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Characterising NADPH oxidase in marine diatoms

Characterising NADPH oxidase in marine diatoms
Characterising NADPH oxidase in marine diatoms
NADPH oxidase (NOX) is a widespread enzyme that catalyses the transmembrane reduction of oxygen, generating extracellular reactive oxygen species (ROS). NOX-derived production of ROS is best characterised as a defence mechanism, but a number of functions have been ascribed across eukaryotes, including intercellular signalling, facilitating cell-wall development and nutrient acquisition. Though extracellular ROS (eROS) production in unicellular algae is increasingly researched, characterisation of an enzyme source is relatively limited. This thesis addressed this by examining NOX distribution and function in marine diatoms, reporting several key findings. Firstly, a detailed screen of diatoms was carried out to identify NOX protein distribution. Compared to established NOX proteins in plants and animals, diatom NOX proteins are shown to be unusually diverse, with three structurally and phylogenetically distinct Classes of NOX protein and an atypical NOX-like Class. Secondly, the dynamics of eROS production and transmembrane electron transport were examined in three ecologically distinct marine diatoms. Baseline production rates differed significantly between species and changes to light intensity generated species-specific effects. NOX activity was inferred to be responsible for ROS production in two of the species tested. Finally, the function for NOX in the model diatom Phaeodactylum tricornutum was investigated. Following chemical inhibition of NOX, significant reductions to growth and photophysiology were observed, alongside an increase in cytosolic H2O2 (detected using a genetically encoded biosensor roGFP2-Orp1). This thesis proposes that NOX acts as a photoprotective mechanism by dissipating excess electrons and preventing over-reduction of chloroplast photosystems. Together, this study greatly improves understanding of NOX proteins in diatoms. By focusing on an understudied group, compared to animals or plants, knowledge of NOX distribution is expanded, with implications for NOX evolution. Furthermore, active eROS production can be beneficial to marine algae, and an electron dissipation function may explain the widespread use of eROS production by phytoplankton. While enzymatic sources of eROS are surprisingly diverse in diatoms, NOX likely represents the most common source. Thus, greater understanding of NOX function and distribution in diatoms may provide insights into understanding unique diatom photoprotection mechanisms, helping explain how diatoms can respond to changing environmental conditions.
University of Southampton
Dickenson, Jack, Edward
69720403-4aa2-423f-bc02-d3a15be06f44
Dickenson, Jack, Edward
69720403-4aa2-423f-bc02-d3a15be06f44
Moore, Christopher
7ec80b7b-bedc-4dd5-8924-0f5d01927b12
Brownlee, Colin
2af37c1c-b2bf-4832-8370-d9c35e7b3385

Dickenson, Jack, Edward (2021) Characterising NADPH oxidase in marine diatoms. University of Southampton, Doctoral Thesis, 219pp.

Record type: Thesis (Doctoral)

Abstract

NADPH oxidase (NOX) is a widespread enzyme that catalyses the transmembrane reduction of oxygen, generating extracellular reactive oxygen species (ROS). NOX-derived production of ROS is best characterised as a defence mechanism, but a number of functions have been ascribed across eukaryotes, including intercellular signalling, facilitating cell-wall development and nutrient acquisition. Though extracellular ROS (eROS) production in unicellular algae is increasingly researched, characterisation of an enzyme source is relatively limited. This thesis addressed this by examining NOX distribution and function in marine diatoms, reporting several key findings. Firstly, a detailed screen of diatoms was carried out to identify NOX protein distribution. Compared to established NOX proteins in plants and animals, diatom NOX proteins are shown to be unusually diverse, with three structurally and phylogenetically distinct Classes of NOX protein and an atypical NOX-like Class. Secondly, the dynamics of eROS production and transmembrane electron transport were examined in three ecologically distinct marine diatoms. Baseline production rates differed significantly between species and changes to light intensity generated species-specific effects. NOX activity was inferred to be responsible for ROS production in two of the species tested. Finally, the function for NOX in the model diatom Phaeodactylum tricornutum was investigated. Following chemical inhibition of NOX, significant reductions to growth and photophysiology were observed, alongside an increase in cytosolic H2O2 (detected using a genetically encoded biosensor roGFP2-Orp1). This thesis proposes that NOX acts as a photoprotective mechanism by dissipating excess electrons and preventing over-reduction of chloroplast photosystems. Together, this study greatly improves understanding of NOX proteins in diatoms. By focusing on an understudied group, compared to animals or plants, knowledge of NOX distribution is expanded, with implications for NOX evolution. Furthermore, active eROS production can be beneficial to marine algae, and an electron dissipation function may explain the widespread use of eROS production by phytoplankton. While enzymatic sources of eROS are surprisingly diverse in diatoms, NOX likely represents the most common source. Thus, greater understanding of NOX function and distribution in diatoms may provide insights into understanding unique diatom photoprotection mechanisms, helping explain how diatoms can respond to changing environmental conditions.

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Jack Dickenson_24856339_PhD_Thesis_July21 - Version of Record
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Available under License University of Southampton Thesis Licence.

More information

Published date: 29 July 2021

Identifiers

Local EPrints ID: 450588
URI: http://eprints.soton.ac.uk/id/eprint/450588
PURE UUID: e034e55c-2e5b-4850-bb55-95ccab75241b
ORCID for Christopher Moore: ORCID iD orcid.org/0000-0002-9541-6046

Catalogue record

Date deposited: 04 Aug 2021 16:35
Last modified: 19 Nov 2021 02:36

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Contributors

Author: Jack, Edward Dickenson
Thesis advisor: Christopher Moore ORCID iD
Thesis advisor: Colin Brownlee

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