The effect of cadmium on the net N2O production of deep-sea bacteria

Abstract

Deep-sea bacteria have high environmental importance due to their active role on nutrient cycling, among other activities. Some of these bacteria are responsible for maintaining low natural levels of nitrous oxide (N2O), a powerful greenhouse gas and ozone-depleting, by reducing it to dinitrogen gas (N2), through the denitrification pathway. The growth of deep-sea mining activities may increase the exposure of marine bacteria to toxic concentrations of metals such as cadmium and lead, as well as other rare elements. However, the susceptibility of the N2O-reducing pathway to metal exposure is relatively unknown, especially in deep-sea conditions. The aim of this dissertation is to understand potential impacts of cadmium exposure on net N2O production, by deep-sea bacteria. For this, we used mostly the strain Shewanella loihica PV-4 as a model, a piezotolerant deep-sea isolate with genetic potential to produce and remove N2O. Cadmium exposure experiments started with synthetic air flowing to allow S. loihica PV-4 aerobic growth inside closed bioreactors. When growth reached mid or late exponential phase, gas supply was changed to N2 to create an anoxic environment towards stimulating denitrifying conditions. Headspace gas sampling was performed at seven time points during anoxia to quantify N2O accumulation. Over the same period of time, four liquid culture samples were taken for RNA extraction to assess nirK and nosZ relative expression, two key genes for net N2O production. In the mid-exponential experiment, net N2O production showed a positive flux of 2,307 ± 1,428 ppm/min in the control and a null flux for cadmium treatment. For the control treatment, the ratio nirK/nosZ was 10x times higher than in the cadmium treatment (nirK/nosZCADMIUM = 1,384; nirK/nosZCONTROL = 13,415), supporting the positive flux in this treatment. In the late-exponential experiment, net N2O production fluxes were zero for both treatments, but the background levels of N2O in the cadmium treatment were higher than in the control. Relative expression of nosZ was much higher than nirK (nirK/nosZCADMIUM = 0,008; nirK/nosZCONTROL = 0,007) in the late-exponential stage, explaining the low N2O values quantified in both treatments, when compared to the mid-exponential. Either in the mid-exponential experiment, where cadmium appears to inhibit net N2O production, or in the late-exponential experiment, where cadmium might increase N2O production, this implies that the nitrogen cycle can be altered by cadmium exposure in marine environments.

Date of Award	16 Dec 2022
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Miguel Albergaria Furtado Semedo (Supervisor), Catarina Magalhães (Co-Supervisor) & Maria Fátima Carvalho (Co-Supervisor)