Mixed-culture electroPioreactor (MEP)

The mixed-culture electroPioreactor is where all of this started: a standard Pioreactor vial with an electrode pair through the cap and a CO₂ line, and none of the sealing that makes the AEP aseptic. It is cheaper and simpler, and it suits the job we first built it for, which is enrichment. Take a spoonful of mud or river water, put it in a chloride-free medium with hydrogen and CO₂ as the only food, and see what grows. Whatever keeps growing through a few rounds of that, and stops when the electrolysis stops, is very likely a hydrogen-oxidising bacterium, and it might be one nobody has grown before.

The MEP was the only electroPioreactor until the AEP came along. We picked it up again in 2026 as the low-cost version, and built two (MEP 0.3, known to us as ed04 and ed05) for the School of Biological Sciences at the University of Edinburgh, for its 2026 MSc industrial projects.

MEP on GitHub

Pinch sliders: a few pence of PETG that closes a silicone line without a valve. Photo: AMYBO, CC BY-SA 4.0.

Documents

Written for the Edinburgh students, so they assume you have never seen a Pioreactor before:

• Assembly
• Calibration
• Operation: 14 ml of medium in a 20 ml vial, and what to do each day
• Pre-transport check: what we test before a unit leaves the workshop
• Card bundle for installing the plugin from the SD card

A guide with a video and drawings for each step will follow the AEP guide at docs.electropioreactor.org, sharing the steps the two variants have in common.

History

The 2024 electroPioreactor v0.02, the design the MEP grew from, is written up in our earlier notes with a full bill of materials, along with the 2024 electroPioreactor page and the first experiments. The home enrichment protocol is written for an AEP 0.1, but the stages are the same on an MEP.