Inorganic Carbon Acquisition in Coastal Pacific Phytoplankton Communities

Despite significant advances in the understanding of carbon acquisition in eukaryotic algae and cyanobacteria, very little information is available on the mechanisms of C uptake in natural phytoplankton communities or the effects of CO2variations on marine primary productivity. In this article, we present the results of a 3-yr study of C acquisition in coastal Pacific phytoplankton populations and their responses to experimental CO2manipulations. Diatom-dominated phytoplankton assemblages collected without incubation showed photosynthetic characteristics indicative of a carbon concentrating mechanism. Cells possessed a high affinity for external inorganic C (apparent K$_m \sim1 \mu$M CO2) and accumulated internal inorganic C pools that were ∼3-5.5-fold higher than those in the external medium. Evidence of in situ carbonic anhydrase expression was found in some of the phytoplankton populations we examined, and inhibitor experiments showed that this enzyme was essential for C fixation by cells. The presence of carbon concentrating mechanisms enabled the phytoplankton to maintain rapid growth rates over a wide range of CO2concentrations (3-32 μM). In five of six long-term (∼2-5-d) CO2manipulation experiments, no difference in growth rates could be detected across treatments. However, a significant decrease in growth rate (30%) was observed in one experiment at the lowest CO2level tested (3 μM). Although phytoplankton growth rates were generally unaffected by the CO2manipulations, significant C02-dependent changes occurred in the cellular bio-chemistry and physiology of two assemblages that were examined. Cells grown at low CO2showed higher short-term rates of C uptake (indicative of transport system up-regulation), as well as enhanced expression of Rubisco and carbonic anhydrase. In one of these two incubation experiments, lower C: N and carbohydrate: protein ratios were observed at low CO2. Phytoplankton from both incubations showed low C isotope discrimination relative to the13C/12C of the available CO2. Photosynthetic fractionation factors (εp) ranged from ∼3.5 permil to 7.5 permil and were independent of both cellular growth rates and aqueous CO2concentrations. Our data indicate that nutrient-replete, rapidly growing coastal phytoplankton can use carbon concentrating mechanisms and respond physiologically and biochemically to changing dissolved CO2concentrations. Future field studies should examine the effects of CO2on the growth of nutrient-limited phyto