Carbon and energy fluxes during haloadaptation of Halomonas sp. EF11 growing on phenol

The haloalkaliphile Halomonas sp. EF11 can grow on phenol as sole source for carbon and energy, while maintaining an osmotic equilibrium predominantly by adjusting levels of a certain compatible solute. To determine the energy costs of haloadaptation and the fate of substrate-carbon, the strain was grown continuously in an isothermal compensation calorimeter, keeping all conditions constant except salinity. As salinity increased, slight linear reductions in exothermic heat flow and biomass formation occurred, and 1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid (ectoine) synthesis increased linearly. However, beyond a certain salinity threshold the stationary phenol concentration increased exponentially, while heat flow fell sharply, indicating intoxication or wash-out. The clear transition point between the phases, where ectoine formation peaked, suggests that calorimetric measurements could be used to control the conversion of growth-inhibiting substrates (like phenol) into ectoine and to optimize the process. Enthalpy balance and chemical determinations revealed acetate and formate were formed as side products when the C/N ratio in the feed was low, while 2-muconic acid semialdehyde and formate were produced when the ratio was high. These findings indicate that phenol assimilation occurs via the meta pathway. However, enzyme assays implied that assimilation occurs via the ortho and meta pathways at a low C/N ratio and exclusively via the meta pathway at a high C/N ratio.