Environmental regulation in a network of simulated microbial ecosystems

Abstract

The Earth possesses a number of regulatory feedback mechanisms involving life. In the absence of a population of competing biospheres, it has proved hard to find a robust evolutionary mechanism that would generate environmental regulation. It has been suggested that regulation must require altruistic environmental alterations by organisms and, therefore, would be evolutionarily unstable. This need not be the case if organisms alter the environment as a selectively neutral by-product of their metabolism, as in the majority of biogeochemical reactions, but a question then arises: Why should the combined by-product effects of the biota have a stabilizing, rather than destabilizing, influence on the environment? Under certain conditions, selection acting above the level of the individual can be an effective adaptive force. Here we present an evolutionary simulation model in which environmental regulation involving higher-level selection robustly emerges in a network of interconnected microbial ecosystems. Spatial structure creates conditions for a limited form of higher-level selection to act on the collective environment-altering properties of local communities. Local communities that improve their environmental conditions achieve larger populations and are better colonizers of available space, whereas local communities that degrade their environment shrink and become susceptible to invasion. The spread of environment-improving communities alters the global environment toward the optimal conditions for growth and tends to regulate against external perturbations. This work suggests a mechanism for environmental regulation that is consistent with evolutionary theory.evolution ͉ Gaia theory ͉ microbial ecology ͉ multilevel selection

Social evolution in multispecies biofilms

“…S9). The potential for such higher-level selection to shape microbial communities was seen in a large-scale simulation of microbial species growing and dispersing among a series of 10 interconnected flasks (70). Over time, sets of species that limited harm to their local environment-a form of cooperation-were favored over more rapacious species that limited group productivity, something also seen in experiments that artificially selected for group productivity in real microbial communities (71).…”

Multiple states of environmental regulation in well-mixed model biospheres

“…In the Flask model [31] [29], flasks contain an abiotic environment with parameters (that can be thought of as temperature, pH, salinity), and nutrients which are the substrates for metabolism, with a constant inflow and outflow of these abiotic parameters. The flasks are seeded with 'microbes' which consume the nutrients available and affect the abiotic parameters as a side effect of their metabolism.…”

“…During reproduction there is a small constant probability of mutation per locus P mut so that over time new species arise via mutation. A spatial version of the model connected multiple local environments by inflows and outflows [31] [29]. Stabilising environmental regulation still emerged and this model argues for spatial structure creating conditions where limited higher-level selection can take place.…”

“…For our single flask Flask model we closely follow the implementation detailed in [29] limiting the system to a single flask. We present a model of self-regulation of a purely global environment arising via evolution.…”

“…The Flask model [30] [29] [31] removed a limiting assumption of Daisyworld and the Guild model that traits selected for at the individual level always improve the global environment. Instead the organisms in the system affect the abiotic environment as a byproduct of their metabolism, making these effects selectively neutral at the individual level.…”