Investigating the potential for nitrous oxide (N2O) production from sediment microbiota, and its potential effect on growth of cobalamin-dependent microalgae

Abstract

Microalgae are important primary producers of marine ecosystems. Many species require cobalamin (Vitamin B12), which is primarily related to the synthesis of methionine, where cobalamin is cofactor for the enzyme methionine synthase (METH). A gap of knowledge is the required amount of cobalamin needed for growth of cobalamin-dependent organisms and the potential environmental factors that can inactivate cobalamin. Nitrous oxide (N2O) has shown to be a potent inactivator of cobalamin in clinical studies. The greenhouse gas is biologically produced by marine microorganisms, and it is not known whether the concentrations of N2O usually found in marine environments can inhibit growth of cobalamin-dependent microalgae. The aim of this thesis was to investigate the effect of cobalamin depletion and N2O exposure on growth of cobalamin-dependent microalgae.

The effect of different concentrations of N2O on growth of cobalamin-dependent E.coli was established in an E.coli model system. Further, the potential for N2O production from microbiota of enriched sediments was investigated in air-tight vials. Growth of cobalamin-dependent I.galbana and T.lutea upon cobalamin depletion was investigated by growing the microalgae in medium without cobalamin. Lastly, N2O effect on algal cultures were tested through injecting growing cultures into air-tight vials containing N2O, followed by measurements of oxygen levels.

It was found inactivation of growth of cobalamin-dependent E.coli when presence of saturated concentrations of N2O. This concentration was substantially above the N2O concentrations associated with marine environments and the observed accumulation from enriched sediments. The growth of the cobalamin-dependent microalgae was surprisingly not affected by the absence of cobalamin in medium. Through 16S rRNA analysis, the presence of bacteria was detected in the cultures of these microalgae. Lastly, it was not succeeded to evaluate the effect of N2O on oxygen production from microalgae with the current experimental set-up.

In conclusion, saturated concentrations of N2O was shown to inactivate growing cobalamin-dependent E.coli, but the exact inactivation concentration remains undetermined and requires further investigation. As the observed inactivation level was far above the N2O concentrations associated with marine environments, the results suggests that the N2O concentrations usually found in such environments are insufficient to impact the growth of resident cobalamin-dependent organisms. Further, if the non-observed effect of cobalamin depletion on growth of cobalamin-dependent microalgae is caused by the presence of cobalamin-producers or other factors could be investigated further. Lastly, as it was not possible to evaluate the effect of N2O on microalgal growth, this should be investigated further.