2018-08, Ewe, Daniela, Tachibana, Masaaki, Kikutani, Sae, Gruber, Ansgar, Río Bártulos, Carolina, Konert, Grzegorz, Kaplan, Aaron, Matsuda, Yusuke, Kroth, Peter G.

Diatoms are unicellular algae and important primary producers. The process of carbon fixation in diatoms is very efficient even though the availability of dissolved CO₂ in sea water is very low. The operation of a carbon concentrating mechanism (CCM) also makes the more abundant bicarbonate accessible for photosynthetic carbon fixation. Diatoms possess carbonic anhydrases as well as metabolic enzymes potentially involved in C4 pathways; however, the question as to whether a C4 pathway plays a general role in diatoms is not yet solved. While genome analyses indicate that the diatom Phaeodactylum tricornutum possesses all the enzymes required to operate a C4 pathway, silencing of the pyruvate orthophosphate dikinase (PPDK) in a genetically transformed cell line does not lead to reduced photosynthetic carbon fixation. In this study, we have determined the intracellular location of all enzymes potentially involved in C4-like carbon fixing pathways in P. tricornutum by expression of the respective proteins fused to green fluorescent protein (GFP), followed by fluorescence microscopy. Furthermore, we compared the results to known pathways and locations of enzymes in higher plants performing C3 or C4 photosynthesis. This approach revealed that the intracellular distribution of the investigated enzymes is quite different from the one observed in higher plants. In particular, the apparent lack of a plastidic decarboxylase in P. tricornutum indicates that this diatom does not perform a C4-like CCM.

2008, Bowler, Chris, Allen, Andrew E., Badger, Jonathan H., Grimwood, Jane, Jabbari, Kamel, Kuo, Alan, Maheswari, Uma, Martens, Cindy, Maumus, Florian, Otillar, Robert P., Rayko, Edda, Salamov, Asaf, Vandepoele, Klaas, Beszteri, Bank, Gruber, Ansgar, Heijde, Marc, Katinka, Michael, Mock, Thomas, Valentin, Klaus, Verret, Fréderic, Berges, John A., Brownlee, Colin, Cadoret, Jean-Paul, Chiovitti, Anthony, Choi, Chang Jae, Coesel, Sacha, De Martino, Alessandra, Detter, John Chris, Durkin, Colleen, Falciatore, Angela, Fournet, Jérome, Haruta, Miyoshi, Huysman, Marie J. J., Jenkins, Bethany D., Jiroutova, Katerina, Jorgensen, Richard E., Joubert, Yolaine, Kaplan, Aaron, Kröger, Nils, Kroth, Peter G., La Roche, Julie, Lindquist, Erica, Lommer, Markus, Martin Jézéquel, Véronique, Lopez, Pascal J., Lucas, Susan, Mangogna, Manuela, McGinnis, Karen, Medlin, Linda K., Montsant, Anton, Oudot Le Secq, Marie-Pierre, Napoli, Carolyn, Obornik, Miroslav, Schnitzler Parker, Micaela, Petit, Jean-Louis, Porcel, Betina M., Poulsen, Nicole, Robison, Matthew, Rychlewski, Leszek, Rynearson, Tatiana A., Schmutz, Jeremy, Shapiro, Harris, Siaut, Magali, Stanley, Michele S., Sussman, Michael R., Taylor, Alison R., Vardi, Assaf, Dassow, Peter von, Vyverman, Wim, Willis, Anusuya, Wyrwicz, Lucjan S., Rokhsar, Daniel S., Weissenbach, Jean, Armbrust, E. Virginia, Green, Beverley R., Van de Peer, Yves, Grigoriev, Igor V.

Diatoms are photosynthetic secondary endosymbionts found throughout marine and freshwater environments, and are believed to be responsible for around one-fifth of the primary productivity on Earth. The genome sequence of the marine centric diatom Thalassiosira pseudonana was recently reported, revealing a wealth of information about diatom biology. Here we report the complete genome sequence of the pennate diatom Phaeodactylum tricornutum and compare it with that of T. pseudonana to clarify evolutionary origins, functional significance and ubiquity of these features throughout diatoms. In spite of the fact that the pennate and centric lineages have only been diverging for 90 million years, their genome structures are dramatically different and a substantial fraction of genes (40%) are not shared by these representatives of the two lineages. Analysis of molecular divergence compared with yeasts and metazoans reveals rapid rates of gene diversification in diatoms. Contributing factors include selective gene family expansions, differential losses and gains of genes and introns, and differential mobilization of transposable elements. Most significantly, we document the presence of hundreds of genes from bacteria. More than 300 of these gene transfers are found in both diatoms, attesting to their ancient origins, and many are likely to provide novel possibilities for metabolite management and for perception of environmental signals. These findings go a long way towards explaining the incredible diversity and success of the diatoms in contemporary oceans.

2013-01, Haimovich-Dayan, Maya, Garfinkel, Nitsan, Ewe, Daniela, Marcus, Yehouda, Gruber, Ansgar, Wagner, Heiko, Kroth, Peter G., Kaplan, Aaron

Diatoms are important players in the global carbon cycle. Their apparent photosynthetic affinity for ambient CO(2) is much higher than that of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), indicating that a CO(2)-concentrating mechanism (CCM) is functioning. However, the nature of the CCM, a biophysical or a biochemical C(4), remains elusive. Although (14)C labeling experiments and presence of complete sets of genes for C(4) metabolism in two diatoms supported the presence of C(4), other data and predicted localization of the decarboxylating enzymes, away from Rubisco, makes this unlikely. We used RNA-interference to silence the single gene encoding pyruvate-orthophosphate dikinase (PPDK) in Phaeodactylum tricornutum, essential for C(4) metabolism, and examined the photosynthetic characteristics. The mutants possess much lower ppdk transcript and PPDK activity but the photosynthetic K(1/2) (CO(2)) was hardly affected, thus clearly indicating that the C(4) route does not serve the purpose of raising the CO(2) concentration in close proximity of Rubisco in P. tricornutum. The photosynthetic V(max) was slightly reduced in the mutant, possibly reflecting a metabolic constraint that also resulted in a larger lipid accumulation. We propose that the C(4) metabolism does not function in net CO(2) fixation but helps the cells to dissipate excess light energy and in pH homeostasis.

2008, Kroth, Peter G., Chiovitti, Anthony, Gruber, Ansgar, Martin-Jezequel, Veronique, Mock, Thomas, Schnitzler Parker, Micaela, Stanley, Michele S., Kaplan, Aaron, Caron, Lise, Weber, Till, Maheswari, Uma, Armbrust, Elisabeth Virginia, Bowler, Chris, Kroymann, Juergen

Background:
Diatoms are unicellular algae responsible for approximately 20% of global carbon fixation. Their evolution by secondary endocytobiosis resulted in a complex cellular structure and metabolism compared to algae with primary plastids.
Methodology/Principal Findings:
The whole genome sequence of the diatom Phaeodactylum tricornutum has recently been completed. We identified and annotated genes for enzymes involved in carbohydrate pathways based on extensive EST support and comparison to the whole genome sequence of a second diatom, Thalassiosira pseudonana. Protein localization to mitochondria was predicted based on identified similarities to mitochondrial localization motifs in other eukaryotes, whereas protein localization to plastids was based on the presence of signal peptide motifs in combination with plastid localization motifs previously shown to be required in diatoms. We identified genes potentially involved in a C4-like photosynthesis in P. tricornutum and, on the basis of sequence-based putative localization of relevant proteins, discuss possible differences in carbon concentrating mechanisms and CO2 fixation between the two diatoms. We also identified genes encoding enzymes involved in photorespiration with one interesting exception: glycerate kinase was not found in either P. tricornutum or T. pseudonana. Various Calvin cycle enzymes were found in up to five different isoforms, distributed between plastids, mitochondria and the cytosol. Diatoms store energy either as lipids or as chrysolaminaran (a β-1,3-glucan) outside of the plastids. We identified various β-glucanases and large membrane-bound glucan synthases. Interestingly most of the glucanases appear to contain C-terminal anchor domains that may attach the enzymes to membranes.
Conclusions/Significance:
Here we present a detailed synthesis of carbohydrate metabolism in diatoms based on the genome sequences of Thalassiosira pseudonana and Phaeodactylum tricornutum. This model provides novel insights into acquisition of dissolved inorganic carbon and primary metabolic pathways of carbon in two different diatoms, which is of significance for an improved understanding of global carbon cycles.