• Spontaneous aggregation of GO and Shewanella cells into condensed entities occurs within 36 hours, influencing the bioreduction process.
• A positive linear correlation exists among aggregation potential, bacterial reduction ability, and the reduction degree (ID/IG) of graphene oxide.
• The study provides mechanistic insights into the rate-limiting contact step in microbial reduction of GO, advancing green nanomaterial synthesis.
• Findings support the use of Shewanella for environmentally friendly production of reduced graphene oxide with competitive properties.