Session: Session 4

Understanding the function(s) of iota-carbonic anhydrase in the diatom Phaeodactylum tricornutum: the role of zinc

Introduction: Diatoms possess several carbonic anhydrases (CA) including iota-CA, an enzyme recently discovered in diatoms and also present in other phytoplankton organisms [1,2]. Iota-CA is involved in the CO2 concentrating mechanism (CCM) since this enzyme is overexpressed at low CO2 concentration in the centric diatom Thalassiosira pseudonana [3].

Methodology and Results: We have determined the conditions of expression of the iota-CA from the pennate diatom Phaeodactylum tricornutum. We observed that unlike T. pseudonana, the expression is enhanced in either low CO2 levels or low Zn concentration. The depletion of Zn is a more important environmental cue than low CO2 for the expression enhancement. Label free quantitative proteomics of P. tricornutum shows that iota-CA is the protein more differentially overexpressed under both low CO2 and Zn depletion. In addition, high CO2 concentrations repress the expression of iota-CA even at low Zn concentration. The oligomerization state of this enzyme was tetrameric using size exclusion chromatography (SEC). Using SEC-Small Angle X ray Scattering, a mixture of tetramer (37 angstroms) and octamer (50 angstroms) was observed while using ESI mass spectrometry, we observed a dimer, an octamer and a tetramer of 132.5 kDa, this latter being the most abundant.

Conclusion: These results indicate that iota-CA is not a metalloenzyme which may play a role in the CCM of P. tricornutum when zinc in the environment is scarce, unlike most CAs.

1. Jensen, E.L. et al. A New Widespread Subclass of Carbonic Anhydrase in Marine Phytoplankton. ISME J 2019, 13, 2094–2106

2. Jensen, E.L. et al. Structural Contour Map of the Iota Carbonic Anhydrase from the Diatom Thalassiosira pseudonana Using a Multiprong Approach. Int. J. of Mol. Sci.s 2021, 22, 8723

3. Clement, R.et al. Responses of the Marine Diatom Thalassiosira pseudonana to Changes in CO2 Concentration: A Proteomic Approach. Sci Rep 2017, 7, 42333