An Integrated Response of Trichodesmium erythraeum IMS101 Growth and Photo-Physiology to Iron, CO2, and Light Intensity.

Tobias G Boatman,Kevin Oxborough,Richard J Geider,Tracy Lawson,Martha Gledhill

doi:10.3389/fmicb.2018.00624

Abstract

We have assessed how varying CO2 (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m−2 s−1) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe′) concentrations between 20 and 9,600 pM. We found significant iron dependencies of growth rate and the initial slope and maximal relative PSII electron transport rates (rPm). Under iron-limiting concentrations, high-light increased growth rates and rPm; possibly indicating a lower allocation of resources to iron-containing photosynthetic proteins. Higher CO2 increased growth rates across all iron concentrations, enabled growth to occur at lower Fe′ concentrations, increased rPm and lowered the iron half saturation constants for growth (Km). We attribute these CO2 responses to the operation of the CCM and the ATP spent/saved for CO2 uptake and transport at low and high CO2, respectively. It seems reasonable to conclude that T. erythraeum IMS101 can exhibit a high degree of phenotypic plasticity in response to CO2, light intensity and iron-limitation. These results are important given predictions of increased dissolved CO2 and water column stratification (i.e., higher light exposures) over the coming decades.

Highlights

In vast regions of the oligotrophic tropical and sub-tropical open oceans, input of new nitrogen is primarily dependent on the N2-fixing capabilities of diazotrophic cyanobacteria, including unicellular cyanobacteria such as UCYN-A (Martinez-Perez et al, 2016) and filamentous cyanobacteria such as Trichodesmium spp. (Carpenter and Capone, 1992; Capone et al, 1997; Campbell et al, 2005)
Our aim was to assess the response of T. erythraeum IMS101 growth, relative photosystem II (PSII) electron transport rates and photophysiology to free iron (Fe′), and investigate how the integrated effect of CO2 and light intensity influence this response
Acclimation to high CO2 enabled growth to occur at comparatively lower Fe′ concentrations at both low and high light (Figures 1A,B)

Summary

Introduction

In vast regions of the oligotrophic tropical and sub-tropical open oceans, input of new nitrogen is primarily dependent on the N2-fixing capabilities of diazotrophic cyanobacteria, including unicellular cyanobacteria such as UCYN-A (Martinez-Perez et al, 2016) and filamentous cyanobacteria such as Trichodesmium spp. (Carpenter and Capone, 1992; Capone et al, 1997; Campbell et al, 2005). Trichodesmium spp. are a fundamentally important organism as they represent up to 50% of new nitrogen in some regions (Karl et al, 1997; Capone et al, 2005), and contribute between 80 and 110 Tg of fixed N2 to the open ocean ecosystems per year (Capone et al, 1997). Nitrogenase contains 19 iron atoms per heterodimeric protein molecule (Shi et al, 2007). This is important because iron is a cofactor for a whole range of enzymes involved in photosynthetic and respiratory electron transport, nitrate and nitrite reduction, chlorophyll synthesis and other biosynthetic or degradative reactions (Geider and La Roche, 1994). Iron availability may be critical in controlling rates of nitrogen fixation in large areas of the open ocean (Rueter, 1988; Rueter et al, 1992; Falkowski and Raven, 1997; Wu et al, 2000)

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