Biofilms promote altruism
Overview
The origin of altruism is a fundamental problem in evolution, and the maintenance of biodiversity is a fundamental problem in ecology. These two problems combine with the fundamental microbiological question of whether it is always advantageous for a unicellular organism to grow as fast as possible. 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 project on "One-step Nitrification").
This page mainly serves as a companion to my 2004 paper in Microbiology and provides the movies promised there. A poster presented at the "Biofilms 2003" conference in Victoria, B.C., Canada, can serve as a short version of this paper.
Summary of results with links to movies
Bacteria with two fixed and heritable strategies, Ego (Egoistic: high growth rate but low yield) and Eco (Economic: high yield but low growth rate), competed in an individual-based biofilm model where they grow as clusters in gradients of limiting substrate. Ego always wins in chemostats due to its higher growth rate. Eco wins in biofilms either if not encircled too closely or if the density of the besieging Ego is so high that they outcompete themselves. Also, Eco may win due to a clustering effect. Eco clusters can be invaded by single Ego cells but not groups, inversely, Ego clusters can be invaded by Eco groups but not single cells.
Watch the biofilm grow with one frame per 1,000 min. All frames were rendered with POV-Ray. Each cell is shown as a sphere. The grey box at the bottom is the inert substratum. Note the wrapped around boundaries in the horizontal direction. Scale: total width of the system = 200 µm. The Ego strategists are blue, while the Eco strategists are red.
All against one
One Eco cell surrounded by Ego, medium density (20 cells total)
One Ego cell surrounded by Eco, medium density (20 cells total)
Side-by-side
Side-by-side arrangement, medium density (20 cells total)
Effect of density
Alternating Ego, Eco, Ego, ..., at low density (10 cells total)
Alternating Ego, Eco, Ego, ..., at medium density (20 cells total)
Alternating Ego, Eco, Ego, ..., at medium density (40 cells total)
Alternating Ego, Eco, Ego, ..., at high density (100 cells total)
Invasion by groups
Flat biofilm with random mix of Ego and Eco
