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Life detection on Europa from a lander: metabolic signatures
Daniel Prieur UniversitИ de Bretagne Occidentale Brest, France


Starting Hypothesis
· If Life exists on Europa
­ Living Entities are small-sized and not visible from an Orbiter ­ Living Entities are organized like Cells and have properties similar to living Cells on Earth ­ These Cell-like Entities carry out Carbon Chemistry in liquid Water


How to detect microbial Cells from an Orbiter
· We cannot look at them · We cannot measure their metabolic activity · We can search for Life Macromolecules, but there is a serious detection limit problem: a single cell dry weight is about 10-12 g. · But we can search for metabolic signatures


Cell Life on Earth


The best example
· To fix carbon dioxide via the Calvin cycle, oxygenic photosynthetic organisms need to reduce it into carbohydrates · The reducing power (electrons from Hydrogen) comes from photolysis of water · This photolysis also produces O2, which accumulated in Earth 's atmosphere


Diversity of metabolisms involved in energy production
· Phototrophy · Chemotrophy
­ Aerobic respiration
· Organic or inorganic electron donnors

­ Anaerobic respiration
· Organic or inorganic electron donnors · Organic or inorganic electron acceptors

­ Fermentation of organic compounds


Phototrophy
· Oxygenic photosynthesis
­ O2 detection

· Anoxygenic photosynthesis · Bacteriorhodopsin-like processes · In all cases: pigments


Respirations


Hydrogen oxidation

H2 + 1/2 O2 => H2O G0 '= -237 kJ


Fe
2Fe
++

++

oxidation

+ 1/2O2 + 2H+ => 2Fe+++ + H2O

Non soluble iron hydroxide

FeCO3 + 10 H2O => 4 Fe(OH)3 + 3 HCO3 + 3H+


Oxidation of reduced sulphur compounds

H2S + 2O2 => SO4-- + 2 H+ HS- + 1/2O2 + H+ => S° + H2O S° + H2O + 1/2O2 => SO4-- + 2 H+ S2O3-- + H2O + 2O2 => 2SO4-- + 2 H+ Environment becomes acidic


Ammonium oxidation=>Nitrite


Nitrite Oxidation=>Nitrate


Methane & C1 compounds oxidation
CH4 => CH3OH => CH2O => HCOO- => CO2


Nitrate respiration=>NO, N2O,NO2, N

2


Sulphate reduction=> H2S

4 H2 + SO4-- + H+ => HS- + 4 H2O CH3COO- + SO4-- + 3 H+ => 2 CO2 + H2S + 2 H2O


CO2: electron acceptor


Other electron acceptors
· · · · · · · Chlorate (ClO3-) Mn4+ Fe3+ Selenate Arsenate DMSO Fumarate => => => => => => => Chloride Mn2+ Fe2+ Selenite Arsenite DMS Succinate


Fermentations


Fermentation products (1)


Fermentation products(2)


Conclusions (1)
There is a variety of compounds that cells (prokaryotic) may use to obtain energy. This energy is used by cells to build macromolecules and biomass. If the amount of energy produced by unit of substrate is low, then the amount of biomass produced may be undetectable. But the quantity of metabolic products may be high.


Conclusions (2)
Some metabolic products are volatile and may accumulate in the atmosphere: H2, CO2, CH4, O2, N2, NO, N2O, H2S, organics, etc. Some others may accumulate in the liquid phase, dissolved (nitrate, nitrite) or not (iron hydroxide), or change the pH of the environment (sulphuric acid). Detection of concentration anomalies of such compounds may indicate the existence of life-mediated chemical reactions.