# Label-free metabolic imaging and energy costs in Chlamydomonas

## TL;DR

A new label-free imaging method reveals metabolic activity in algae cells and shows higher energy costs in a mutant strain.

## Contribution

A novel optical tomography method is introduced for measuring metabolic signals without labels in live cells.

## Key findings

- The dynamic signal in Chlamydomonas mutants was 5 to 10 times higher than in wild-type strains under dark conditions.
- The metabolic signal correlates with ATPeq consumption linked to starch overproduction in the ΔrbcL mutant.
- The method is simple to implement and could be useful for studying phytoplankton or virus-infected cells.

## Abstract

We developed a label-free optical microscopy method to study movements of different frequencies and amplitudes within a cell. We use optical transmission tomography (OTT) that operates in transmission, and we record the changes of signal values of all the pixels of movies taken for a few seconds (dynamic signal). This signal is a metabolic signal in algae as it decreased in the presence of photosystem II inhibitors or when samples were illuminated at wavelengths where the photoreceptors are poorly operative. We used as model organism Chlamydomonas for which mutants are available. We used a mutant deleted of the chloroplastic gene encoding the large subunit of the Rubisco, ΔrbcL. This mutant is unable to fix atmospheric CO2 and is devoid of pyrenoid. We compared the dynamic signal between wild-type strain and ΔrbcL mutant of Chlamydomonas grown in dark condition and found it to be 5 to 10 times higher. This mutant overproduced starch, and we tempted to associate the metabolic signal to the cost in ATPeq consumption for building starch. The method is easy to implement and could be very valuable for studies of phytoplankton in situ or virus-infected cells.

This graphical abstract illustrates the Gouy phase shift showing destructive and constructive interferences. With thisphase shift we generate two types of tomographic images: the static image informs for intracellular structures and thedynamic image provides information on the metabolic activity of the cell (all images are the bacteria Escherichia coli)

This graphical abstract illustrates the Gouy phase shift showing destructive and constructive interferences. With thisphase shift we generate two types of tomographic images: the static image informs for intracellular structures and thedynamic image provides information on the metabolic activity of the cell (all images are the bacteria Escherichia coli)

The online version contains supplementary material available at 10.1140/epje/s10189-025-00499-y.