Abstract
Detection of extant microbial life on Earth and elsewhere in the Solar System requires the ability to identify and enumerate micrometer-scale, essentially featureless cells. On Earth, bacteria are usually enumerated by culture plating or epifluorescence microscopy. Culture plates require long incubation times and can only count culturable strains, and epifluorescence microscopy requires extensive staining and concentration of the sample and instrumentation that is not readily miniaturized for space. Digital holographic microscopy (DHM) represents an alternative technique with no moving parts and higher throughput than traditional microscopy, making it potentially useful in space for detection of extant microorganisms provided that sufficient numbers of cells can be collected. Because sample collection is expected to be the limiting factor for space missions, especially to outer planets, it is important to quantify the limits of detection of any proposed technique for extant life detection. Here we use both laboratory and field samples to measure the limits of detection of an off-axis digital holographic microscope (DHM). A statistical model is used to estimate any instrument's probability of detection at various bacterial concentrations based on the optical performance characteristics of the instrument, as well as estimate the confidence interval of detection. This statistical model agrees well with the limit of detection of 103 cells/mL that was found experimentally with laboratory samples. In environmental samples, active cells were immediately evident at concentrations of 104 cells/mL. Published estimates of cell densities for Enceladus plumes yield up to 104 cells/mL, which are well within the off-axis DHM's limits of detection to confidence intervals greater than or equal to 95%, assuming sufficient sample volumes can be collected. The quantitative phase imaging provided by DHM allowed minerals to be distinguished from cells. Off-axis DHM's ability for rapid low-level bacterial detection and counting shows its viability as a technique for detection of extant microbial life provided that the cells can be captured intact and delivered to the sample chamber in a sufficient volume of liquid for imaging. Key Words: In situ life detection—Extant microorganisms—Holographic microscopy—Ocean Worlds—Enceladus—Imaging. Astrobiology 17, 913–925.
Highlights
The icy moons of Jupiter and Saturn may be the most likely places in the Solar System for extraterrestrial life, and NASA has as an explicit goal the search for signs of life on Europa (Prieto-Ballesteros et al, 2011; Gleeson et al, 2012), Enceladus, and Titan (Carr et al, 2013; McKay et al, 2014; Konstantinidis et al, 2015; NASA, 2016)
Digital holographic microscopy (DHM) represents an alternative technique with no moving parts and higher throughput than traditional microscopy, making it potentially useful in space for detection of extant microorganisms provided that sufficient numbers of cells can be collected
We demonstrate a limit of detection of 103 cells/mL using a custom off-axis digital holographic microscope (DHM) designed for field and astrobiology applications (Wallace et al, 2015a)
Summary
The icy moons of Jupiter and Saturn may be the most likely places in the Solar System for extraterrestrial life, and NASA has as an explicit goal the search for signs of life on Europa (Prieto-Ballesteros et al, 2011; Gleeson et al, 2012), Enceladus, and Titan (Carr et al, 2013; McKay et al, 2014; Konstantinidis et al, 2015; NASA, 2016). Such life is expected to be prokaryotic, anaerobic, and present at potentially very low densities. Chemical means of detection may have difficulty distinguishing extant life from
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