Plastics have quickly become an integral part of modern life. Due to excessive production and improper waste disposal, they are recognized as contaminants present in practically all habitat types. Although there are several polymers, polyethylene terephthalate (PET) is of particular concern due to its abundance in the environment. There is a need for a solution that is both cost-effective and ecologically friendly to address this pollutant. The use of microbial depolymerizing enzymes could offer a biological avenue for plastic degradation, though the full potential of these enzymes is yet to be uncovered. The purpose of this study was to use (1) plate-based screening methods to investigate the plastic degradation potential of marine bacteria from the order Enterobacterales collected from various organismal and environmental sources, and (2) perform genome-based analysis to identify polyesterases potentially related to PET degradation. 126 bacterial isolates were obtained from the strain collection of RD3, Research Unit Marine Symbioses-GEOMAR-and sequentially tested for esterase and polyesterase activity, in combination here referred to as PETase–like activity. The results show that members of the microbial families Alteromonadaceae, Shewanellaceae, and Vibrionaceae, derived from marine sponges and bryozoans, are the most promising candidates within the order Enterobacterales. Furthermore, 389 putative hydrolases from the α/β superfamily were identified in 23 analyzed genomes, of which 22 were sequenced for this study. Several candidates showed similarities with known PETases, indicating underlying enzymatic potential within the order Enterobacterales for PET degradation.
Citation: Denisse Galarza–Verkovitch, Onur Turak, Jutta Wiese, Tanja Rahn, Ute Hentschel, Erik Borchert. Bioprospecting for polyesterase activity relevant for PET degradation in marine Enterobacterales isolates[J]. AIMS Microbiology, 2023, 9(3): 518-539. doi: 10.3934/microbiol.2023027
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Plastics have quickly become an integral part of modern life. Due to excessive production and improper waste disposal, they are recognized as contaminants present in practically all habitat types. Although there are several polymers, polyethylene terephthalate (PET) is of particular concern due to its abundance in the environment. There is a need for a solution that is both cost-effective and ecologically friendly to address this pollutant. The use of microbial depolymerizing enzymes could offer a biological avenue for plastic degradation, though the full potential of these enzymes is yet to be uncovered. The purpose of this study was to use (1) plate-based screening methods to investigate the plastic degradation potential of marine bacteria from the order Enterobacterales collected from various organismal and environmental sources, and (2) perform genome-based analysis to identify polyesterases potentially related to PET degradation. 126 bacterial isolates were obtained from the strain collection of RD3, Research Unit Marine Symbioses-GEOMAR-and sequentially tested for esterase and polyesterase activity, in combination here referred to as PETase–like activity. The results show that members of the microbial families Alteromonadaceae, Shewanellaceae, and Vibrionaceae, derived from marine sponges and bryozoans, are the most promising candidates within the order Enterobacterales. Furthermore, 389 putative hydrolases from the α/β superfamily were identified in 23 analyzed genomes, of which 22 were sequenced for this study. Several candidates showed similarities with known PETases, indicating underlying enzymatic potential within the order Enterobacterales for PET degradation.
Baló's concentric sclerosis (BCS) was first described by Marburg [1] in 1906, but became more widely known until 1928 when the Hungarian neuropathologist Josef Baló published a report of a 23-year-old student with right hemiparesis, aphasia, and papilledema, who at autopsy had several lesions of the cerebral white matter, with an unusual concentric pattern of demyelination [2]. Traditionally, BCS is often regarded as a rare variant of multiple sclerosis (MS). Clinically, BCS is most often characterized by an acute onset with steady progression to major disability and death with months, thus resembling Marburg's acute MS [3,4]. Its pathological hallmarks are oligodendrocyte loss and large demyelinated lesions characterized by the annual ring-like alternating pattern of demyelinating and myelin-preserved regions. In [5], the authors found that tissue preconditioning might explain why Baló lesions develop a concentric pattern. According to the tissue preconditioning theory and the analogies between Baló's sclerosis and the Liesegang periodic precipitation phenomenon, Khonsari and Calvez [6] established the following chemotaxis model
˜uτ=DΔX˜u⏟diffusion ofactivated macrophages−∇X⋅(˜χ˜u(ˉu−˜u)∇˜v)⏟chemoattractant attractssurrounding activated macrophages+μ˜u(ˉu−˜u)⏟production of activated macrophages,−˜ϵΔX˜v⏟diffusion of chemoattractant=−˜α˜v+˜β˜w⏟degradation∖production of chemoattractant,˜wτ=κ˜uˉu+˜u˜u(ˉw−˜w)⏟destruction of oligodendrocytes, | (1.1) |
where ˜u, ˜v and ˜w are, respectively, the density of activated macrophages, the concentration of chemoattractants and density of destroyed oligodendrocytes. ˉu and ˉw represent the characteristic densities of macrophages and oligodendrocytes respectively.
By numerical simulation, the authors in [6,7] indicated that model (1.1) only produces heterogeneous concentric demyelination and homogeneous demyelinated plaques as χ value gradually increases. In addition to the chemoattractant produced by destroyed oligodendrocytes, "classically activated'' M1 microglia also can release cytotoxicity [8]. Therefore we introduce a linear production term into the second equation of model (1.1), and establish the following BCS chemotaxis model with linear production term
{˜uτ=DΔX˜u−∇X⋅(˜χ˜u(ˉu−˜u)∇˜v)+μ˜u(ˉu−˜u),−˜ϵΔX˜v+˜α˜v=˜β˜w+˜γ˜u,˜wτ=κ˜uˉu+˜u˜u(ˉw−˜w). | (1.2) |
Before going to details, let us simplify model (1.2) with the following scaling
u=˜uˉu,v=μˉu˜ϵD˜v,w=˜wˉw,t=μˉuτ,x=√μˉuDX,χ=˜χ˜ϵμ,α=D˜α˜ϵμˉu,β=˜βˉw,γ=˜γˉu,δ=κμ, |
then model (1.2) takes the form
{ut=Δu−∇⋅(χu(1−u)∇v)+u(1−u),x∈Ω,t>0,−Δv+αv=βw+γu,x∈Ω,t>0,wt=δu1+uu(1−w),x∈Ω,t>0,∂ηu=∂ηv=0,x∈∂Ω,t>0,u(x,0)=u0(x),w(x,0)=w0(x),x∈Ω, | (1.3) |
where Ω⊂Rn(n≥1) is a smooth bounded domain, η is the outward normal vector to ∂Ω, ∂η=∂/∂η, δ balances the speed of the front and the intensity of the macrophages in damaging the myelin. The parameters χ,α and δ are positive constants as well as β,γ are nonnegative constants.
If δ=0, then model (1.3) is a parabolic-elliptic chemotaxis system with volume-filling effect and logistic source. In order to be more line with biologically realistic mechanisms, Hillen and Painter [9,10] considered the finite size of individual cells-"volume-filling'' and derived volume-filling models
{ut=∇⋅(Du(q(u)−q′(u)u)∇u−q(u)uχ(v)∇v)+f(u,v),vt=DvΔv+g(u,v). | (1.4) |
q(u) is the probability of the cell finding space at its neighbouring location. It is also called the squeezing probability, which reflects the elastic properties of cells. For the linear choice of q(u)=1−u, global existence of solutions to model (1.4) in any space dimension are investigated in [9]. Wang and Thomas [11] established the global existence of classical solutions and given necessary and sufficient conditions for spatial pattern formation to a generalized volume-filling chemotaxis model. For a chemotaxis system with generalized volume-filling effect and logistic source, the global boundedness and finite time blow-up of solutions are obtained in [12]. Furthermore, the pattern formation of the volume-filling chemotaxis systems with logistic source and both linear diffusion and nonlinear diffusion are shown in [13,14,15] by the weakly nonlinear analysis. For parabolic-elliptic Keller-Segel volume-filling chemotaxis model with linear squeezing probability, asymptotic behavior of solutions is studied both in the whole space Rn [16] and on bounded domains [17]. Moreover, the boundedness and singularity formation in parabolic-elliptic Keller-Segel volume-filling chemotaxis model with nonlinear squeezing probability are discussed in [18,19].
Very recently, we [20] investigated the uniform boundedness and global asymptotic stability for the following chemotaxis model of multiple sclerosis
{ut=Δu−∇⋅(χ(u)∇v)+u(1−u),χ(u)=χu1+u,x∈Ω,t>0,τvt=Δv−βv+αw+γu,x∈Ω,t>0,wt=δu1+uu(1−w),x∈Ω,t>0, |
subject to the homogeneous Neumann boundary conditions.
In this paper, we are first devoted to studying the local existence and uniform boundedness of the unique classical solution to system (1.3) by using Neumann heat semigroup arguments, Banach fixed point theorem, parabolic Schauder estimate and elliptic regularity theory. Then we discuss that exponential asymptotic stability of the positive equilibrium point to system (1.3) by constructing Lyapunov function.
Although, in the pathological mechanism of BCS, the initial data in model (1.3) satisfy 0<u0(x)≤1,w0(x)=0, we mathematically assume that
{u0(x)∈C0(ˉΩ)with0≤,≢u0(x)≤1inΩ,w0(x)∈C2+ν(ˉΩ)with0<ν<1and0≤w0(x)≤1inΩ. | (1.5) |
It is because the condition (1.5) implies u(x,t0)>0 for any t0>0 by the strong maximum principle.
The following theorems give the main results of this paper.
Theorem 1.1. Assume that the initial data (u0(x),w0(x)) satisfy the condition (1.5). Then model (1.3) possesses a unique global solution (u(x,t),v(x,t),w(x,t)) satisfying
u(x,t)∈C0(ˉΩ×[0,∞))∩C2,1(ˉΩ×(0,∞)),v(x,t)∈C0((0,∞),C2(ˉΩ)),w(x,t)∈C2,1(ˉΩ×[0,∞)), | (1.6) |
and
0<u(x,t)≤1,0≤v(x,t)≤β+γα,w0(x)≤w(x,t)≤1,inˉΩ×(0,∞). |
Moreover, there exist a ν∈(0,1) and M>0 such that
‖u‖C2+ν,1+ν/2(ˉΩ×[1,∞))+‖v‖C0([1,∞),C2+ν(ˉΩ))+‖w‖Cν,1+ν/2(ˉΩ×[1,∞))≤M. | (1.7) |
Theorem 1.2. Assume that β≥0,γ≥0,β+γ>0 and
χ<{min{2√2αβ,2√2αγ},β>0,γ>0,2√2αβ,β>0,γ=0,2√2αγ,β=0,γ>0. | (1.8) |
Let (u,v,w) be a positive classical solution of the problem (1.3), (1.5). Then
‖u(⋅,t)−u∗‖L∞(Ω)+‖v(⋅,t)−v∗‖L∞(Ω)+‖w(⋅,t)−w∗‖L∞(Ω)→0,ast→∞. | (1.9) |
Furthermore, there exist positive constants λ=λ(χ,α,γ,δ,n) and C=C(|Ω|,χ,α,β,γ,δ) such that
‖u−u∗‖L∞(Ω)≤Ce−λt,‖v−v∗‖L∞(Ω)≤Ce−λt,‖w−w∗‖L∞(Ω)≤Ce−λt,t>0, | (1.10) |
where (u∗,v∗,w∗)=(1,β+γα,1) is the unique positive equilibrium point of the model (1.3).
The paper is organized as follows. In section 2, we prove the local existence, the boundedness and global existence of a unique classical solution. In section 3, we firstly establish the uniform convergence of the positive global classical solution, then discuss the exponential asymptotic stability of positive equilibrium point in the case of weak chemotactic sensitivity. The paper ends with a brief concluding remarks.
The aim of this section is to develop the existence and boundedness of a global classical solution by employing Neumann heat semigroup arguments, Banach fixed point theorem, parabolic Schauder estimate and elliptic regularity theory.
Proof of Theorem 1.1 (ⅰ) Existence. For p∈(1,∞), let A denote the sectorial operator defined by
Au:=−Δuforu∈D(A):={φ∈W2,p(Ω)|∂∂ηφ|∂Ω=0}. |
λ1>0 denote the first nonzero eigenvalue of −Δ in Ω with zero-flux boundary condition. Let A1=−Δ+α and Xl be the domains of fractional powers operator Al,l≥0. From the Theorem 1.6.1 in [21], we know that for any p>n and l∈(n2p,12),
‖z‖L∞(Ω)≤C‖Al1z‖Lp(Ω)forallz∈Xl. | (2.1) |
We introduce the closed subset
S:={u∈X|‖u‖L∞((0,T);L∞(Ω))≤R+1} |
in the space X:=C0([0,T];C0(ˉΩ)), where R is a any positive number satisfying
‖u0(x)‖L∞(Ω)≤R |
and T>0 will be specified later. Note F(u)=u1+u, we consider an auxiliary problem with F(u) replaced by its extension ˜F(u) defined by
˜F(u)={F(u)uifu≥0,−F(−u)(−u)ifu<0. |
Notice that ˜F(u) is a smooth globally Lipshitz function. Given ˆu∈S, we define Ψˆu=u by first writing
w(x,t)=(w0(x)−1)e−δ∫t0˜F(ˆu)ˆuds+1,x∈Ω,t>0, | (2.2) |
and
w0≤w(x,t)≤1,x∈Ω,t>0, |
then letting v solve
{−Δv+αv=βw+γˆu,x∈Ω,t∈(0,T),∂ηv=0,x∈∂Ω,t∈(0,T), | (2.3) |
and finally taking u to be the solution of the linear parabolic problem
{ut=Δu−χ∇⋅(ˆu(1−ˆu)∇v)+ˆu(1−ˆu),x∈Ω,t∈(0,T),∂ηu=0,x∈∂Ω,t∈(0,T),u(x,0)=u0(x),x∈Ω. |
Applying Agmon-Douglas-Nirenberg Theorem [22,23] for the problem (2.3), there exists a constant C such that
‖v‖W2p(Ω)≤C(β‖w‖Lp(Ω)+γ‖ˆu‖Lp(Ω))≤C(β|Ω|1p+γ(R+1)) | (2.4) |
for all t∈(0,T). From a variation-of-constants formula, we define
Ψ(ˆu)=etΔu0−χ∫t0e(t−s)Δ∇⋅(ˆu(1−ˆu)∇v(s))ds+∫t0e(t−s)Δˆu(s)(1−ˆu(s))ds. |
First we shall show that for T small enough
‖Ψ(ˆu)‖L∞((0,T);L∞(Ω))≤R+1 |
for any ˆu∈S. From the maximum principle, we can give
‖etΔu0‖L∞(Ω)≤‖u0‖L∞(Ω), | (2.5) |
and
∫t0‖etΔˆu(s)(1−ˆu(s))‖L∞(Ω)ds≤∫t0‖ˆu(s)(1−ˆu(s))‖L∞(Ω)ds≤(R+1)(R+2)T | (2.6) |
for all t∈(0,T). We use inequalities (2.1) and (2.4) to estimate
χ∫t0‖e(t−s)Δ∇⋅(ˆu(1−ˆu)∇v(s))‖L∞(Ω)ds≤C∫t0(t−s)−l‖et−s2Δ∇⋅(ˆu(1−ˆu)∇v(s))‖Lp(Ω)ds≤C∫t0(t−s)−l−12‖(ˆu(1−ˆu)∇v(s)‖Lp(Ω)ds≤CT12−l(R+1)(R+2)(β|Ω|1p+γ(R+1)) | (2.7) |
for all t∈(0,T). This estimate is attributed to T<1 and the inequality in [24], Lemma 1.3 iv]
‖etΔ∇z‖Lp(Ω)≤C1(1+t−12)e−λ1t‖z‖Lp(Ω)forallz∈C∞c(Ω). |
From inequalities (2.5), (2.6) and (2.7) we can deduce that Ψ maps S into itself for T small enough.
Next we prove that the map Ψ is a contractive on S. For ˆu1,ˆu2∈S, we estimate
‖Ψ(ˆu1)−Ψ(ˆu2)‖L∞(Ω)≤χ∫t0(t−s)−l−12‖[ˆu2(s)(1−ˆu2(s))−ˆu1(s)(1−ˆu1(s))]∇v2(s)‖Lp(Ω)ds+χ∫t0‖ˆu1(s)(1−ˆu1(s))(∇v1(s)−∇v2(s))‖Lp(Ω)ds+∫t0‖e(t−s)Δ[ˆu1(s)(1−ˆu1(s))−ˆu2(s)(1−ˆu2(s))]‖L∞(Ω)ds≤χ∫t0(t−s)−l−12(2R+1)‖ˆu1(s)−ˆu2(s)‖X‖∇v2(s)‖Lp(Ω)ds+χ∫t0(R+1)(R+2)(β‖w1(s)−w2(s)‖Lp(Ω)+γ‖ˆu1(s)−ˆu2(s)‖Lp(Ω))ds+∫t0(2R+1)‖ˆu1(s)−ˆu2(s)‖Xds≤χ∫t0(t−s)−l−12(2R+1)‖ˆu1(s)−ˆu2(s)‖X‖∇v2(s)‖Lp(Ω)ds+2βδχ∫t0(R+1)(R+2)t‖ˆu1(s)−ˆu2(s)‖Lp(Ω)+γ‖ˆu1(s)−ˆu2(s)‖Lp(Ω)ds+∫t0(2R+1)‖ˆu1(s)−ˆu2(s)‖Xds≤(CχT12−l(2R+1)(β|Ω|1p+γ(R+1))+2βδχT(R2+3R+γ+2)+T(2R+1))‖ˆu1(s)−ˆu2(s)‖X. |
Fixing T∈(0,1) small enough such that
(CχT12−l(2R+1)(β|Ω|1p+γ(R+1))+2βδχT(R2+3R+γ+2)+T(2R+1))≤12. |
It follows from the Banach fixed point theorem that there exists a unique fixed point of Ψ.
(ⅱ) Regularity. Since the above of T depends on ‖u0‖L∞(Ω) and ‖w0‖L∞(Ω) only, it is clear that (u,v,w) can be extended up to some maximal Tmax∈(0,∞]. Let QT=Ω×(0,T] for all T∈(0,Tmax). From u∈C0(ˉQT), we know that w∈C0,1(ˉQT) by the expression (2.2) and v∈C0([0,T],W2p(Ω)) by Agmon-Douglas-Nirenberg Theorem [22,23]. From parabolic Lp-estimate and the embedding relation W1p(Ω)↪Cν(ˉΩ),p>n, we can get u∈W2,1p(QT). By applying the following embedding relation
W2,1p(QT)↪Cν,ν/2(ˉQT),p>n+22, | (2.8) |
we can derive u(x,t)∈Cν,ν/2(ˉQT) with 0<ν≤2−n+2p. The conclusion w∈Cν,1+ν/2(ˉQT) can be obtained by substituting u∈Cν,ν/2(ˉQT) into the formulation (2.2). The regularity u∈C2+ν,1+ν/2(ˉQT) can be deduced by using further bootstrap argument and the parabolic Schauder estimate. Similarly, we can get v∈C0((0,T),C2+ν(ˉΩ)) by using Agmon-Douglas-Nirenberg Theorem [22,23]. From the regularity of u we have w∈C2+ν,1+ν/2(ˉQT).
Moreover, the maximal principle entails that 0<u(x,t)≤1, 0≤v(x,t)≤β+γα. It follows from the positivity of u that ˜F(u)=F(u) and because of the uniqueness of solution we infer the existence of the solution to the original problem.
(ⅲ) Uniqueness. Suppose (u1,v1,w1) and (u2,v2,w2) are two deferent solutions of model (1.3) in Ω×[0,T]. Let U=u1−u2, V=v1−v2, W=w1−w2 for t∈(0,T). Then
12ddt∫ΩU2dx+∫Ω|∇U|2dx≤χ∫Ω|u1(1−u1)−u2(1−u2)|∇v1||∇U|+u2(1−u2)|∇V||∇U|dx+∫Ω|u1(1−u1)−u2(1−u2)||U|dx≤χ∫Ω|U||∇v1||∇U|+14|∇V||∇U|dx+∫Ω|U|2dx≤∫Ω|∇U|2dx+χ232∫Ω|∇V|2dx+χ2K2+22∫Ω|U|2dx, | (2.9) |
where we have used that |∇v1|≤K results from ∇v1∈C0([0,T],C0(ˉΩ)).
Similarly, by Young inequality and w0≤w1≤1, we can estimate
∫Ω|∇V|2dx+α2∫Ω|V|2dx≤β2α∫Ω|W|2dx+γ2α∫Ω|U|2dx, | (2.10) |
and
ddt∫ΩW2dx≤δ∫Ω|U|2+|W|2dx. | (2.11) |
Finally, adding to the inequalities (2.9)–(2.11) yields
ddt(∫ΩU2dx+∫ΩW2dx)≤C(∫ΩU2dx+∫ΩW2dx)forallt∈(0,T). |
The results U≡0, W≡0 in Ω×(0,T) are obtained by Gronwall's lemma. From the inequality (2.10), we have V≡0. Hence (u1,v1,w1)=(u2,v2,w2) in Ω×(0,T).
(ⅳ) Uniform estimates. We use the Agmon-Douglas-Nirenberg Theorem [22,23] for the second equation of the model (1.3) to get
‖v‖C0([t,t+1],W2p(Ω))≤C(‖u‖Lp(Ω×[t,t+1])+‖w‖Lp(Ω×[t,t+1]))≤C2 | (2.12) |
for all t≥1 and C2 is independent of t. From the embedded relationship W1p(Ω)↪C0(ˉΩ),p>n, the parabolic Lp-estimate and the estimation (2.12), we have
‖u‖W2,1p(Ω×[t,t+1])≤C3 |
for all t≥1. The estimate ‖u‖Cν,ν2(ˉΩ×[t,t+1])≤C4 for all t≥1 obtained by the embedded relationship (2.8). We can immediately compute ‖w‖Cν,1+ν2(ˉΩ×[t,t+1])≤C5 for all t≥1 according to the regularity of u and the specific expression of w. Further, bootstrapping argument leads to ‖v‖C0([t,t+1],C2+ν(ˉΩ))≤C6 and ‖u‖C2+ν,1+ν2(ˉΩ×[t,t+1])≤C7 for all t≥1. Thus the uniform estimation (1.7) is proved.
Remark 2.1. Assume the initial data 0<u0(x)≤1 and w0(x)=0. Then the BCS model (1.3) has a unique classical solution.
In this section we investigate the global asymptotic stability of the unique positive equilibrium point (1,β+γα,1) to model (1.3). To this end, we first introduce following auxiliary problem
{uϵt=Δuϵ−∇⋅(uϵ(1−uϵ)∇vϵ)+uϵ(1−uϵ),x∈Ω,t>0,−Δvϵ+αvϵ=βwϵ+γuϵ,x∈Ω,t>0,wϵt=δu2ϵ+ϵ1+uϵ(1−wϵ),x∈Ω,t>0,∂ηuϵ=∂ηvϵ=0,x∈∂Ω,t>0,uϵ(x,0)=u0(x),wϵ(x,0)=w0(x),x∈Ω. | (3.1) |
By a similar proof of Theorem 1.1, we get that the problem (3.1) has a unique global classical solution (uϵ,vϵ,wϵ), and there exist a ν∈(0,1) and M1>0 which is independent of ϵ such that
‖uϵ‖C2+ν,1+ν/2(ˉΩ×[1,∞))+‖vϵ‖C2+ν,1+ν/2(ˉΩ×[1,∞))+‖wϵ‖Cν,1+ν/2(ˉΩ×[1,∞))≤M1. | (3.2) |
Then, motivated by some ideas from [25,26], we construct a Lyapunov function to study the uniform convergence of homogeneous steady state for the problem (3.1).
Let us give following lemma which is used in the proof of Lemma 3.2.
Lemma 3.1. Suppose that a nonnegative function f on (1,∞) is uniformly continuous and ∫∞1f(t)dt<∞. Then f(t)→0 as t→∞.
Lemma 3.2. Assume that the condition (1.8) is satisfied. Then
‖uϵ(⋅,t)−1‖L2(Ω)+‖vϵ(⋅,t)−v∗‖L2(Ω)+‖wϵ(⋅,t)−1‖L2(Ω)→0,t→∞, | (3.3) |
where v∗=β+γα.
Proof We construct a positive function
E(t):=∫Ω(uε−1−lnuϵ)+12δϵ∫Ω(wϵ−1)2,t>0. |
From the problem (3.1) and Young's inequality, we can compute
ddtE(t)≤χ24∫Ω|∇vϵ|2dx−∫Ω(uϵ−1)2dx−∫Ω(wϵ−1)2dx,t>0. | (3.4) |
We multiply the second equations in system (3.1) by vϵ−v∗, integrate by parts over Ω and use Young's inequality to obtain
∫Ω|∇vϵ|2dx≤γ22α∫Ω(uϵ−1)2dx+β22α∫Ω(wϵ−1)2dx,t>0, | (3.5) |
and
∫Ω(vϵ−v∗)2dx≤2γ2α2∫Ω(uϵ−1)2dx+2β2α2∫Ω(wϵ−1)2dx,t>0. | (3.6) |
Substituting inequality (3.5) into inequality (3.4) to get
ddtE(t)≤−C8(∫Ω(uϵ−1)2dx+∫Ω(wϵ−1)2dx),t>0, |
where C8=min{1−χ2β28α,1−χ2γ28α}>0.
Let f(t):=∫Ω(uϵ−1)2+(wϵ−1)2dx. Then
∫∞1f(t)dt≤E(1)C8<∞,t>1. |
It follows from the uniform estimation (3.2) and the Arzela-Ascoli theorem that f(t) is uniformly continuous in (1,∞). Applying Lemma 3.1, we have
∫Ω(uϵ(⋅,t)−1)2+(wϵ(⋅,t)−1)2dx→0,t→∞. | (3.7) |
Combining inequality (3.6) and the limit (3.7) to obtain
∫Ω(vϵ(⋅,t)−v∗)2dx→0,t→∞. |
Proof of Theorem 1.2 As we all known, each bounded sequence in C2+ν,1+ν2(ˉΩ×[1,∞)) is precompact in C2,1(ˉΩ×[1,∞)). Hence there exists some subsequence {uϵn}∞n=1 satisfying ϵn→0 as n→∞ such that
limn→∞‖uϵn−u∗‖C2,1(ˉΩ×[1,∞))=0. |
Similarly, we can get
limn→∞‖vϵn−v∗‖C2(ˉΩ)=0, |
and
limn→∞‖wϵn−w∗‖C0,1(ˉΩ×[1,∞))=0. |
Combining above limiting relations yields that (u∗,v∗,w∗) satisfies model (1.3). The conclusion (u∗,v∗,w∗)=(u,v,w) is directly attributed to the uniqueness of the classical solution of the model (1.3). Furthermore, according to the conclusion, the strong convergence (3.3) and Diagonal line method, we can deduce
‖u(⋅,t)−1‖L2(Ω)+‖v(⋅,t)−v∗‖L2(Ω)+‖w(⋅,t)−1‖L2(Ω)→0,t→∞. | (3.8) |
By applying Gagliardo-Nirenberg inequality
‖z‖L∞≤C‖z‖2/(n+2)L2(Ω)‖z‖n/(n+2)W1,∞(Ω),z∈W1,∞(Ω), | (3.9) |
comparison principle of ODE and the convergence (3.8), the uniform convergence (1.9) is obtained immediately.
Since limt→∞‖u(⋅,t)−1‖L∞(Ω)=0, so there exists a t1>0 such that
u(x,t)≥12forallx∈Ω,t>t1. | (3.10) |
Using the explicit representation formula of w
w(x,t)=(w0(x)−1)e−δ∫t0F(u)uds+1,x∈Ω,t>0 |
and the inequality (3.10), we have
‖w(⋅,t)−1‖L∞(Ω)≤e−δ6(t−t1),t>t1. | (3.11) |
Multiply the first two equations in model (1.3) by u−1 and v−v∗, respectively, integrate over Ω and apply Cauchy's inequality, Young's inequality and the inequality (3.10), to find
ddt∫Ω(u−1)2dx≤χ232∫Ω|∇v|2dx−∫Ω(u−1)2dx,t>t1. | (3.12) |
∫Ω|∇v|2dx+α2∫Ω(v−v∗)2dx≤β2α∫Ω(w−1)2dx+γ2α∫Ω(u−1)2dx,t>0. | (3.13) |
Combining the estimations (3.11)–(3.13) leads us to the estimate
ddt∫Ω(u−1)2dx≤(χ2γ232α−1)∫Ω(u−1)2dx+χ2β232αe−δ3(t−t1),t>t1. |
Let y(t)=∫Ω(u−1)2dx. Then
y′(t)≤(χ2γ232α−1)y(t)+χ2β232αe−δ3(t−t1),t>t1. |
From comparison principle of ODE, we get
y(t)≤(y(t1)−3χ2β232α(3−δ)−χ2γ2)e−(1−χ2γ232α)(t−t1)+3χ2β232α(3−δ)−χ2γ2e−δ3(t−t1),t>t1. |
This yields
∫Ω(u−1)2dx≤C9e−λ2(t−t1),t>t1, | (3.14) |
where λ2=min{1−χ2γ232α,δ3} and C9=max{|Ω|−3χ2β232α(3−δ)−χ2γ2,3χ2β232α(3−δ)−χ2γ2}.
From the inequalities (3.11), (3.13) and (3.14), we derive
∫Ω(v−β+γα)2dx≤C10e−λ2(t−t1),t>t1, | (3.15) |
where C10=max{2γ2α2C9,2β2α2}. By employing the uniform estimation (1.7), the inequalities (3.9), (3.14) and (3.15), the exponential decay estimation (1.10) can be obtained.
The proof is complete.
In this paper, we mainly study the uniform boundedness of classical solutions and exponential asymptotic stability of the unique positive equilibrium point to the chemotactic cellular model (1.3) for Baló's concentric sclerosis (BCS). For model (1.1), by numerical simulation, Calveza and Khonsarib in [7] shown that demyelination patterns of concentric rings will occur with increasing of chemotactic sensitivity. By the Theorem 1.1 we know that systems (1.1) and (1.2) are {uniformly} bounded and dissipative. By the Theorem 1.2 we also find that the constant equilibrium point of model (1.1) is exponentially asymptotically stable if
˜χ<2ˉw˜β√2Dμ˜α˜ϵˉu, |
and the constant equilibrium point of the model (1.2) is exponentially asymptotically stable if
˜χ<2√2Dμ˜α˜ϵˉumin{1ˉw˜β,1ˉu˜γ}. |
According to a pathological viewpoint of BCS, the above stability results mean that if chemoattractive effect is weak, then the destroyed oligodendrocytes form a homogeneous plaque.
The authors would like to thank the editors and the anonymous referees for their constructive comments. This research was supported by the National Natural Science Foundation of China (Nos. 11761063, 11661051).
We have no conflict of interest in this paper.
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