owing to a higher specific nutrient affinity due to reduced size. We thus found no clear trade-offs, rather the opposite. However, indirect ecological costs that do not manifest under laboratory conditions may be important. Inducing appropriate defence traits in response to threat-specific warning signals may also prevent larger cumulative costs from expressing several defensive traits simultaneously.

Morphological evolution of silica scales in the freshwater genus Synura (Stramenopiles)

A high degree of morphological variability is expressed between the ornately sculptured siliceous scales formed by species in the chrysophycean genus, Synura. In this study, we aimed to uncover the general principles and trends underlying the evolution of scale morphology in this genus. We assessed the relationships among thirty extant Synura species using a robust molecular analysis that included six genes, coupled with morphological characterization of the species-specific scales. The analysis was further enriched with addition of morphological information from fossil specimens and by including the unique modern species, Synura punctulosa. We inferred the phylogenetic position of the morphologically unique S. punctulosa, to be an ancient Synura lineage related to S. splendida in the section Curtispinae. Some morphological traits, including development of a keel or a labyrinth ribbing pattern on the scale, appeared once in evolution, whereas other structures, such as a hexagonal meshwork pattern, originated independently several times over geologic time. We further uncovered numerous construction principles governing scale morphology and evolution, as follows: (i) scale roundness and pore diameter decreased during evolution; (ii) elongated scales became strengthened by a higher number of struts or ribs; (iii) as a consequence of scale biogenesis, scales with spines possessed smaller basal holes than scales with a keel and; and (iv) the keel area was proportional to scale area, indicating its potential value in strengthening the scale against breakage.

Aquatic Insects and Benthic Diatoms: A History of Biotic Relationships in Freshwater Ecosystems

The most important environmental characteristic in streams is flow. Due to the force of water current, most ecological processes and taxonomic richness in streams mainly occur in the riverbed. Benthic algae (mainly diatoms) and benthic macroinvertebrates (mainly aquatic insects) are among the most important groups in running water biodiversity, but relatively few studies have investigated their complex relationships. Here, we review the multifaceted interactions between these two important groups of lotic organisms. As the consumption of benthic algae, especially diatoms, was one of the earliest and most common trophic habits among aquatic insects, they then had to adapt to the particular habitat occupied by the algae. The environmental needs of diatoms have morphologically and behaviorally shaped their scrapers, leading to impressive evolutionary convergences between even very distant groups. Other less evident interactions are represented by the importance of insects, both in preimaginal and adult stages, in diatom dispersion. In addition, the top-down control of diatoms by their grazers contributes to their spatial organization and functional composition within the periphyton. Indeed, relationships between aquatic insects and diatoms are an important topic of study, scarcely investigated, the onset of which, hundreds of millions of years ago, has profoundly influenced the evolution of stream biological communities.

Diatoms for Carbon Sequestration and Bio-Based Manufacturing

Carbon dioxide (CO2) is a major greenhouse gas responsible for climate change. Diatoms, a natural sink of atmospheric CO2, can be cultivated industrially in autotrophic and mixotrophic modes for the purpose of CO2 sequestration. In addition, the metabolic diversity exhibited by this group of photosynthetic organisms provides avenues to redirect the captured carbon into products of value. These include lipids, omega-3 fatty acids, pigments, antioxidants, exopolysaccharides, sulphated polysaccharides, and other valuable metabolites that can be produced in environmentally sustainable bio-manufacturing processes. To realize the potential of diatoms, expansion of our knowledge of carbon supply, CO2 uptake and fixation by these organisms, in conjunction with ways to enhance metabolic routing of the fixed carbon to products of value is required. In this review, current knowledge is explored, with an evaluation of the potential of diatoms for carbon capture and bio-based manufacturing.

Phylogeny and cultivation of the holocarpic oomycete Diatomophthora perforans comb. nov., an endoparasitoid of marine diatoms

Oomycetes infecting diatoms are biotrophic parasitoids and live in both marine and freshwater environments. They are ubiquitous, but the taxonomic affinity of many species remains unclear and the majority of them have not been studied for their molecular phylogeny. Only recently, the phylogenetic and taxonomic placement of some diatom-infecting, early-diverging oomycetes was resolved, including the genera Ectrogella, Miracula, Olpidiopsis, and Pontisma. A group of holocarpic diatom parasitoids with zoospores swarming within the sporangium before release were found to be unrelated to the known genera with diatom-infecting species, and were re-classified to a new genus, Diatomophthora. However, about a dozen species of holocarpic diatom parasitoids with unclear affinity remained unsequenced, which includes a commonly occurring species so far identified as Ectrogella perforans. However, this assignment to Ectrogella is doubtful, as the species was not reported to feature a clear-cut diplanetism, a hallmark of Ectrogella s. str. and the whole class Saprolegniomycetes. It was the aim of the current study to clarify the phylogenetic affinities of the species and if the rather broad host range reported is correct or a reflection of cryptic species. By targeted screening, the parasitoid was rediscovered from Helgoland Roads, North Sea and Oslo Fjord, Southern Norway and investigated for its phylogenetic placement using small ribosomal subunit (18S) sequences. Stages of its life cycle on different marine diatoms were described and its phylogenetic placement in the genus Diatomophthora revealed. A stable host-parasite axenic culture from single spore strains of the parasitoid were established on several strains of Pleurosigma intermedium and Coscinodiscus concinnus. These have been continuously cultivated along with their hosts for more than 2 years, and cultural characteristics are reported. Cross-infection trials revealed the transferability of the strains between hosts under laboratory conditions, despite some genetic distance between the pathogen strains. Thus, we hypothesise that D. perforans might be in the process of active radiation to new host species.