(Very) Small is Beautiful

A New Emerging View on Microbiology and its Implications for Global Ecology

Jelte P. Harnmeijer & Kenneth H. Nealson

What, you might be wondering, is an article dealing with microbes doing in The Bulletin? In an era of genetically modified agricultural products on supermarket shelves and plans – in direct contravention of international legislation – for spraying engineered fungal agents on rural communities in Colombia, readers may well be forgiven for raising their eyebrows. However, as you are about to discover, humankind – and western society in particular – has much to learn from the way microbes treat their world.

Prokaryotes (comprised of Bacteria and Archaea) are organisms whose cells, unlike ours, lack a membrane-bound nucleus. The vast majority are extremely small (one-thousandth of a millimetre or less in diameter) single cells, similar is size and shape to the well-studied bacteria Shewanella shown in Figure 1. Like all living things, bacteria metabolize: they take in substances for energy and growth from their surroundings, and release waste products into their environment. Most are unable to feed directly on other organisms (including other microbes), so these small chemical factories specialize in eating and breathing almost anything else the environment has to offer. Thus, although invisible to the naked eye, microbes play an important role in virtually every known ecosystem on our planet.

Figure 1: The bacteria Shewanella seen under high magnification through a microscope.

In parallel with most political decision-making today, scientists always assumed that microbes maximize their metabolic rate (the rate at which they ‘breath’ and ‘eat’) in order to achieve the highest possible growth rate. After all, faster growth results in more offspring and less resources available for competing organisms. As a result of this thinking, treatises on microbial metabolism often read like IMF country reports. Performance is judged solely by GNP per capita (metabolic rate) and ‘optimum growth’ occurs where economic productivity (the turnover of resources for metabolism) is maximized. Other indicators, including long-term survivability and ecosystem stability, are not considered. Figure 2 shows how growth rate (measured by microbiologists as the number of times the population doubles per hour) varies with temperature for a species of bacteria called Psychrobacter pegella. Note that according to standard microbial notation, the ‘optimal growth temperature’ (Topt) is defined as the temperature where population growth is the fastest.

Figure 2: Growth rate versus temperature for Psychrobacter pegella.

But bacteria know what’s good for them. After all, they’ve had at least three and a half billion years to learn: much longer, thankfully, than the IMF has been imposing it’s SAPs (Structural Adjustment Programs) on the Third World through the application of artificial, and often misleading, indicators of ‘progress’. And guess what? Recent developments in microbiology indicate that bacteria, given the choice, are more comfortable using available resources sparingly even if it means attaining a lower growth rate. In fact, at temperatures corresponding to maximum growth, microbes show every sign of being unhappy. In a phenomenon called stress response, they start producing special proteins to fight the detrimental effects of higher temperature. Cellular reactions become inefficient and wasteful. Increase the temperature by a few degrees, and certain cellular reactions cease to function. Increase the temperature further, and cells begin to die. No surprise, then, that most bacteria are found in their natural environment at temperatures ten to fifteen degrees Celsius below their so-called ‘optimum growth temperature’! Looking at how efficiently resources are used, rather than just by how fast, thus provides new insights into microbial behavior. The view, expressed as ‘yield’ and calculated on the basis of output per unit input, is shown in Figure 3.

Figure 3: Yield per mole of substrate for Psychrobacter pegella.

Bacteria, most of which live on the edge of life in a permanent state of chronic starvation, have evolved under immense selective pressure to not treat resources in their environment as boundless. As a result, the stability and diversity of their environment, and the resources available for their progeny, are conserved. We can only hope that our governments will come to treat our forests, fish, oil, water, air and topsoil – to mention a few – with the foresight and wisdom that our miniscule single-celled cousins have learned to treat the scraps of phosphate and nitrates on which they depend.

Bacteria, after all, will probably survive the world our governments are creating. But our children’s children may not.

July 14th, 2003;  Wrigley Marine Science Institute, Catalina Island.

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