What HEPES reveals about cultivating with light

A cyanobacterial growth recipe looks modular. BG-11 supplies nutrients, HEPES holds the pH steady, light drives photosynthesis, the cyanobacterium grows. Each ingredient has one job.

The organism never encounters the word buffer. It encounters a molecule in a chemical environment, and when the lights come on that environment changes.

Illuminated medium containing HEPES generates hydrogen peroxide. That was reported in 1985, in a paper asking why cells kept dying in light-exposed medium (Zigler et al., 1985). How much depends on the buffer’s amine. Tertiary amines produce the most and primary amines almost none, which is why Tris sits on the no-buffer baseline and HEPES sits far above it (Liu et al., 2023).

Whether that matters is a biological question, not a chemical one.

Prochlorococcus is the clearest case. It lacks catalase, and neighbouring heterotrophs keep it alive by pulling peroxide out of the shared water (Morris et al., 2011). In illuminated seawater buffered with HEPES, axenic cultures died. They lived if a helper bacterium was present, or if the buffer was swapped. Cutting the HEPES until it made 75% less peroxide did not save them.

Losing your own defences only counts as a deficiency if you are alone.

Cell density settles the same question in the lab. Li and colleagues found dense cyanobacterial cultures untroubled by the peroxide in their medium while dilute ones died within two days (Li et al., 2017). Their fix was another organism. In their hands, and in Morris’s, a living partner outperformed purified catalase.

Sometimes the way to hold a system steady is to put something else living into it, rather than take a variable out.

This is where it reaches design. Designers change cell loading, thickness, porosity, geometry and illumination. Those read as fabrication variables. For a living system they are environmental ones. Printed microalgal hydrogels are described as habitats that shape light and mass transport (Dawiec-Liśniewska et al., 2026). Alginate scaffolds swap ions with the medium and loosen, which changes gas exchange and then carbon fixation (Levä et al., 2023). Photosynthetic beads lose pigment where light cannot reach the cells (Armaly et al., 2026).

Change the geometry and you have changed what the buffer does.

Which is why “HEPES-buffered” does not specify an experiment. Published methods give concentration but not spectrum, or density but not whether peroxide was measured (Tóth et al., 2022; Wang et al., 2026; Morris & Zinser, 2013). Nothing in them is false. The record is not sufficient to tell whether this chemistry was running.

Biodesign teaching is already close to the right posture. Living Pigments asks students to treat organisms as collaborators and to respond to biological variability rather than suppress it (Nerlich et al., 2026). The HEPES case adds one step. Ask how the environment changes the organism, then ask how it changes the things you introduced to control the organism.

The first check is cheap. Put your medium under your own light, with no cells in it, and measure peroxide over your own timescale. One assay, no biology, and it tells you whether there is anything worth chasing.

Reading 05, paper by paper

This is the short version. The full Reading works through the evidence paper by paper, with page-level sourcing, six plates, and the cases where the effect does not appear.

Members also get the Teaching Notes: four questions for auditing any control in a protocol, a printable audit card, candidate second pathways in fabricated living systems, and the reagent record a machine would need before it could reason about a protocol.