Fosfor ve glikojen depolayan organizmaların metabolik ve matematik modelleri

Metabolik and mathematical modeling of phosphate and glycogen accumulating organism

Özet

Fosfor depolayan organizmalar (PAO) ve glikojen depolayan organizmalar (GAO) hücre içi enerji kaynakları dışında benzer metabolizmaya sahiptirler. Her iki organizmada anaerobik şartlarda asetatı hücre içinde PHA olarak depolarlar. PAO'lar hem glikoliz hem de Poly-P kırılması ile içsel enerjilerini elde ederken GAO'lar sadece glikoliz ile enerji sağlarlar.Bu çalışmada, fosfor ve glikojen depolayan organizma karışık kültürünün anaerobik koşullarda asetat alımı için bir metabolik model önerilmektedir. Anaerobik şartlar için önerilen biyokimyasal modele ait temel reaksiyon ifadeleri tanımlanmıştır. Önerilen model stokiyometrisi, esas alınan metabolik yolların organik karbon, enerji ve indirgeme kuvvetlerinin dengelenmesi suretiyle elde edilmiştir. Bu model, fosfor depolayan organizmaların PHA sentezini gerçekleştirmek üzere gerekli indirgeme kuvvetlerini glioksilat metabolik yolunu kullanarak elde edebilecekleri esasına dayanmaktadır. Bu nedenle, model değişken bir stokiyometriye sahiptir. Ayrıca bu çalışmada, biyolojik aşırı fosfor gideriminin mevcut matematik model, asetat için rekabet eden glikojen ve fosfor depolayan organizmaların mekanistik ifadesi ve daha sonra glikojen metabolizmasının modele ilave edilmesi ile geliştirilmiştir. Heterotroflar, ototroflar ve fosfor depolayan organizmaların tanımlandığı. ASM2d modeli esas alınmış, glikojen depolayan organizmaların prosesleri ve fosfor depolayan organizmaların glikojen mekanizmaları modele dahil edilmiştir. Önceden geliştirilen metabolik kavram ve modellere dayanılarak karışık kültürler için yeni bir proses stokiyometrisi tanımlanmıştır. Geliştirilen matematik modelde yukarıda önerilen metabolik model ile uyumlu olmak üzere PAO'lar ve GAO'lar arasında asetatın paylaştırılması da tanımlanmıştır.

Abstract

It is now widely accepted that EBPR occurs as a result of the predominance of a group of bacteria, commonly called phosphate accumulating organisms (PAOs), with the capability of storing poly-phosphate within the cell. EBPR is only sustained when the activated sludge system is operated in an anaerobic/aerobic sequence and the biomass is fed with short chain fatty acids (SCFAs) like acetate, during the anaerobic phase. In this phase, PAOs activate the metabolic tools to take up acetate and to store it as polyhydroxyalkanoates (PHAs), mainly polyhydroxybutyrate (PHB). PHA storage proceeds using the internal polyphosphate (poly-P) pool as energy source resulting in a release of orthophos-phate (Pi). In the subsequent aerobic stage, PAOs grow on the internally stored PHA and take up orthophosphate for the replenishment of poly-P reserves that increase the P content of sludge. Higher P removal is achieved by withdrawing excess sludge with high P-content. Some observations given in the literature may be related to the presence of other types of bacteria, later called glycogen accumulating organisms (GAOs), that are capable of storing substrate under anaerobic conditions without using energy from P release. This implies the involvement of an energy source other than poly-P in EBPR systems, thus leading to a drastic decrease in the P released/substrate uptake ratio. It should be noted that during anaerobic acetate metabolism in GAOs, energy and reducing power are provided only by glycogen degradation without any poly-P involvement. This necessitates a metabolic pathway for the regeneration of the surplus NADH2 produced during glycolysis to maintain the redox balance inside the cell. This paper proposes a biochemical model for the acetate uptake by a mixed culture of PAOs and GAOs under anaerobic conditions. The proposed model is used to establish basic stoichiometric balances for organic carbon, ATP and reducing power, through appropriate metabolic pathways. PAOs and GAOs have similar metabolism except for their internal energy sources. Both of them uptake acetate anaerobically and store it as PHA. During PHA storage, the redox balance is regulated by the consumption of glycogen. Their metabolisms basically differ in the energy source utilized: While PAOs have the metabolic complement to use both poly-P cleavage and glycolysis as their energy supply; GAOs solely depend on glycogen for this purpose. Basic reactions of the metabolic model are given involving the same sequence for the two types of microorganisms, except for the formation of propionyl-CoA: Acetate uptake and activation to Acetyl-CoA, glycogen degradation to pyruvate, oxidative decar-boxylation of pyruvate, formation of propionyl-CoA from pyruvate, formation of propionyl-CoA from acetyl-CoA, PHB synthesis, PHV synthesis and Poly-P cleavage. With this model, behavior of enhanced biological phosphorus systems is better evaluated in terms of the resulting overall stoichiometry of a mixed culture of PAOs and GAOs competing for the same substrate under anaerobic conditions. Besides, the modeling of enhanced biological phosphate removal is improved by introducing the mechanistic description of a mixed culture of glycogen and phosphate accumulating organisms competing for acetate, and glycogen metabolism of the latter. A new process stoichiometry is defined for the mixed culture based upon previously developed metabolic concepts and models. Thus, ASM2d is considered as a base-model since it basically embodies three different types of microorganisms: heterotrophs, autotrophs and phosphorus accumulating organisms describing the nutrient removal processes in activated sludge systems. In ASM2d, the phosphorus accumulating organisms, XPAOs takes up acetate in the availability of acetate and store only in the form of polyhydroxyalkanoate, XPHA with the stoichiometric ratio of HAc/PHA: 1. In subsequent aerobic or anoxic conditions, the processes of XPAOs growth and the phosphate uptake utilize XPHA. The stoichiometry and processes for the autotrophs and heterotrophs are taken as the same in ASM2d, accordingly. Glycogen metabolism of PAOs and GAO metabolism included into the model, individually. It is assumed that under anaerobic condition, the PHA pool is replenished by the consumption of both external acetate and glycogen with the stoichiometry of YSA and 1-YSA, respectively by assuming PHA is equal to unit COD. Model development mainly relied on evaluation of critical coefficients based on proposed metabolic relationships related to the interactive growth of GAOs and PAOs in the mixed cultures. The model is able to describe the split of acetate utilization between PAOs and GAOs, accurately as predicted by related metabolic relationship.