How diversity is structured has been a central goal of microbial ecology. In freshwater ecosystems, selection has been found to be the main driver shaping bacterial communities. However, its relative importance compared with other processes (dispersal, drift, diversification) may depend on spatial heterogeneity and the dispersal rates within a metacommunity. Still, a decrease in the role of selection is expected with increasing dispersal homogenization. Here, we investigate the main ecological processes modulating bacterial assembly in contrasting scenarios of environmental heterogeneity. We carried out a spatiotemporal survey in the floodplain system of the Paraná River. The bacterioplankton metacommunity was studied using both statistical inferences based on phylogenetic and taxa turnover as well as co-occurrence networks. We found that selection was the main process determining community assembly even at both extremes of environmental heterogeneity and homogeneity, challenging the general view that the strength of selection is weakened due to dispersal homogenization. The ecological processes acting on the community also determined the connectedness of bacterial networks associations. Heterogeneous selection promoted more interconnected networks increasing β-diversity. Finally, spatiotemporal heterogeneity was an important factor determining the number and identity of the most highly connected taxa in the system. Integrating all these empirical evidences, we propose a new conceptual model that elucidates how the environmental heterogeneity determines the action of the ecological processes shaping the bacterial metacommunity.

如何构建多样性一直是微生物生态学的中心目标。在淡水生态系统中，选择是形成细菌群落的主要驱动力。然而，与其他过程（扩散、漂移、多样化）相比，其相对重要性可能取决于空间异质性和元群落内的扩散率。尽管如此，随着分散同质化的增加，选择的作用预计会减弱。在这里，我们研究了在环境异质性对比情况下调节细菌组装的主要生态过程。我们对巴拉那河漫滩系统进行了时空调查。使用基于系统发育和分类群更替以及共现网络的统计推断来研究浮游细菌元群落。我们发现，即使在环境异质性和同质性的两个极端下，选择也是决定群落组装的主要过程，这挑战了选择强度因分散同质化而减弱的普遍观点。作用于群落的生态过程也决定了细菌网络关联的连通性。异质选择促进了更加互联的网络，增加了β多样性。最后，时空异质性是决定系统中关联度最高的类群的数量和身份的重要因素。整合所有这些经验证据，我们提出了一个新的概念模型，阐明环境异质性如何决定塑造细菌元群落的生态过程的作用。