Taming wild microbes

Karen Houghton checking growth  of acidophiles in her culture tubes

On the surface of Earth, most biological communities are fueled by organisms that use light energy to convert inorganic carbon (CO₂) into organic compounds (i.e. biomass) in a process known as photosynthesis. This is what plants do. In deep-sea hydrothermal environments, there is no sunlight so instead these communities are supported by chemosynthesis – a process by which microorganisms use chemical energy to convert CO₂ into biomass. The chemical energy for chemosynthesis comes in the form of reduced chemical compounds (H₂, Fe²⁺, H₂S, CH₄) that are relatively abundant in hydrothermal fluids. Organisms that perform chemosynthesis are known as chemolithoautotrophs.  Both aerobic microorganisms, which require oxygen (O₂) to grow and anaerobic microorganisms, which use alternative electron acceptors (NO₃^-, S⁰, SO₃^-2), are found in hydrothermal environments.  Within the Lau Basin and other deep-sea vent fields, microbial communities are heavily influenced by steep chemical, pH and temperature gradients.  For this reason, microbiologists on board the Revelle hope to isolate several novel acid-loving (acidophiles), high-temperature (thermophile) species on this expedition.

90 degree Celsius oven filled with enrichment cultures from samples collected at ABE and Mariner

The isolation of chemolithoautotrophs is driven by the principles of growth selection.  In practice, synthetic media is prepared that mimics the conditions in the deep-sea but only the necessary energy (e.g. reduced inorganic chemical), carbon (e.g. CO₂) and electron acceptors (O₂ or NO₃^-, S⁰, SO₃^-2) required for a specific type of chemosynthetic metabolism are supplied.  By restricting alternative metabolisms and by exposure to stringent pH and temperatures, microorganisms with desirable traits out-compete undesirable species and become enriched.  Over time, with the application of sufficiently selective growth conditions, it is possible to remove all undesired microorganisms; resulting in a “pure” isolate.

In our lab on the Revelle, microbiologists are working hard to isolate high temperature, acid-loving hydrogen-oxidizers and methane oxidizing species; in addition to heterotrophic bacteria capable of eating organic compounds (amino acids, peptides, chitin).  In the weeks to come, both microbial community and metagenomic data will be used to refine the selective media used in isolations.    

Contributed by Carlo Carere and Gilbert Flores