Undergraduate Honors Theses

Thesis Defended

Spring 2017

Document Type


Type of Thesis

Departmental Honors


Molecular, Cellular, & Developmental Biology

First Advisor

Sebastian Kopf

Second Advisor

Sanjoy Som


(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.