Slow life

All living organisms must obtain energy from the environment to grow, to maintain a metabolic steady state, or simply to preserve viability.  The availability of energy sources in the environment thus represents a key factor in determining the size, distribution, and activity of biological populations. And it ultimately constrains the possibility of life itself.

The biological demand for energy

Energy, however, is everywhere.  To develop an incisive tool for understanding the interaction between microorganisms and their world requires that the biological demand for energy be understood and quantified.  A variety of approaches – theoretical, biochemical, organic geochemical, culture-based, and environmental – can be brought to bear on this question, but results have not converged to yield a single “answer”.

International workshops

In order to motivate research on the many different aspects of slow life, the CfG has taken initiative to organize international workshops on “Microbial Life under Extreme Energy Limitation”. The first two workshops were held in Aarhus in 2007 and 2012 while the third workshop was held at Sandbjerg Manor in southern Denmark.

Extremely low energy fluxes

Prokaryotic cells in the deep seabed may comprise more than half of all the microorganisms in the ocean, yet they have access to less than 1% of the energy fixed by photosynthesis. These organisms live at the interface between the inhabited and uninhabited realms of our planet and represent the ultimate biological arbiters of chemical exchange between those spheres.  At several hundred meters below the sea floor, the energy flux and the theoretical growth rate of bacteria are orders of magnitude below anything we can understand from research on cultivated microorganisms. The Center strives to understand how prokaryotic cells maintain complex functions at an energy flux that barely allows cell growth over tens to thousands of years.

We use two independent approaches to determine the mean metabolic rates and growth rates of the sub-seafloor communities. One is to model or experimentally measure the rates of organic carbon mineralization and relate this rate to the microbial community size. The terminal mineralization can be sulfate reduction or methanogenesis, while the community quantification may be of the total cell numbers or of specific physiological types, e.g. sulfate reducers or methanogens. The other approach is to analyze the ratio between the two stereo-isomers of specific amino acids in the bulk sediment and, based on knowledge about the rate constant for their physical racemization to the opposite stereo-isomer, to model the turnover rate of bulk amino acids and thereby of the living microbial biomass.

The two approaches yield similar rates of microbial biomass turnover in the sub-seafloor, in the range of 10 to 100 years for coastal sediments and 100 to >1000 years for oceanic sediments. These calculated mean rates may, however, conceal large differences in the growth rate of individual cells of which some may be dormant while others are active. Through the analysis of a specific spore component, dipicolinic acid, we have found that the most resistant dormant stage of bacteria, endospores, are as abundant as vegetative cells in the deep biosphere. Does this dormant stage represent a real survival strategy in low energy environments or is the formation of endospores a dead end that eventually will lead to extinction.