Graduate Thesis Or Dissertation
 

A study on the effects of enhanced pCO2 on open ocean diazotrophic assemblages

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wm117r403

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  • Biological di-nitrogen (N₂) fixation is a key process in open-ocean ecosystems, where the new nitrogen (N) provided by marine diazotrophs can support a large fraction of primary productivity and carbon (C) drawdown. Recent laboratory studies have shown that elevated pCO₂ enhances the rate of N₂ fixation by select laboratory isolates of Trichodesmium and Crocosphaera. While both N₂-fixing cyanobacteria, these groups differ widely in their cell size, maximal growth rates, and diel periodicity of N₂ fixation. It is unclear whether the CO₂ enhancement shown by these species in laboratory settings will hold for diverse diazotrophic communities under natural ocean environments. The aim of this thesis was to investigate how open ocean diazotrophs respond to elevated pCO₂ and whether this response is modulated by environmental conditions. I used laboratory and field approaches to address this problem. Laboratory experiments were designed to test the impact of changing pCO₂ on growth, C and N accumulation by Crocosphaera strain WH8501. Elevating pCO₂ above present-day conditions increased growth rates and net C and N accumulation rates. Monitoring C and N accumulation over a diurnal cycle showed that cultures had a distinct rhythm of C and N accumulation; elevating pCO₂ changed this pattern by extending the period of high N accumulation rates. However, the ecologically relevant question is not how a single organism responds to elevated pCO₂ in a controlled setting, but rather how diverse microbial communities will respond in their natural habitats. Accordingly, field experiments were conducted on three cruises at Station ALOHA in order to study how natural assemblages of Trichodesmium and co-associated organisms respond to elevated pCO₂. In contrast to the laboratory results, we observed no consistent pCO₂ enhancement of C or N₂ fixation rates on these cruises. This response was not affected by amending seawater with P or Fe or by changing the light levels of deckboard incubations. Ultimately, DNA sequences from Trichodesmium colonies provided evidence suggesting that biological diversity of marine diazotrophs in our samples is the underlying reason for the lack of pCO₂ enhancement observed in the field. In other words, pCO₂ may upregulate metabolic processes in certain species but not others, leading to no measurable CO₂ enhancement of N₂ fixation by real-environment, diverse diazotrophic assemblages.
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