Effects of global change on the physiology and biogeochemistry of the dinitrogen-fixing cyanobacteria Trichodesmium erythraeum and Crocosphaera watsonii

Approximately half of natural global biological dinitrogen (N2) fixation takes places in the oceans. Estimates suggest that cyanobacteria including the filamentous genus Trichodesmium and unicellular groups like Crocosphaera collectively contribute the majority of oceanic N2 fixation. Rapidly changing environmental factors such as the rising atmospheric partial pressure of carbon dioxide (pCO2), shallower mixed layers (higher light intensities) and changes in nutrient fluxes to the euphotic zone (from both deep water and atmospheric inputs) will likely affect N 2-fixation rates in the future ocean. Several studies using laboratory cultures of Trichodesmium erythraeum and Crocosphaera watsonii have documented increased N2-fixation rates when pCO2 was doubled from present-day atmospheric concentrations (∼380 ppm) to 100-year projected future levels (∼750 ppm). Because marine N and C biogeochemistry are tightly linked, this potential impact on the N cycle will likely have important consequences for the C cycle. These findings provided impetus for examining effects of elevated pCO2 on N2- fixation rates in combination with other environmental factors like iron (Fe), phosphorus (P), and light. Thus, I examined interactive effects of light and pCO2 on growth, N2- and CO2-fixation rates by two strains of T. erythraeum (GBRTRLI101 and IMS101) in laboratory semi-continuous cultures. The effect of elevated pCO2 on gross N2-fixation rates was high in cultures (GBRTRLI101 and IMS101) growing under low (38 micromol quanta m-2 s -1) and mid irradiances (100 micromol quanta m-2 s-1), but this effect was reduced at high light (220 micromol quanta m-2 s-1). This study suggests that elevated pCO2 may have a strong positive effect on gross N2 fixation by Trichodesmium in intermediate and bottom layers of the euphotic zone, but perhaps not in light-saturated upper layers of the oceans. I also examined the combined effects of irradiance and pCO2 on growth, N2- and CO2-fixation rates in two western tropical Atlantic Ocean isolates of C. watsonii (WH0401 and WH0402). In both strains, cellular growth, gross N2- and CO2-fixation rates were reduced in low-pCO2-acclimated cultures (190 ppm) relative to present-day (∼385 ppm) or future (∼750 ppm) pCO2 treatments. Unlike previous reports for C. watsonii (WH8501), however, N2-fixation rates did not increase further in cultures acclimated to 750 ppm relative to those maintained at present-day pCO2. Both increasing irradiance (p<0.001) and pCO2 (p<0.03) had a significant negative effect on gross:net N2- fixation rates in WH0402 and trends were similar in WH0401, implying that retention of fixed N was enhanced under elevated irradiance and pCO2. These results also imply that growth rates and N2- fixation rates of WH0401 and WH0402 respond differently to changing pCO2. These data, along with previously reported results, suggest that C. watsonii may have wide-ranging, strain-specific responses to changing irradiance and pCO2, emphasizing the need to examine a range of isolates within this genus. In the third chapter, I examine three-way interactions between pCO2, P availability, and irradiance in a Pacific Ocean isolate of C. watsonii (WH0003). First, I document P requirements for growth, N2- and CO 2-fixation rates by generating Monod functional response curves under high and low pCO2 and light conditions. The effect of elevated pCO2 on these physiological rates was greatly enhanced under low P conditions in comparison with Preplete cultures, due to a high threshold concentration of P for these rates in low-pCO2-acclimated cultures. This trend was consistent under low (40 micromol quanta m-2 s-1) and high (150 micromol quanta m -2 s-1) irradiance, and suggests that the P demand for growth of C. watsonii decreases with increasing pCO 2. The effect of elevated pCO2 on N2-fixation rates was reduced 8-fold under low light in comparison with high-light-acclimated cultures (p<0.05), reflecting a light-pCO2 interactive trend opposite to that documented for Trichdodesmium. This difference in the interactive effect of light and pCO2 on N2 fixation by Crocosphaera when compared with Trichodesmium might be caused by the different N2-fixation strategies that they use (temporal vs. spatial separation of N2 and CO 2 fixation). These studies emphasize the need to examine interactive effects of multiple environmental variables on a variety of oceanic N 2 fixers to accurately predict effects of global change on the N and C cycles.