title: Resource Availability Modulates the Cooperative and Competitive Nature of a Microbial Cross-Feeding Mutualism.
abstract: Mutualisms between species play an important role in ecosystem function and stability. However, in some environments, the competitive aspects of an interaction may dominate the mutualistic aspects. Although these transitions could have far-reaching implications, it has been difficult to study the causes and consequences of this mutualistic-competitive transition in experimentally tractable systems. Here, we study a microbial cross-feeding mutualism in which each yeast strain supplies an essential amino acid for its partner strain. We find that, depending upon the amount of freely available amino acid in the environment, this pair of strains can exhibit an obligatory mutualism, facultative mutualism, competition, parasitism, competitive exclusion, or failed mutualism leading to extinction of the population. A simple model capturing the essential features of this interaction explains how resource availability modulates the interaction and predicts that changes in the dynamics of the mutualism in deteriorating environments can provide advance warning that collapse of the mutualism is imminent. We confirm this prediction experimentally by showing that, in the high nutrient competitive regime, the strains rapidly reach a common carrying capacity before slowly reaching the equilibrium ratio between the strains. However, in the low nutrient regime, before collapse of the obligate mutualism, we find that the ratio rapidly reaches its equilibrium and it is the total abundance that is slow to reach equilibrium. Our results provide a general framework for how mutualisms may transition between qualitatively different regimes of interaction in response to changes in nutrient availability in the environment.
authors.affiliation: Hubrecht Institute, The Royal Netherlands Academy of Arts and Sciences (KNAW) and University Medical Center Utrecht, Utrecht, the Netherlands.
authors.forename: Tim A
authors.initials: TA
authors.lastname: Hoek
authors.affiliation: Biophysics PhD Program, Harvard University, Cambridge, Massachusetts, United States of America.
authors.forename: Kevin
authors.initials: K
authors.lastname: Axelrod
authors.affiliation: Physics of Living Systems, Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
authors.forename: Tommaso
authors.initials: T
authors.lastname: Biancalani
authors.affiliation: Physics of Living Systems, Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
authors.forename: Eugene A
authors.initials: EA
authors.lastname: Yurtsev
authors.affiliation: Physics of Living Systems, Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
authors.forename: Jinghui
authors.identifier: 0000-0002-8549-8359
authors.initials: J
authors.lastname: Liu
authors.affiliation: Physics of Living Systems, Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
authors.forename: Jeff
authors.initials: J
authors.lastname: Gore
mesh_terms: D000465:Algorithms; D002455:Cell Division / Q000187:drug effects / Q000235:genetics; D003470:Culture Media / Q000378:metabolism / Q000494:pharmacology; D017753:Ecosystem; D005434:Flow Cytometry; D007930:Leucine / Q000235:genetics / Q000378:metabolism*; D060847:Metabolic Engineering / Q000379:methods; D008954:Models, Biological; D012441:Saccharomyces cerevisiae / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D013045:Species Specificity; D013053:Spectrophotometry; D013559:Symbiosis; D013997:Time Factors; D014364:Tryptophan / Q000235:genetics / Q000378:metabolism*
chemical_list: D003470:Culture Media; D014364:Tryptophan; D007930:Leucine