title: Low CO2 results in a rearrangement of carbon metabolism to support C4 photosynthetic carbon assimilation in Thalassiosira pseudonana.
abstract: The mechanisms of carbon concentration in marine diatoms are controversial. At low CO2 , decreases in O2 evolution after inhibition of phosphoenolpyruvate carboxylases (PEPCs), and increases in PEPC transcript abundances, have been interpreted as evidence for a C4 mechanism in Thalassiosira pseudonana, but the ascertainment of which proteins are responsible for the subsequent decarboxylation and PEP regeneration steps has been elusive. We evaluated the responses of T. pseudonana to steady-state differences in CO2 availability, as well as to transient shifts to low CO2 , by integrated measurements of photosynthetic parameters, transcript abundances and quantitative proteomics. On shifts to low CO2 , two PEPC transcript abundances increased and then declined on timescales consistent with recoveries of Fv /Fm , non-photochemical quenching (NPQ) and maximum chlorophyll a-specific carbon fixation (Pmax ), but transcripts for archetypical decarboxylation enzymes phosphoenolpyruvate carboxykinase (PEPCK) and malic enzyme (ME) did not change. Of 3688 protein abundances measured, 39 were up-regulated under low CO2 , including both PEPCs and pyruvate carboxylase (PYC), whereas ME abundance did not change and PEPCK abundance declined. We propose a closed-loop biochemical model, whereby T. pseudonana produces and subsequently decarboxylates a C4 acid via PEPC2 and PYC, respectively, regenerates phosphoenolpyruvate (PEP) from pyruvate in a pyruvate phosphate dikinase-independent (but glycine decarboxylase (GDC)-dependent) manner, and recuperates photorespiratory CO2 as oxaloacetate (OAA).
authors.affiliation: Earth and Environmental Sciences, Rutgers University, 101 Warren Street, Newark, NJ, 07102, USA.
authors.forename: Adam B
authors.initials: AB
authors.lastname: Kustka
authors.affiliation: Department of Botany and Plant Pathology, Oregon State University, 2082 Cordley Hall, Corvallis, OR, 97331, USA.
authors.forename: Allen J
authors.initials: AJ
authors.lastname: Milligan
authors.affiliation: Biological Mass Spectrometry Facility, Rutgers University, 174 Frelinghuysen Road, Piscataway, NJ, 08854, USA.
authors.forename: Haiyan
authors.initials: H
authors.lastname: Zheng
authors.affiliation: Earth and Environmental Sciences, Rutgers University, 101 Warren Street, Newark, NJ, 07102, USA.
authors.forename: Ashley M
authors.initials: AM
authors.lastname: New
authors.affiliation: Earth and Environmental Sciences, Rutgers University, 101 Warren Street, Newark, NJ, 07102, USA.
authors.forename: Colin
authors.initials: C
authors.lastname: Gates
authors.affiliation: Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ, 08901, USA.
authors.forename: Kay D
authors.initials: KD
authors.lastname: Bidle
authors.affiliation: Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, NJ, 08901, USA.
authors.forename: John R
authors.initials: JR
authors.lastname: Reinfelder
mesh_terms: D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism* / Q000494:pharmacology*; D017377:Diatoms / Q000187:drug effects* / Q000502:physiology*; D010788:Photosynthesis / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide; D002244:Carbon
keywords: C4 metabolism; fatty acid metabolism; glycine decarboxylase; marine diatoms; pentose phosphate pathway; pyruvate carboxylase; pyruvate phosphate dikinase (PPDK); quantitative proteomics