The abundance of certain microscopic particulates (for example, pollen or diatoms) in natural climate archives, such as sediment cores, can provide information about past environmental conditions. Traditionally, analysts estimate these abundances by examining the sample under a microscope. This process is often time-consuming, taking a trained analyst 2–10 hours to count 150–500 pollen grains; therefore, a small portion of the total sample material is examined and population estimates are obtained via scaling. Although devices such as slide scanners help to automate this process by digitizing slides, taking 1–3 hours per slide, a fast method to count the particles in an entire sample remains elusive.

Imaging flow cytometry (IFC) can precisely and rapidly quantify particulates (~1–100 μm diameter) in almost an entire sample. Samples are sieved to remove particles that are over 100 μm in diameter and then suspended in ≤200 μl of deionized water instead of being mounted on a slide. The prepared sample is loaded into the instrument, which uses a sheath fluid to hydrodynamically align the particles into a single-file stream. This stream is directed past a camera, which uses targeted ultraviolet and visible light excitation lasers to achieve high-throughput, multispectral imaging. This approach can analyse an entire 0.5-cm3 fossil pollen sample suspended in 100 μl of water in ~1.5 hours, returning images of tens of thousands pollen grains. Post-acquisition, image-analysis software is used to gate subpopulations using features such as size, location, shape, texture, and signal strength across all targeted wavelengths, improving the ratio of particles of interest to background, and enabling fast quantification.

Credit: Nisha Lamichhane

In palaeoclimatology, IFC can accelerate the generation of high-resolution records and increase statistical robustness by enabling the analysis of most of the total sample material. Additionally, the method can quantify rare particle types that constitute <1% of the assemblage and reveal subtle changes that would be prohibitively time-consuming to detect manually, expanding the range of detectable climate indicators. For example, IFC could be used to track changes in the abundance of rare, non-native wind-blown pollen species in peat, to reconstruct atmospheric dynamics over the last 10,000 years.