Regarding inorganic carbon as 'just another' chemical resource used in the growth of aquatic photolithotrophs, we ask three questions and then attempt to answer them. (1) How common are catalysed chemical changes of the resource outside the cell, and accumulation of the resource inside the cell prior to assimilation, for the diverse chemical resources used? (2) Do acquisition and assimilation meet evolutionary optimality criteria with respect to the use of other resources? (3) Are there clues to the evolutionary origin of inorganic carbon concentrating mechanism (CCMs) in the mechanisms of acquisition of other resources and vice versa? Evidence considered includes molecular genetic similarities between CCM components and components of other resource acquisition mechanisms, and palaeogeochemical evidence on the timing of restrictions on the availability of the resources such that extracellular transformation of materials, and their accumulation within cells prior to assimilation, are needed. Provisional answers to the questions are as follows: (1) Many common chemical resources other than inorganic carbon are subject to extracellular chemical conversion and/or accumulation prior to assimilation, e.g. ammonium, nitrate, urea, amino acids, organic and inorganic phosphate and iron; (2) There is some evidence for optimality of CCMs and of less complex resource acquisition processes, exemplified by NH(4)(+) entry and assimilation, though many more data are needed and (3) There are molecular genetic similarities between CCM components and transporters for other solutes and components of respiratory NADH dehydrogenases that are consistent with their use in CCMs representing a derived evolutionary state. Palaeogeochemical evidence suggests that CCMs evolved later than did at least some of the extracellular chemical transformation and/or accumulation mechanisms for other resources.