Electro-Stimulation of Thermotoga Neapolitana Metabolism
Authors: Cristiani Pierangela , D'Ippolito Giuliana , Squadrito Gaetano , Fontana Angelo 
Published: February 2020

Thermotoga neapolitana (DSM 4359, ATCC 49049), more than others Thermotogales is able to accumulate biohydrogen from waste organic matter, in a selective environment (>80ºC). Thanks to the high temperature, this bacterium is able to use a peculiar anaplerotic biochemical pathway named Capnophilic Lactic Fermentation (CLF) that accomplishes chain elongation of acetate with direct reduction of CO2, producing lactic acid (Pradhan et al. 2015). This is a carbon fixation mechanism which, differently from autotrophism, leads to direct elimination of fixed carbon into excreted metabolites, such as lactic acid, instead of converting and accumulating it into biomass. T. neapolitana is therefore a promising microorganism for recovering added-value chemicals from organic wastes (Fontana, EP14711847.5).
In this work, the possibility of bioelectrostimulate the production of lactic acid by Thermotoga Neapolitana supplying additional electric energy in polarized and unpolarized bioreactors is investigated. For this, an alternating polarization up to ±1.2V was imposed between two identical electrodes of carbon cloth, immersed in the electrochemical bioreactors. Bioreactor without electrodes were also tested, as control. Glass reactors of 250 mL were operated at temperature of 85°C with a culture media of T. neapolitana containing 5 g/L (~30mM) of glucose, in triplicates. Three successive cycles of chemical measurements was carried out every 24 hours for three measurement cycles, to estimate the glucose consume and the acetic/lactic acid production, as well the hydrogen yieldin each tested bioreactor. The solution Optical density (OD) and micrographs produced by scanning electrode microscopy (SEM) showed a strong affinity of bacteria to form biofilm on the polarized and unpolarized electrodes.
Evidence of the lactate/acetate ratio modification with respect the control bioreactors was achieved, especially under higher polarized condition, but also in the bioreactors with unpolarized electrodes.
Glucose fermentation was strongly inhibited in electrochemical bioreactors in the first 24-48 hours test and during the whole three days test in some cases. Only in the control bioreactor glucose was oxidized to acetic acid in the molar rate similar to dark fermentation (glucose:acetic 1:2) and only the first day. Lactic acid in a relative high ratio versus the glucose consume, although at lower concentration with respect acetate, was found especially in some polarized bioereactors.
These results globally confirm the affinity of Thermotoga neapolitana to carbon cloth electrodes and the possibility of electrostimultate their metabolism, opening future perspectives on electro-fermentation with this hyperthermophilic strain.
Pradhan N., Dipasquale L., et al. 2015, Int. J. Mol. Sci. 16, 12578-12600
Fontana EP EP14711847.5. EU patent application: EP14711847.5; Priority IT20130109 24/01/2013
d’Ippolito G, Dipasquale L, Vella FM, Romano I, Gambacorta A, Cutignano A, Fontana A. Int J Hydrogen Energy 2010;35:2290–5. doi:10.1016/j.ijhydene.2009.12.044.