Marine phytoplankton–single-celled photosynthesizing organisms that account for about half of global carbon fixation (Behrenfeld & Falkowski, 1997)–require a suite of nutrient elements including carbon (C), nitrogen (N), phosphorous (P), and, in the case of diatoms, silicon (Si). Of these elements, C is the highest molar constituent of phytoplankton and is utilized from the ocean in the form of carbon dioxide (CO2) and bicarbonate (HCO3-). Although HCO3- is present at relatively high concentrations, CO2 is the preferred substrate and comprises <1% of total dissolved inorganic C.
Over the next century, due to anthropogenic fossil fuel use, the partial pressure of atmospheric CO2 (pCO2) is predicted to rise from ~380 ppm (present day) to ~750 ppm (year 2100) (Solomon et al., 2007) resulting in a decrease in carbonate (CO32-) and seawater pH. The consequences of these changes on marine phytoplankton are currently not entirely understood but include potentially deleterious effects for calcifying organisms such as coccolithophores. On the other hand, higher CO2 availability has been shown to relieve CO2 limitation of nitrogen-fixing diazotrophs such as Trichodesmium (Hutchins et al., 2009).
Most marine phytoplankton require CO2 as the substrate for the carboxylating enzyme Rubisco that fixes C into organic matter. Cyanobacteria are believed to have evolved some 3.5 billion years ago when oceanic pCO2 was ~100-fold higher than present (~4000 ppm) and were not limited by CO2 availability (Kump et al., 2009). As CO2 concentrations declined a few hundred million years ago, carbon concentrating mechanisms (CCM) were believed to have evolved (Badger & Price, 2003) and enabled phytoplankton to more efficiently transport and retain CO2 and HCO3-, and to convert HCO3– to CO2 via carbonic anhydrase, a metalloenzyme that uses trace metal zinc (Zn), and, in the case of diatoms, cobalt (Co) or cadmium (Cd) (Morel & Price, 2003). Under lower CO2 availability, the reliance on CCMs is believed to increase iron (Fe) requirements of the photosystem that provides metabolic energy (ATP and NADH) for CCMs (Raven 1990). There is therefore a biochemical connection between the supply of CO2 and the requirement for trace metals that are very low in concentration in the present day oceans.
The CO2-trace metal connection is evident in culture-based studies with the centric diatom Attheya (family Chaetocerotaceae) grown semi-continuously at 200 ppm (approximate last glacial maximum), 370 ppm (approx. present day), and 670 ppm (year 2100 prediction) pCO2. Higher CO2 availability resulted in increases in growth rate and carbon fixation, and decreases Fe and Zn requirements (mmol metal:mol P; Figure 1; King et al., submitted). A study with the diatom Thalassiosira weissflogii also observed a decrease in μmol Fe:mol C from low to high CO2 (Shi et al., 2010). The supply and availability of trace metals in present day surface oceans are relatively low and ~30% of the world’s oceans are limited by Fe in high nutrient, low chlorophyll regimes. Fe is also relatively low in coastal upwelling and transition zones where Fe availability controls phytoplankton nitrate utilization (Hutchins et al., 1998). Under high CO2 conditions, a lower Fe requirement could possibly relieve phytoplankton Fe limitation currently observed in these regions and could result in higher carbon fixation and export (to the deep ocean and fish production).
Although the rate and magnitude of predicted change in pCO2 over the next century is indeed troubling, it is important to consider past and present day CO2 variability in the oceans. For instance, coastal upwelling water masses off California and Oregon have been recently reported to be as high as ~800 ppm (Feely et al., 2008). Conversely, high productivity and drawdown of CO2 could result in transient periods of low pCO2. The varying availability of CO2 relative to changing supply of nutrients and trace metals are collectively expected to affect phytoplankton growth and success.
Acknowledgements
This work was in collaboration with Dr. Feixue Fu, Dr. Dave Hutchins, Dr. Sergio Sañudo-Wilhelmy, and Dr. Karine Leblanc; funding for this research was provided by the National Science Foundation (DH and SSW).
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