Biofilms promote altruism

The common basis for these three themes is a trade-off between growth rate and growth yield, which in turn is based on irreversible thermodynamics. The trade-off creates an evolutionary alternative between two strategies: high growth yield at low growth rate versus high growth rate at low growth yield. High growth yield at low growth rate is a case of an altruistic strategy because it increases the fitness of the group by using resources economically at the cost of decreased fitness, or growth rate, of the individual. The group beneficial behaviour is advantageous in the long-term while the high growth rate strategy is advantageous in the short-term. Coexistence of species requires differences between their niches, and niche space is typically divided into four 'axes' (time, space, resources, predators). This neglects survival strategies based on cooperation, which extend the possibilities of coexistence, arguing for the inclusion of cooperation as the fifth 'axis'. The individual-based model simulations below show that spatial structure, as in, e.g., biofilms, is necessary for the origin and maintenance of this 'primitive' altruistic strategy and that the common belief that growth rate but not yield decide the outcome of competition is based on chemostat models and experiments. This evolutionary perspective on life in biofilms can explain long known biofilm characteristics, such as the structural organisation into microcolonies, the often observed lack of mixing among microcolonies, and the shedding of single cells, as promoting the origin and maintenance of the altruistic strategy. While biofilms enrich altruists, enrichment cultures, microbiology's paradigm for isolating bacteria into pure culture, select for highest growth rate.

Selection for higher yield in biofilms is the basis for postulating that a nitrifying bacterium exists that combines two short and fast pathways into one longer and slower pathway, because the longer pathway of complete ammonia oxidation results in a higher yield (See the paper on Why is metabolic labour divided in nitrification?).

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