CO2 modulation of the rates of photosynthesis and light-dependent O2 consumption in Trichodesmium.

Tobias G Boatman,Phillip A Davey,Tracy Lawson,Richard J Geider

doi:10.1093/jxb/ery368

Tobias G Boatman, Phillip A Davey + Show 2 more

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https://doi.org/10.1093/jxb/ery368

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Abstract

As atmospheric CO2 concentrations increase, so too does the dissolved CO2 and HCO3- concentrations in the world's oceans. There are still many uncertainties regarding the biological response of key groups of organisms to these changing conditions, which is crucial for predicting future species distributions, primary productivity rates, and biogeochemical cycling. In this study, we established the relationship between gross photosynthetic O2 evolution and light-dependent O2 consumption in Trichodesmium erythraeum IMS101 acclimated to three targeted pCO2 concentrations (180 µmol mol-1=low-CO2, 380 µmol mol-1=mid-CO2, and 720 µmol mol-1=high-CO2). We found that biomass- (carbon) specific, light-saturated maximum net O2 evolution rates (PnC,max) and acclimated growth rates increased from low- to mid-CO2, but did not differ significantly between mid- and high-CO2. Dark respiration rates were five times higher than required to maintain cellular metabolism, suggesting that respiration provides a substantial proportion of the ATP and reductant for N2 fixation. Oxygen uptake increased linearly with gross O2 evolution across light intensities ranging from darkness to 1100 µmol photons m-2 s-1. The slope of this relationship decreased with increasing CO2, which we attribute to the increased energetic cost of operating the carbon-concentrating mechanism at lower CO2 concentrations. Our results indicate that net photosynthesis and growth of T. erythraeum IMS101 would have been severely CO2 limited at the last glacial maximum, but that the direct effect of future increases of CO2 may only cause marginal increases in growth.

Highlights

The ocean is one of the largest readily exchangeable reservoirs of inorganic carbon on Earth and is a major sink for anthropogenic CO2 emissions (Sabine et al, 2004).The ocean’s capacity to sequester atmospheric CO2 is strongly mediated by biological processes (Raven and Falkowski, 1999), where organic matter production and export drive CO2 sequestration.This is important as future emission scenarios predict that atmospheric CO2 will increase from present concentrations (~400 μmol mol–1) to 750 μmol mol–1 or 1000 μmol mol–1 by the end of this century (Raven et al, 2005)
Our results indicate that net photosynthesis and growth of T. erythraeum IMS101 would have been severely CO2 limited at the last glacial maximum, but that the direct effect of future increases of CO2 may only cause marginal increases in growth
As the photon efficiency of ATP production by pseudocyclic photophosphorylation is the same as that of photophosphorylation driven by linear photosynthetic electron transport (LPET) (Baker et al, 2007), our results suggest that 26% more ATP than can be generated by LPET is required by cells growing under high-CO2, increasing to 55% more ATP in cells growing under mid-CO2 and 75% more ATP in cells growing under low-CO2

Summary

Introduction

The ocean is one of the largest readily exchangeable reservoirs of inorganic carbon on Earth and is a major sink for anthropogenic CO2 emissions (Sabine et al, 2004).The ocean’s capacity to sequester atmospheric CO2 is strongly mediated by biological processes (Raven and Falkowski, 1999), where organic matter production and export drive CO2 sequestration.This is important as future emission scenarios predict that atmospheric CO2 will increase from present concentrations (~400 μmol mol–1) to 750 μmol mol–1 or 1000 μmol mol–1 by the end of this century (Raven et al, 2005). Trichodesmium plays a significant role in the N cycle of the oligotrophic oceans; fixing nitrogen in an area corresponding to half of the Earth’s surface (Davis and McGillicuddy, 2006) and representing up to 50% of new production in some oligotrophic tropical and subtropical oceans (Capone, 2005).The annual marine N2 fixation is currently estimated at between 100 Tg and 200 Tg N per year (Gruber and Sarmiento, 1997; Karl et al, 2002), of which Trichodesmium spp. contribute between 80 Tg and 110 Tg of fixed N2 to open ocean ecosystems (Capone et al, 1997)

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