Insights into Ecological Features of Microbial Dark Matter Within the Symbiotic Community During <i>Alexandrium pacificum</i> Bloom: Co-Occurrence Interactions and Assembly Processes

The symbiotic microbiome constitutes a consortium that has been persistently domesticated by a specific algal species, fostering a close and enduring association with the host. The majority of microbial taxa remain uncharacterized. These unknown microbes, often referred to as “microbial dark matter (MDM)”, have important ecological contributions. Given the challenges in discerning symbiotic microbes in natural environments, herein, ecological characteristics of MDM and known taxa within symbiotic communities were investigated in a simulated bloom process using <i>Alexandrium pacificum</i> without antibiotic treatment. Specifically, increased diversification was observed in MDM along the bloom process. Higher trophic interaction and less vulnerability of the molecular network were found in MDM taxa. The “bridge” role of MDM species was better than that of known taxa, as shown by higher betweenness centralization. Deterministic processes dominated in MDM taxa, which promote phylogenic diversity of such groups to some extent. The findings highlight that MDM taxa play an important role in sustaining community stability and functioning. This study broadens our understanding of the ecological contribution of MDM under disturbances from dinoflagellate blooms, providing essential theoretical insights and empirical data to inform the management of coastal toxic blooms.