## 4 Discussion and Conclusions  
Thermodynamics can improve the comprehension of the microorganisms growth and, consequently, it can allow us to improve their productivity of chemical compounds useful to biofuels production. The thermodynamic black box model has been proved to be a good tool for the evaluation of the microbial growth. In particular, it is useful to define the conditions of optimal growth, in relation to the interaction between the microorganisms and their environment. However, this interaction is based on fluxes through the microorganisms membrane. Our results improve this approach, by analysing both the thermal and the electric fluxes. Indeed, the comprehension of the effects of the exogenous stimuli represents a fundamental improvement in the understanding of the bacterial electrophysiology [46; 142; 143]. So, biofuel and bioplastic production can be related to the proliferation, which is controlled just by membrane electric potential, driven by ATP synthesis [118; 144; 145; 146]. The fundamental role of the membrane electric potential has been experimentally pointed out, by the analysis of the energy used by _Escherichia coli_ to maintain its membrane electric potential; indeed, it results in around half of its total energy consumption [147]. Our results suggest explanation to some experimental evidence; indeed, an exogenous electrical stimulus has been shown to shape the proliferative capacity of bacteria, by inducing hyperpolarization in the cells [46]. Moreover, within the temperature range of life for the micro-organisms, higher temperature shortens the lag period and stimulates cell growth, in accordance with the microbial metabolism [148; 149; 150; 151; 152]. In our relation, the metabolism is taken into account by the surface temperature, while the environmental conditioning is expressed by the environmental temperature. We highlight that a variation of the micro-organisms behaviour can be induced by the symbiosis between two different species. Indeed, in order to improve the formation of the useful high-value biomolecules, for biofuels production from micro-organisms, an interesting approach is to exploit the _natural_ positive mutual interactions between different species. This could lead to a variation in their metabolites exchanges across their membranes and to a related change of the pH gradient [104]. For example, the occultivation of two different species can bring to an enhancement of the lipid biomolecules inside the cells [153], which are the useful molecules employed for the biodiesel production [154]. Several studies have investigated the occultivation of different micro-organisms, with the aim of improving their lipid concentration:  
* In Ref. [155], the species _Chlorella pyrenoidosa_ and _Rhodospiridium toruloides_ have been cocultivated, obtaining 4.60 \(\g_{\text{lipid}}\) L\({}^{-1}\) (compared respectively to 3.00 \(\g_{\text{lipid}}\) L\({}^{-1}\) and 3.40 \(\g_{\text{lipid}}\) L\({}^{-1}\), respectively for each single species);
* In Ref. [156], the species _Spirulina platensis_ and _Rhodotorula glutinis_ have been cocultivated, obtaining 0.467 \(\g_{\text{lipid}}\) L\({}^{-1}\) (compared respectively to 0.013 \(\g_{\text{lipid}}\) L\({}^{-1}\) and 0.135 \(\g_{\text{lipid}}\) L\({}^{-1}\) for the single species);
* In Ref. [157], the species _Chlorella_ sp. and _Toluruspore_ have been cocultivated, obtaining 2.42 \(\g_{\text{lipid}}\) L\({}^{-1}\) (compared respectively to 0.052 \(\g_{\text{lipid}}\) L\({}^{-1}\) and 1.141 \(\g_{\text{lipid}}\) L\({}^{-1}\) for the single species);
* In Ref. [158], the species _Chlorella_ sp. and _Monoraphidium_ sp. have been cocultivated, obtaining an improved lipid productivity of \(29.52\pm 1.13\) mg\({}_{\text{lipid}}\) L\({}^{-1}\) d\({}^{-1}\) (compared respectively to \(17.99\pm 3.39\) mg\({}_{\text{lipid}}\) L\({}^{-1}\) d\({}^{-1}\) and \(17.70\pm 1.19\) mg\({}_{\text{lipid}}\) L\({}^{-1}\) d\({}^{-1}\) for the single species).