Advantages of the division of labour for the long-term population dynamics of cyanobacteria at different latitudes

Valentina Rossetti,Homayoun C Bagheri

doi:10.1098/rspb.2012.0755

Valentina Rossetti, Homayoun C Bagheri

Open Access

https://doi.org/10.1098/rspb.2012.0755

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Abstract

A fundamental advancement in the evolution of complexity is division of labour. This implies a partition of tasks among cells, either spatially through cellular differentiation, or temporally via a circadian rhythm. Cyanobacteria often employ either spatial differentiation or a circadian rhythm in order to separate the chemically incompatible processes of nitrogen fixation and photosynthesis. We present a theoretical framework to assess the advantages in terms of biomass production and population size for three species types: terminally differentiated (heterocystous), circadian, and an idealized species in which nitrogen and carbon fixation occur without biochemical constraints. On the basis of real solar irradiance data at different latitudes, we simulate population dynamics in isolation and in competition for light over a period of 40 years. Our results show that in isolation and regardless of latitude, the biomass of heterocystous cyanobacteria that optimally invest resources is comparable to that of the idealized unconstrained species. Hence, spatial division of labour overcomes biochemical constraints and enhances biomass production. In the circadian case, the strict temporal task separation modelled here hinders high biomass production in comparison with the heterocystous species. However, circadian species are found to be successful in competition for light whenever their resource investment prevents a waste of fixed nitrogen more effectively than do heterocystous species. In addition, we show the existence of a trade-off between population size and biomass accumulation, whereby each species can optimally invest resources to be proficient in biomass production or population growth, but not necessarily both. Finally, the model produces chaotic dynamics for population size, which is relevant to the study of cyanobacterial blooms.

Highlights

Division of labour generally helps an organism to efficiently integrate distinct cellular activities
We present a theoretical framework to assess the advantages in terms of biomass production and population size for three species types: terminally differentiated, circadian, and an idealized species in which nitrogen and carbon fixation occur without biochemical constraints
We investigated the performance of undifferentiated and differentiated cyanobacteria in terms of population size and biomass production, when they grow in isolation as well as when they compete for light

Summary

INTRODUCTION

Division of labour generally helps an organism to efficiently integrate distinct cellular activities. We investigated the performance of undifferentiated and differentiated cyanobacteria in terms of population size and biomass production, when they grow in isolation as well as when they compete for light This can elucidate some of the advantages that each strategy for division of labour provides for the resolution of the biochemical incompatibility between nitrogen and carbon fixation. This hypothetical species type represents an ideal cyanobacterial species in which biochemical constraints owing to the inhibition of oxygen on nitrogenase are absent This implies that nitrogen and carbon fixations can take place in each cell and at any time, as soon as the necessary resources are available. The presence of a filled diamond on a grid point indicates that a species at a given latitude reaches its best performance in terms of population size at the corresponding (a, r) pair.

12 N latitude

CONCLUSIONS