代谢互养关系在维持微生物物种多样性中的作用

互养关系（cross-feeding）是微生物物种之间普遍存在的一种相互关系,其中物种利用环境中其他成员的代谢产物以促进自身生长的情形称为代谢互养关系,这种关系对物种间的竞争结果往往有很大影响,甚至会改变种群结构。为了研究代谢互养关系在维持微生物物种多样性中的作用,构建包含不同代谢互养关系的资源竞争模型,这些模型既体现了微生物物种竞争资源时种群密度及资源量的动态,也展示了物种利用其他竞争者的代谢资源对自身生存状况的影响。数值模拟结果显示：（1）考虑微生物中不同的代谢互养关系结构：两物种间单向互养、双向互养以及多物种间的互养,不同的互养关系都可以促进竞争物种稳定共存,竞争中处于劣势的物种通过利用其他竞争成员的代谢产物,打破外界资源量对其生长的限制,改变原本消亡的命运;而处于优势的物种则通过利用其他竞争成员的代谢产物,增大种群密度。（2）多物种竞争同一种有限资源时,不是所有物种都能共存,在四物种模拟中,原本处于最劣势的物种灭绝,其余三者共存。物种产生代谢资源对其本身是"不利"的,如果在模拟中物种利用代谢资源的能力相同,那么物种竞争外界资源的劣势就很可能无法被抵消。通过改变资源利用率发现只有互养关系中代谢资源的利用可以弥补劣势种在竞争外界资源时的不足,多物种才可以全部共存。（3）验证数值模拟结果的普遍性,分析参数变化对共存的影响,结果表明代谢互养关系促进的共存对代谢资源相关参数不敏感,参数的改变只影响平衡态时物种的种群密度。所以,代谢互养关系可以促进相互竞争的微生物物种共存,即微生物之间的互养关系很可能是维持物种多样性的一种机制。

The role of metabolic cross-feeding in maintaining microbial species diversity

Cross-feeding is a common relationship among microbial species, in which species use metabolites of other members in the environment to promote their own growth is called metabolic cross-feeding relationship. Metabolic cross-feeding relationship often has a great influence on the competition between species and even changes the population structure. In order to study the role of metabolic cross-feeding relationships in maintaining microbial species diversity, we constructed the resource competition models with different cross-feeding relationships. These models not only reflect the dynamics of population size and resource quantity when microbial species compete for resources, but also show the influence of species'' utilization of metabolic resources from other competitors on their own living status. Then we carried out numerical simulation on the models, and the results show that:(1) we consider different cross-feeding structures in microorganisms:unidirectional cross-feeding and bidirectional cross-feeding between two species and multidirectional cross-feeding between multiple species. The simulation results indicate that different cross-feeding relationships can promote the stable coexistence of competing species. On the one hand, by using the metabolites of other competitive members, the disadvantaged species can break the restriction of external resources on their growth and change the fate of their original extinction. The dominant species, on the other hand, increases population size by utilizing metabolites from competing members. (2) When multiple species compete for the same limited resource, not all species can coexist. In a four-species simulation, the least advantaged species dies out and the remaining three coexist. The generation of metabolic resources by species is disadvantageous to the species itself. If the ability of species to utilize metabolic resources is the same in the simulation, the disadvantage of species in competing for external resources may not be offset. We found that when the utilization of metabolic resources in the cross-feeding relationship can make up for the deficiency of the inferior species in the competition for external resources, multiple species can all coexist. (3) We verified the universality of numerical simulation results by analyzing the influence of changes in metabolic resource-related parameters on coexistence. The results showed that the co-existence promoted by the metabolic cross-feeding relationship is insensitive to the metabolic resource related parameters, and the change of parameters only affected the population density of the species at equilibrium state. It has been found that metabolic cross-feeding relationships can promote the coexistence of competing microbial species, that is, the cross-feeding relationships among microorganisms may be a mechanism to maintain species diversity.