Microbial Life Detection

The search for life beyond Earth relies on biosignatures: measurable features that can provide evidence for past or present life. One promising class of biosignatures is biological pigments. These molecules give organisms their characteristic colours by selectively absorbing certain wavelengths of light. Pigments are responsible for the green colour of plants, but they are also found throughout the microbial world, including in algae, bacteria, and archaea. In microorganisms, pigments can serve several important functions, such as capturing light for photosynthesis, protecting cells from harmful radiation, or helping organisms adapt to their environment.

In SenseLife, we are particularly interested in photosynthetic microorganisms and their pigments. Photosynthetic microorganisms use light as an energy source and are among the oldest and most widespread forms of life on Earth. Their ability to harvest energy from starlight makes them particularly interesting when considering how life might survive on the surface of other worlds. Our work focuses mainly on two groups with very different photosynthetic strategies: cyanobacteria, which perform oxygen-producing photosynthesis similar to plants, and green sulfur bacteria, which use a different form of photosynthesis and do not produce oxygen. Their pigments absorb light at different wavelengths and are organized within the cells in highly structured ways. This makes them useful organisms for investigating whether we can recognize microbial life from a distance.

Our key question is whether we can use the optical properties of biological pigments to distinguish them from non-biological materials. This is where spectropolarimetry becomes important. When light interacts with biological molecules and their organized structures, it can acquire a small amount of circular polarization. This signal is related to the molecular asymmetry that is characteristic of biological systems and can therefore provide information that standard measurements of colour or light absorption cannot.

Using FlyPol (our spectroplarimeter), we measure both the spectrum of light and its circular polarization after it interacts with microbial samples. We are systematically studying different microorganisms and their pigments to determine which species produce detectable signals, which pigments are responsible, and how environmental conditions and cellular organization influence the strength and shape of the signal. By building a database of spectropolarimetric signatures from a diverse range of microorganisms, we aim to establish how reliably this technique can be used to detect microbial life – not only on Earth, but potentially on the surfaces of other worlds.