Physiological response of Crocosphaera watsonii to enhanced and fluctuating carbon dioxide conditions.

Mary R Gradoville,Angelicque E White,Ricardo M Letelier,Adam Driks

doi:10.1371/journal.pone.0110660

Abstract

We investigated the effects of elevated pCO2 on cultures of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii WH8501. Using CO2-enriched air, cultures grown in batch mode under high light intensity were exposed to initial conditions approximating current atmospheric CO2 concentrations (∼400 ppm) as well as CO2 levels corresponding to low- and high-end predictions for the year 2100 (∼750 and 1000 ppm). Following acclimation to CO2 levels, the concentrations of particulate carbon (PC), particulate nitrogen (PN), and cells were measured over the diurnal cycle for a six-day period spanning exponential and early stationary growth phases. High rates of photosynthesis and respiration resulted in biologically induced pCO2 fluctuations in all treatments. Despite this observed pCO2 variability, and consistent with previous experiments conducted under stable pCO2 conditions, we observed that elevated mean pCO2 enhanced rates of PC production, PN production, and growth. During exponential growth phase, rates of PC and PN production increased by ∼1.2- and ∼1.5-fold in the mid- and high-CO2 treatments, respectively, when compared to the low-CO2 treatment. Elevated pCO2 also enhanced PC and PN production rates during early stationary growth phase. In all treatments, PC and PN cellular content displayed a strong diurnal rhythm, with particulate C:N molar ratios reaching a high of 22∶1 in the light and a low of 5.5∶1 in the dark. The pCO2 enhancement of metabolic rates persisted despite pCO2 variability, suggesting a consistent positive response of Crocosphaera to elevated and fluctuating pCO2 conditions.

Highlights

Anthropogenic emissions and land use change are increasing the concentration of carbon dioxide (CO2) in the atmosphere and surface ocean waters [1]
We present data from experiments tracking the growth, particulate carbon (PC) and particulate nitrogen (PN) production rates of Crocosphaera watsonii WH8501 cultures bubbled with air at three CO2 levels (,400, 750,1000 ppm), while allowing for biologically induced pCO2 variability in the culture medium resulting from photosynthesis and respiration
In agreement with a previous study [9], we found that PC production, PN production, and growth rates were all positively correlated with pCO2 (Table 2, Fig. 2)

Summary

Introduction

Anthropogenic emissions and land use change are increasing the concentration of carbon dioxide (CO2) in the atmosphere and surface ocean waters [1]. Because the carboxylating enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) is typically not saturated under ambient surface seawater pCO2, many phytoplankton groups invest energy in carbon concentrating mechanisms (CCMs) to increase CO2 concentrations at the catalytic site [3]. Under future OA conditions, phytoplankton with low-affinity RuBisCO could potentially down-regulate CCMs and reallocate energy and elemental resources to allow for increased carbon (C) fixation and growth rates [4]. Elevated pCO2 appears to have a strong metabolic enhancement in dinitrogen (N2)-fixing (diazotrophic) cyanobacteria. The unicellular cyanobacterium Crocosphaera strain WH8501 displays increased rates of C and N2 fixation under elevated pCO2 conditions [9]. A global pCO2 enhancement of marine N2 fixation could increase oceanic C uptake and export, producing a negative feedback to climate change [6]

Methods

Results

Conclusion