Morphological and Isotopic Changes of Anabaena cylindrica PCC 7122 in Response to N2 Partial Pressure

Silverman, Shaelyn

Undergraduate Honors Thesis

Morphological and Isotopic Changes of Anabaena cylindrica PCC 7122 in Response to N2 Partial Pressure Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/d504rk823

Abstract

(1) Many filamentous cyanobacteria overcome nitrogen limitation by developing heterocysts, specialized cells that maintain microoxic environments to protect the oxygen-sensitive nitrogen-fixing enzyme nitrogenase. Under current atmospheric conditions (0.79 bar N₂, 0.21 bar O₂), heterocysts form in a well-regulated pattern along the filaments with a separation distance of about 10 to 20 vegetative cells that is maintained as the cells divide. Previous attempts to understand this developmental pattern have focused on the control by genetic expression and nitrogen bioavailability, but no study has examined whether heterocyst development is influenced by the availability of N₂, which scales directly with atmospheric N₂ partial pressure (pN₂) by Henry’s law. Here I have investigated whether pN₂ affects heterocyst spacing in two cyanobacterial species. In Anabaena cylindrica PCC 7122 filaments, heterocyst spacing appears to be regulated by N₂ through its effect on the nitrogen fixation rate. In contrast, it is unclear whether the heterocyst pattern in Anabaena variabilis ATCC 29413 responds to pN₂.

(2) Biological nitrogen fixation contributes the main flux of fixed nitrogen to the global nitrogen cycle. As accumulated biomass moves through the biogeochemical cycles, isotope effects imposed by enzymes lead to a potentially distinct and measurable isotopic signature. In particular, fractionation (δ¹⁵N) of newly fixed nitrogen by Mo-nitrogenase is –1 ± 1‰, which highlights its preference for ¹⁴N, the lighter nitrogen isotope. These measurements have been reported for nitrogenase operating under the nitrogen replete conditions of the present atmosphere (0.79 bar N₂), but the effect of N₂ availability on overall cellular isotope fractionation has not been explored in nitrogen fixation. Here I show that Mo-nitrogenase in Anabaena cylindrica PCC 7122 produces fixed nitrogen with more positive δ¹⁵N values (i.e. weaker fractionation) under N₂ limited conditions, which suggests that its inherent isotope discrimination is compromised during substrate deficiency.

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