ELVIS: Multi-Modal Microscopy for Autonomous Life Detection
Microscopy remains one of the most direct tools for detecting microbial life, capable of revealing morphology, motility, and fluorescence signatures that serve as strong biosignatures. Recent interest in ocean worlds such as Europa and Enceladus, and subglacial water on Mars, has intensified the need for instruments that can operate autonomously under extreme constraints. Estimates for potential microbial densities on Europa suggest as few as 10² cells per milliliter, making detection a formidable challenge. The Extant Life Volumetric Imaging System (ELVIS) was developed to meet these demands, combining high-resolution imaging with high-throughput sample analysis in a compact, autonomous package.
ELVIS integrates digital holographic microscopy (DHM) with a fluorescence light field microscopy system known as the Volume Fluorescence Imager (VFI). DHM, an interferometric technique, captures volumetric information in a single shot by recording the complex electric field at the image plane and numerically propagating it. This method achieves a volumetric throughput two to three orders of magnitude greater than conventional microscopy, compressing 3D data into a 2D hologram. For a 40 µl sample volume—necessary to detect 100 cells/ml—DHM requires only 200 images compared to 80,000 for a conventional system, reducing data storage from hundreds of gigabytes to just 8 GB.
While DHM provides label-free imaging based on absorption and refractive index contrast, it lacks biochemical specificity. VFI addresses this by capturing volumetric fluorescence data using a microlens array to encode both position and direction of light rays, enabling post-capture refocusing. This increases depth of field by up to two orders of magnitude, though at some cost to spatial resolution. By sharing optics and triggers, DHM and VFI record synchronized datasets from the same field of view, with spectral separation eliminating cross-talk. The shared volume of view measures 350 μm × 480 μm × 150 μm, with the depth limited by VFI’s reconstruction capability.
Supporting these imaging modalities is an automated Sample Processing Unit (SPU) and Sample Aliquot Chamber (SAC). The SAC’s dual-channel design allows simultaneous sample and reference beam paths through optical-quality glass. The SPU delivers samples, performs rinsing, and applies up to three fluorescent dyes without manual intervention. Fluid handling is achieved through coordinated operation of fixed-volume pumps and latching solenoid valves, avoiding mechanical damage to cells by preventing sample flow through pump chambers.
Autonomous operation is managed by DHMx acquisition software, built on Allied Vision’s Vimba SDK. DHMx controls exposure, gain, and frame rate, and includes a real-time DHM reconstruction module. Hardware triggering via an Arduino microcontroller synchronizes both cameras to within 0.2 ms, negligible at the system’s 15 fps capture rate. The software supports remote operation over network links, a critical feature for space missions.
Field testing at the Kerckhoff Marine Laboratory in Newport Beach, California, used ocean and tidal pool samples of varying biomass. The workflow involved cleaning the SAC, loading reference fluid, imaging unstained samples, applying dyes such as Acridine Orange and FM1-43, and reimaging. DHM detected both prokaryotic and eukaryotic motility, while VFI initially captured only larger eukaryotic cells. Autofluorescence from native pigments like chlorophyll was observed prior to staining.
Early tests revealed limitations in detecting small bacterial cells with VFI due to high background fluorescence, partly from ABS material in objective mounts. Replacing mounts with anodized aluminum and optimizing dye concentrations improved signal-to-noise ratios, enabling detection of 1 µm bacteria such as Serratia marcescens stained with 50 nM Syto-9. Multi-wavelength DHM now captures absorption differences at 405, 520, and 635 nm in a single shot, while multi-color VFI distinguishes membrane dyes from native pigments in algae samples.
The ELVIS prototype demonstrates a robust, multi-modal approach to life detection in liquid environments. By combining DHM’s high-resolution volumetric imaging with VFI’s biochemical specificity, and integrating autonomous sample handling, the system addresses the stringent mass, power, and data constraints of planetary missions. Its adaptability makes it equally valuable for terrestrial microbiology, offering engineers and researchers a powerful tool for surveying microbial life in challenging environments.
