The Importance of Microcoleus vaginatus in Shaping Bacterial Communities Essential for the Development of Cyanobacterial Biological Soil Crusts

Biological soil crusts (BSCs) are critical ecological components in arid lands. Their formation and stability hinge on the assembly and interactive networks of cyanobacteria-led bacterial communities. Yet, how different functional cyanobacteria shape the underlying microbial structure and assembly rules is poorly understood. Here, we cultivated artificial algal crusts using two representative cyanobacteria: the nitrogen-fixing <i>Leptolyngbya</i> sp. and the non-nitrogen-fixing <i>Microcoleus vaginatus</i> (<i>M. vaginatus</i> CM01). A total of six treatments were established based on the presence or absence of spraying with in situ BSCs leachate: a control group without inoculation of algae or bacteria (soil, S); a treatment group sprayed only with bacterial suspension (soil + bacteria, SB); a treatment group sprayed only with <i>M. vaginatus</i> CM01 (soil + <i>M. vaginatus</i> CM01, SM); a treatment group co-inoculated with both BSCs leachate and <i>M. vaginatus</i> CM01 (soil + <i>M. vaginatus</i> CM01 + bacteria, SMB); a treatment group inoculated only with <i>Leptolyngbya</i> sp. CT01 (soil + <i>Leptolyngbya</i> sp. CT01, SL); and a treatment group co-inoculated with <i>Leptolyngbya</i> sp. CT01 and biocrust leachate (soil + <i>Leptolyngbya</i> sp. CT01 + bacteria, SLB). By integrating 16S rRNA gene sequencing, neutral community modeling (NCM), and structural equation modeling (SEM), we dissected differences in Cyano-BSCs development, bacterial community composition, co-occurrence networks, and assembly mechanisms. Inoculation with <i>M. vaginatus</i> CM01 (SM, SMB) superiorly promoted Cyano-BSCs development: the SM group achieved the highest coverage (23.33%), while the SMB group showed marked increases in organic matter (OM, 4.10 g·kg^-1) and chlorophyll <i>a</i> (Chl<i>a</i>, 13.40 μg·g^-1), alongside a >5-fold rise in bacterial, cyanobacterial, and nitrogen-fixation gene abundances versus controls. The mechanism centers on extracellular polymeric substances (EPS) secreted by <i>M. vaginatus</i>, which homogenized the microenvironment, suppressed stochastic bacterial dispersal (NCM, SM: R² = 0.698), and enhanced deterministic selection. This process forged a highly cooperative network (89.74% positive links, average degree 34.71) that directionally enriched Cyanobacteria (relative abundance 40.40%). The Shannon index of Cyano-BSCs from the group (SMB) reached 7.72 ± 0.09, reflecting high microbial community diversity. SEM confirmed <i>M. vaginatus</i> directly regulated bacterial assembly (path coefficient = 0.59, <i>p</i> < 0.05) and indirectly improved the soil environment (path coefficient = 0.64, <i>p</i> < 0.05), establishing a "cyanobacteria-community-environment" feedback loop. Conversely, the <i>Leptolyngbya</i> sp. groups (SL, SLB), despite enriching nitrogen-fixing bacteria and fungi, exhibited low carbon fixation efficiency (notably 1.26 g·kg^-1 OM in SL) and lack of EPS; communities remained stochastic (NCM, SL: R² = 0.751) with no effective regulatory pathway-a pattern mirrored in S and SB groups. Our findings demonstrate that <i>M. vaginatus</i> acts as a core engineer of biological soil Cyano-BSCs formation via an "EPS-mediated habitat filtering-functional group enrichment-cooperative network assembly" cascade, enforcing deterministic community construction. <i>Leptolyngbya</i> sp., with limited niche-constructing ability, fails to exert comparable control. This work provides a targeted framework for the artificial restoration of Cyano-BSCs in arid zones.