Extant life detection using label-free video microscopy in analog aquatic environments

The ability of microbial morphology, active motion, and refractive index to serve as biosignatures was investigated by in situ video microscopy in a wide range of extreme field sites where such imaging had not previously been performed. These sites allowed for sampling seawater, sea ice brines, cryopeg brines, hypersaline pools and seeps, hyperalkaline springs, and glaciovolcanic cave ice. In all samples, except the cryopeg brine, active motion was observed without any sample treatment. Active motion was observed in the cryopeg brines when samples were subjected to temperature gradient above in situ. Levels of prokaryotic motility were, in general, low in the field samples collected at temperatures < 4ºC. Non-motile cells could be distinguished from microminerals by differences in passive motion (e.g., density measured by sinking/floating), refractive index and/or absorbance, or morphology in the case of larger eukaryotes. Dramatic increases in the fraction of motile cells were seen ..., Field Sites and Samples
Sites analyzed ranged across western North America and also included West Greenland (Fig 1). Field sites were visited that exhibited one or more environmental parameters that could be considered extreme: temperature, salinity, pH, oxygen concentration, or light availability. Several of them are known to be useful analogs for potential near-term lander missions, and others are emerging sites of interest. A summary of the sites is given in Table 1, and full descriptions with context images and references to geochemical analyses where available are provided in Supplementary Information text and Fig S1-S5.
Materials and Methods
Digital holographic microscope (DHM). The off-axis DHM used throughout this work has been described previously. Its design has been optimized for easy adaptability to spaceflight (e.g., no compound objective lenses) and for balance between resolution and depth of field in order to obtain a limit of detection of ~103 bacteria per millilite..., , # Data from: Extant life detection using label-free video microscopy in analog aquatic environments

https://doi.org/10.5061/dryad.70rxwdc41

## Overview

This data package contains raw digital holographic microscopy (DHM) video recordings collected from a range of extreme aquatic field environments across western North America. These recordings were used to investigate whether active microbial motion, cell morphology, and optical properties can serve as biosignatures of extant life detectable by label-free video microscopy. Data are provided in support of the associated publication in PLoS ONE (DOI: 10.1371/journal.pone.0318239).

All videos are raw hologram recordings acquired by a custom off-axis DHM instrument (SHAMU or Son of SHAMU) operating at a 405 nm laser wavelength, with a maximum acquisition rate of 15 frames per second. Each hologram video can be reconstructed into amplitude and phase image stacks using the angular spectrum method via the associated Fiji plug-ins described ..., ,