A role for diatom-like silicon transporters in calcifying coccolithophores.
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ABSTRACT:
SUBMITTER: Durak GM
PROVIDER: S-EPMC4742977 | biostudies-literature | 2016 Feb
REPOSITORIES: biostudies-literature
Publications
A role for diatom-like silicon transporters in calcifying coccolithophores.
Durak Grażyna M GM Taylor Alison R AR Walker Charlotte E CE Probert Ian I de Vargas Colomban C Audic Stephane S Schroeder Declan D Brownlee Colin C Wheeler Glen L GL
Nature communications 20160204
Biomineralization by marine phytoplankton, such as the silicifying diatoms and calcifying coccolithophores, plays an important role in carbon and nutrient cycling in the oceans. Silicification and calcification are distinct cellular processes with no known common mechanisms. It is thought that coccolithophores are able to outcompete diatoms in Si-depleted waters, which can contribute to the formation of coccolithophore blooms. Here we show that an expanded family of diatom-like silicon transport ...[more]
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Project description:The utilization of silicon by diatoms has both global and small-scale implications, from oceanic primary productivity to nanotechnological applications of their silica cell walls. The sensing and transport of silicic acid are key aspects of understanding diatom silicon utilization. At low silicic acid concentrations (<30 μM), transport mainly occurs through silicic acid transport proteins (SITs), and at higher concentrations it occurs through diffusion. Previous analyses of the SITs were done either in heterologous systems or without a distinction between individual SITs. In the present study, we examined individual SITs in Thalassiosira pseudonana in terms of transcript and protein abundance in response to different silicic acid regimes and examined knockdown lines to evaluate the role of the SITs in transport, silica incorporation, and lipid accumulation resulting from silicon starvation. SIT1 and SIT2 were localized in the plasma membrane, and protein levels were generally inversely correlated with cellular silicon needs, with a distinct response being found when the two SITs were compared. We developed highly effective approaches for RNA interference and antisense knockdowns, the first such approaches developed for a centric diatom. SIT knockdown differentially affected the uptake of silicon and the incorporation of silicic acid and resulted in the induction of lipid accumulation under silicon starvation conditions far earlier than in the wild-type cells, suggesting that the cells were artificially sensing silicon limitation. The data suggest that the transport role of the SITs is relatively minor under conditions with sufficient silicic acid. Their primary role is to sense silicic acid levels to evaluate whether the cell can proceed with its cell wall formation and division processes.
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Project description: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 HCO?? 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 H(v)1 H+ channels, which function as voltage-gated 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 H(v)1 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 CO? generation. Furthermore, the presence of H(v)1 class ion channels in a wide range of extant eukaryote groups indicates they evolved in an early common ancestor.
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Project description:Diatom cell walls, made of nanostructured silica, are of interest in diverse areas ranging from cellular structure, to hierarchical organization in biomineralization, to nanotechnology. Thus far, only cell surface proteins and proteins tightly associated with silica matrix have been characterized, and essential components of the silica deposition vesicle (SDV) are unknown, including components of the SDV membrane, cytoskeletal-interacting proteins, and proteins involved in trafficking associated with the SDV. Thus, an understanding of most of the molecular components and the dynamics of cellular processes involved in cell wall synthesis is lacking. In this work we report the first whole-cell response analysis using whole genome microarrays to identify genes potentially involved in diverse aspects of diatom cell division or cell wall synthesis. Thalassiosira pseudonana transcript profiles from precise time points, known to be associated with specific cell wall formation processes in cell-cycle synchronized cultures, suggests that this gene set includes extracellular proteins, silica matrix proteins, and proteins involved in signal transduction, vesicle trafficking, and transport. Protein localization experiments further confirm the first discovery of proteins associated with the SDV membrane. We propose that these proteins provide the interface between extra-SDV organization by the cytoskeleton and intra-SDV organization of silica polymerization determinants, which lead to the higher order organization of diatom silica structure. Analyzed mRNA from 0, 2, 4, 7, 8 and 9 hr of synchronized cell cycle using the Affymetrix GeneChip whole genome tiling array. Initial analysis of gene level expression was performed using Affymetrix Expression Console Software, version 1.1. No biological replicates were performed. 0 hr is used as reference point.
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Project description:Photosynthesis in marine diatoms is a vital fraction of global primary production empowered by CO(2)-concentrating mechanisms. Acquisition of HCO(3)(-) from seawater is a critical primary step of the CO(2)-concentrating mechanism, allowing marine photoautotrophic eukaryotes to overcome CO(2) limitation in alkaline high-salinity water. However, little is known about molecular mechanisms governing this process. Here, we show the importance of a plasma membrane-type HCO(3)(-) transporter for CO(2) acquisition in a marine diatom. Ten putative solute carrier (SLC) family HCO(3)(-) transporter genes were found in the genome of the marine pennate diatom Phaeodactylum tricornutum. Homologs also exist in marine centric species, Thalassiosira pseudonana, suggesting a general occurrence of SLC transporters in marine diatoms. Seven genes were found to encode putative mammalian-type SLC4 family transporters in P. tricornutum, and three of seven genes were specifically transcribed under low CO(2) conditions. One of these gene products, PtSLC4-2, was localized at the plasmalemma and significantly stimulated both dissolved inorganic carbon (DIC) uptake and photosynthesis in P. tricornutum. DIC uptake by PtSLC4-2 was efficiently inhibited by an anion-exchanger inhibitor, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, in a concentration-dependent manner and highly dependent on Na(+) ions at concentrations over 100 mM. These results show that DIC influx into marine diatoms is directly driven at the plasmalemma by a specific HCO(3)(-) transporter with a significant halophilic nature.
| S-EPMC3562803 | biostudies-literature