The Holobiont Bioreactor: Microbiome Dynamics, Biosafety Paradoxes, and Ecological Engineering in Black Soldier Fly Bioconversion

The black soldier fly (Hermetia illucens L.) has emerged as a cornerstone technology for the circular economy, valorising organic waste into high-value insect protein and biofertiliser. The efficiency, safety, and adaptability of this bioconversion process are fundamentally dictated by the complex interplay between the insect host and its gut microbiome, which together function as an integrated ‘holobiont bioreactor’. This reactor is characterised by partially vertically transmitted core taxa and a dynamic, horizontally acquired microbial community. This review critically synthesises high-impact original research identified through a systematic literature search published predominantly between 2021 and 2026, incorporating foundational landmark studies, to provide a mechanistic overview of this holobiont’s function. We first dissect the architecture of the BSFL microbiome, examining the ‘nature vs. nurture’ debate of egg-associated vertical inheritance versus horizontal acquisition and highlighting the critical, yet often overlooked, role of the fungal mycobiome. We then explore the larval gut as a spatio-temporally compartmentalised bioreactor, where extreme physicochemical gradients and host–microbe metabolic crosstalk drive nutrient extraction and immunity. A central focus is placed on the ‘Bioremediation vs. Biosafety Paradox’, wherein the system’s capacity to degrade chemical xenobiotics and sanitise pathogens is contrasted with its propensity to amplify antimicrobial resistance genes (ARGs) and bioaccumulate contaminants like heavy metals and microplastics under specific substrate and exposure conditions. Finally, we evaluate the frontier of microbiome engineering from the deployment of native probiotics and synthetic microbial consortia (SynComs) to enhance performance to the use of prebiotics for mitigating ammonia emissions. This synthesis reveals that while BSFL technology holds immense promise, its industrial maturation is constrained by significant methodological limitations in the current literature. We conclude by advocating for a field-wide shift towards integrated multi-omics and viability-based analytical frameworks to ensure the development of safe, efficient, and regulatory-compliant bioconversion systems.