The symbiontic nature of metabolic evolution.
Kooijman, S. A. L. M. and Hengeveld, R. 2005.
The symbiontic nature of metabolic evolution.
In: Reydon, A. C. and Hemerik, L. (eds)
Current Themes in Thoeretical Biology: A Dutch perspective.
Kluwer, Dordrecht. p 159 - 202.
Abstract
We discuss evolutionary aspects of metabolism, right from the
beginning of life to the present day at various levels of
organization, thereby including quantitative aspects on the basis of
the Dynamic Energy Budget (DEB) theory. After an extended initial
phase of prokaryotic diversification, cycles of exchange of
metabolites between partners in a symbiosis, integration of partners
into new individuals and new specializations led to forms of symbiosis
of various intensity, ranging from loosely living together in species
aggregates to several forms of endosymbiosis. While the prokaryotic
metabolism evolved into a considerable chemical diversity, the
eukaryotic metabolic design remained qualitatively the same, but shows
a large organizational diversity. Homeostasis of biomass evolved,
introducing stoichiometric constraints on production and excretion of
products that can be re-utilized; carbohydrates and inorganic nitrogen
being the most important ones. This stimulates the formation of
symbioses, since most are based on syntrophy.
A remarkable property of DEB theory for metabolic organization is that
organisms of two species that exchange products, and thereby follow
the DEB rules, can together follow a symbiogenic route such that the
symbiosis behaves as a new organism that itself follows the DEB
rules. This property of the reserve dynamics in the DEB theory also
explains a possible evolutionary route to homeostasis.
The reserve dynamics in DEB theory also plays a key role in linking
the kinetics of metabolic pathways to needs of metabolites at the
cellular level. Reserve kinetics, in combination with other DEB
elements, explains how metabolic performance depends on body size and
why such relationships work out differently within and between
species. Apart from the key role of reserves, the dynamic interaction
between surface areas and volumes is a basic feature of the DEB theory
at all levels of organization (molecules, individuals,
ecosystems). The explicit mass and energy balances of the DEB theory
facilitates ecosystem modelling as it depends on nutrient
exchange. The theoretical interest in this topic concerns the huge
range in space-time scales that is involved to understanding the
significance of the actions of life within the context of metabolic
organization.