<p>Microbe-driven arsenite oxidation is crucial for arsenic (As) biogeochemical cycling and bioremediation. Understanding the mechanisms regulating this process is a research priority. Although As stress levels and exogenous carbon input are recognized as potential determinants, how microbial community responds to the changes in these environmental conditions can influence the arsenite oxidation remains elusive. Here, we established a cross-gradient experimental system to investigate the responses of overall community, key taxa, and microbial networks to the synergistic effect of arsenite concentration and carbon source diversity. We found that microbial network complexity, rather than overall diversity, community structure, or key species abundance, was the primary factor influencing arsenite oxidation efficiency. Severer As stress simplified network topology, while diversifying carbon sources alleviated this adverse effect by increasing network complexity and positive correlations. More complex and cooperative network associations promoted more efficient arsenite oxidation, which was attributed to the input of more diverse carbon sources. Our results highlight that microbial network topology is a key indicator and more responsive to As-biotransformation than simple diversity metrics. Diversifying carbon sources optimizes microbial networks by enhancing complexity and cooperative associations. This study provides a safe and sustainable strategy for As bioremediation, particularly in highly contaminated sites.</p>