Abstract
Abstract. The surface ocean absorbs large quantities of the CO2 emitted to the atmosphere from human activities. As this CO2 dissolves in seawater, it reacts to form carbonic acid. While this phenomenon, called ocean acidification, has been found to adversely affect many calcifying organisms, some photosynthetic organisms appear to benefit from increasing [CO2]. Among these is the cyanobacterium Trichodesmium, a predominant diazotroph (nitrogen-fixing) in large parts of the oligotrophic oceans, which responded with increased carbon and nitrogen fixation at elevated pCO2. With the mechanism underlying this CO2 stimulation still unknown, the question arises whether this is a common response of diazotrophic cyanobacteria. In this study we therefore investigate the physiological response of Nodularia spumigena, a heterocystous bloom-forming diazotroph of the Baltic Sea, to CO2-induced changes in seawater carbonate chemistry. N. spumigena reacted to seawater acidification/carbonation with reduced cell division rates and nitrogen fixation rates, accompanied by significant changes in carbon and phosphorus quota and elemental composition of the formed biomass. Possible explanations for the contrasting physiological responses of Nodularia compared to Trichodesmium may be found in the different ecological strategies of non-heterocystous (Trichodesmium) and heterocystous (Nodularia) cyanobacteria.
Highlights
Massive anthropogenic emissions caused atmospheric CO2 concentrations to rise from an interglacial level of 280 ppm in preindustrial times, (Indermuehle et al, 1999) to presently 385 ppm (Keeling et al, 2008)
We aim to determine whether the stimulating effects of elevated [CO2] on carbon and nitrogen fixation found in Trichodesmium represent a general phenomenon among diazotrophic cyanobacteria and apply to heterocystic Nodularia
In the present study a mono-specific culture of Nodularia spumigena revealed a decrease in division rates in response to increasing pCO2
Summary
Massive anthropogenic emissions caused atmospheric CO2 concentrations to rise from an interglacial level of 280 ppm in preindustrial times, (Indermuehle et al, 1999) to presently 385 ppm (Keeling et al, 2008). As photosynthetic CO2 fixation is substrate-limited under current atmospheric CO2/O2 ratios all photoautotrophic organisms evolved active carbon concentrating mechanisms (CCM), providing elevated [CO2] at the site of carboxylation. Rising CO2 concentrations have been shown to enhance carbon fixation in several single species experiments (Hinga, 2002; Riebesell, 2004; Rost et al, 2008) and in natural plankton communities (Hein and Sand-Jensen, 1997; Tortell et al, 2002b; Riebesell et al, 2007)
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