On organism: environment buffers and their ecological significance

Abstract

We consider, from a physical perspective, the case where the interface between an organism and its environment becomes large enough that it acts as a buffer regulating their matter and energy exchanges. We illustrate the physiological and evolutionary role of buffers through the example of lungfish estivation. Then we ponder the relevance of buffers of this kind to the quest for a general definition of concepts like niche construction, the extended phenotype, and related ones, whose meaning is conveyed at present mostly through particular examples. Finally, we comment on the potential significance of buffers to organism—environment codetermination in the sense originally suggested by Lewontin.

Appendix

Here we outline the thermodynamic analysis of matter and energy fluxes among the three nested objects in Fig. 1b.

First, we assume an interface that allows such exchanges at the organism–buffer boundary, plus a rigid, impermeable interface that isolates the buffer from the environment. Under such conditions we effectively have a two-body situation like that described by Eq. 1 above, thus obtaining for the work performed during this process,

$$ \Updelta W^{{({\text{ob}})}} = - T_{{\text{b}}} \Updelta S^{{({\text{ob}})}}_{{{\text{total}}}} + (P_{{\text{o}}} - P_{{\text{b}}} )\Updelta V^{{({\text{ob}})}}_{{\text{o}}} - (T_{{\text{o}}} - T_{{\text{b}}} )\Updelta S^{{({\text{ob}})}}_{{\text{o}}} - {\sum\limits_j {{\left( {\mu _{{{\text{o}}j}} - \mu _{{{\text{b}}j}} } \right)}\Updelta M^{{({\text{ob}})}}_{{{\text{o}}j}} } } $$

(A-1)

where the subscript b denotes the buffer, and the superscript (ob) indicates that the corresponding exchanges occurred at the organism–buffer interface; $\Updelta S^{{({\text{ob}})}}_{{{\text{total}}}} = \Updelta S^{{({\text{ob}})}}_{{\text{o}}} + \Updelta S^{{({\text{ob}})}}_{{\text{b}}},$ where the superscript indicates that entropy was generated both at the organism and at the buffer when energy and matter transfers occurred at the organism–buffer interface.

Second, we now interchange the interfaces used in the first step, thus obtaining for the amount of work extracted from the ensuing buffer–environment exchange,

$$ \Updelta W^{{({\text{be}})}} = - T_{{\text{e}}} \Updelta S^{{({\text{be}})}}_{{{\text{total}}}} + (P_{{\text{b}}} - P_{{\text{e}}} )\Updelta V^{{({\text{be}})}}_{{\text{b}}} - (T_{{\text{b}}} - T_{{\text{e}}} )\Updelta S^{{({\text{be}})}}_{{\text{b}}} - {\sum\limits_j {{\left( {\mu _{{{\text{b}}j}} - \mu _{{{\text{e}}j}} } \right)}\Updelta M^{{({\text{be}})}}_{{{\text{b}}j}} } } $$

(A-2)

where $\Updelta S^{{({\text{be}})}}_{{{\text{total}}}} = \Updelta S^{{({\text{be}})}}_{{\text{b}}} + \Updelta S^{{({\text{be}})}}_{{\text{e}}};$ the (be) superscript indicates that all transfers now occur at the buffer–environment interface.

The total entropy produced in the whole process, involving simultaneous matter and energy transfers at both interfaces, is given by, $\Updelta S^{{}}_{{{\text{total}}}} = \Updelta S^{{({\text{ob}})}}_{{{\text{total}}}} + \Updelta S^{{({\text{be}})}}_{{{\text{total}}}}.$ The total entropy generated within the buffer is thus, $\Updelta S_{{{\text{b,total}}}} = \Updelta S^{{({\text{ob}})}}_{{\text{b}}} + \Updelta S^{{({\text{be}})}}_{{\text{b}}} ,$ and the net amount of heat that flows through it during the whole exchange of matter and energy in the object–buffer–environment system is, $\Updelta Q^{{}}_{{{\text{b,total}}}} = \Updelta Q^{{({\text{ob}})}}_{{\text{b}}} + \Updelta Q^{{({\text{be}})}}_{{\text{b}}},$ where $\Updelta Q^{{({\text{ob}})}}_{{\text{b}}} = T_{{\text{b}}} \Updelta S^{{({\text{ob}})}}_{{\text{b}}}$ flows at the object–buffer interface, and $\Updelta Q^{{({\text{be}})}}_{{\text{b}}} = T_{{\text{b}}} \Updelta S^{{({\text{be}})}}_{{\text{b}}}$ flows at the buffer–environment one.

Similarly, from Eqs. A-1 and A-2 the total mechanical work performed by the buffer is, $\Updelta W_{{{\text{total}}}} = (P_{{\text{b}}} - P_{{\text{o}}} )\Updelta V_{{\text{b}}} ^{{({\text{ob}})}} + (P_{{\text{b}}} - P_{{\text{e}}} )\Updelta V_{{\text{b}}} ^{{({\text{be}})}},$ and the net change in buffer volume is given by, $\Updelta V_{{\text{b}}} = \Updelta V_{{\text{b}}} ^{{({\text{ob}})}} + \Updelta V_{{\text{b}}} ^{{({\text{be}})}}.$