This study reviews the addition of compatible solutes to biological systems as a strategy to counteract osmolarity and other environmental stresses. At high osmolarity many microorganisms accumulate organic solutes called "compatible solutes" in order to balance osmotic pressure between the cytoplasm and the environment. These organic compounds are called compatible solutes because they can function inside the cell without the need for special adaptation of the intracellular enzymes, and also serve as protein stabilizers in the presence of high ionic strength. Moreover, the compatible solutes strategy is regularly being employed by the cell, not only under osmotic stress at high salinity, but also under other extreme environmental conditions such as low temperature, freezing, heat, starvation, dryness, recalcitrant compounds and solvent stresses. The accumulation of these solutes from the environment has energetically a lower cost than de novo synthesis. Based on this cell mechanism several studies in the field of environmental biotechnology (most of them on biological wastewater treatment) employed this strategy by exogenously adding compatible solutes to the wastewater or medium in order to alleviate environmental stress. This current paper critically reviews and evaluates these studies, and examines the future potential of this approach. In addition to this, a strategy for the successful implementation of compatible solutes in biological systems is proposed.

This study investigated the role of compatible solutes, extracellular polysaccharides (EPS), and nutrients on anaerobic biomass when stressed with salinity. When 1 mM of osmoregulants glycine betaine, α-glutamate and β-glutamate were added separately to serum bottles containing biomass not adapted to sodium, and fed with glucose and 35 g NaCl/L, all the compatible solutes were found to alleviate sodium inhibition, although glycine betaine was found to be the most effective. The effect of glycine betaine on different anaerobic bacterial groups under salinity stress was monitored using VFAs, and showed that methanogens were more protected than propionate utilisers. Moreover, the addition of 1 mM of glycine betaine to anaerobic biomass not adapted to salinity resulted in significantly higher methane production rates compared with anaerobic biomass that was exposed for 4 weeks to 35 g NaCl/L. Interestingly, under saline batch conditions when the medium was replaced totally the culture produced less methane than when only new substrate was added due to compatible solutes cycling between the media and the cell. The elimination of macronutrients from the medium was found to have a more pronounced negative effect on biomass under saline compared with nonsaline conditions, and because of the synthesis of N-compatible solutes sufficient nutrients should always be present. On the other hand, the absence from the medium of micronutrients did not further reduce biomass activity under salinity. Finally, a higher production of EPS was obtained from biomass exposed to higher salt concentrations, and its composition was found to change under different saline conditions and time. As a result, biomass under saline conditions had a slightly higher mean flock size compared with the biomass that was not subjected to salt. © 2008 Elsevier Inc. All rights reserved

This paper deals with the development of a citrus peel waste (CPW) biorefinery that employs low environmental impact technologies for production of ethanol and methane. Three major yeasts were compared for ethanol production in batch fermentations using CPW pretreated through acid hydrolysis and a combination of acid and enzyme hydrolysis. The most efficient conditions for production of CPW-based hydrolyzates included processing at 116 °C for 10 min. Pichia kudriavzevii KVMP10 achieved the highest ethanol production that reached 30.7 g L⁻¹ in fermentations conducted at elevated temperatures (42 °C). A zero-waste biorefinery was introduced by using solid biorefinery residues in repeated batch anaerobic digestion fermentations achieving methane formation of 342 mL g_VS⁻¹ (volatile solids). Methane production applying untreated and dried CPW reached a similar level (339–356 mL g_VS⁻¹) to the use of the side stream, demonstrating that the developed bioprocess constitutes an advanced alternative to energy intensive methods for biofuel production.