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EFFECTS OF GLOBAL CHANGE ON THE PHYSIOLOGY AND BIOGEOCHEMISTRY OF THE N2-FIXING CYANOBACTERIA TRICHODESMIUM ERYTHRAEUM AND CROCOSPHAERA WATSONII By: Nathan Samuel Garcia ____________ A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (BIOLOGY) December 2012 Copyright 2012 Nathan Samuel Garcia
|Title||Effects of global change on the physiology and biogeochemistry of the N₂-fixing cyanobacteria Trichodesmium erythraeum and Crocosphaera watsonii|
|Author||Garcia, Nathan Samuel|
|Degree||Doctor of Philosophy|
|School||College of Letters, Arts And Sciences|
|Advisor (committee chair)||Hutchins, David A.|
|Advisor (committee member)||
Capone, Douglas G.
Caron, David A.
Webb, Eric A.
Berelson, William M.
|Abstract||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 N₂-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 μmol quanta m⁻² s⁻¹) and mid irradiances (100 μmol quanta m⁻² s⁻¹), but this effect was reduced at high light (220 μmol 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 CO₂-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 P-replete cultures, due to a high threshold concentration of P for these rates in low-pCO₂-acclimated cultures. This trend was consistent under low (40 μmol quanta m⁻² s⁻¹) and high (150 μmol quanta m⁻² s⁻¹) irradiance, and suggests that the P demand for growth of C. watsonii decreases with increasing pCO₂. 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 CO₂ fixation). These studies emphasize the need to examine interactive effects of multiple environmental variables on a variety of oceanic N₂ fixers to accurately predict effects of global change on the N and C cycles.|
|Keyword||nitrogen fixation; carbon dioxide; global change; Trichodesmium; Crocosphaera; light; phosphorus|
|Part of collection||University of Southern California dissertations and theses|
|Publisher (of the original version)||University of Southern California|
|Place of publication (of the original version)||Los Angeles, California|
|Publisher (of the digital version)||University of Southern California. Libraries|
|Provenance||Electronically uploaded by the author|
|Legacy record ID||usctheses-m|
|Contributing entity||University of Southern California|
|Rights||Garcia, Nathan Samuel|
|Access conditions||The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given.|
|Repository name||University of Southern California Digital Library|
|Repository address||USC Digital Library, University of Southern California, University Park Campus MC 7002, 106 University Village, Los Angeles, California 90089-7002, USA|
|Contributing entity||University of Southern California|
|Full text||EFFECTS OF GLOBAL CHANGE ON THE PHYSIOLOGY AND BIOGEOCHEMISTRY OF THE N2-FIXING CYANOBACTERIA TRICHODESMIUM ERYTHRAEUM AND CROCOSPHAERA WATSONII By: Nathan Samuel Garcia ____________ A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (BIOLOGY) December 2012 Copyright 2012 Nathan Samuel Garcia|