Detection Of Extraterrestrial Microbial Metabolic Activity

The detection and stydy of possible life forms beyond Earth must be based on absolutely minimal assumptions about the nature of the organism. For example, the organic chemistry, internal structure, and internal components of microbial life might be significantly different from terrestrial microorganisms and cannot be accurately predicted. However two properties of microbial life that are likely to be universal and require no prior assumptions of specific properties are (1) an ability to reproduce itself in a self-regulated form, and (2) the ability to maintain some degree of isolation of its internal compartment from the surrounding environment. In addition, we must assume that any microbial life will require water and carbon.

During the process of reproduction, the organism's metabolism, mediated by its membrane processes, will, by necessity change the surrounding chemical and redox environment. Thus, to study such a process we must be capable of detecting such changes, rapidly and free of extraneous or non-biogenic interferences. To help in this problem, with support from NASA's Astrobiology Science & Technology program, we are developing a new chemically based detection technique, dubbed the Microbial Detection Array (MDA). A substantial portion of the MDA, the sensor array, is heritage-based and already space qualified.

The MDA is designed to provide a response to minute chemical and physical changes occurring in one of two identical chambers via differentially monitored electrochemical sensor arrays. Minimal metabolism will alter the physico-chemical steady state in one chamber such that a difference between the sensor ar-rays will result in a signal. This detection system makes minimal assumptions about the nature of the microorganism, assuming only that after addition of water the it will reproduce and in the process cause changes in its immediate surroundings by consuming or generating, metabolizing, and transporting in both directions, a number of molecules and ionic species.

The experiment begins by placing a homogenized split-sample of soil into each chamber, adding pure water, sterilizing at a high temperature incompatible with any form of terrestrial life, and zeroing. In the absence of any metabolically active organism in either chamber, no signal will be detected. The "inocula-tion" of one chamber with a minimal of viable microorganisms, which proceed to multiply, will produce a significant metabolically generated disequilibrium in the system (compared to the control) to provide a detectable signal. Replication of the experiment and positive results would lead to the conclusion of bio-logically induced changes. Changes resulting from non-biological chemical reactions of whatever type are canceled out by the control. The replication of the procedure, split sample, and minimal inoculation protocol, eliminate non-biogenic causation. In addition to detecting microbes, the sensor array will also characterize the chemical composition and electrochemistry of the sample DA is a differential electrochemical sensor array that will provide the ability to simultaneously monitor a specifically chosen set of chemical and physical parameters in two identical “growth” chambers. The sensitive of the system provided by the two differentially monitored sensor arrays (e-tongues, Figure 1) is such, that minimal growth in one of the chambers will alter the chemistry and ionic properties sufficiently to produce a difference between the two sensor arrays and result in a measurable signal. This life detection system makes minimal assumptions about the nature of any life on Mars. It assumes only that, after addition of water, the microorganism replicates and that in the process will produce small changes in its immediate surroundings by consuming, metabolizing, and excreting a number of molecules and/or ionic species.