Why are there no picoplanktonic O2 evolvers with volumes less than 10−19 m3?

Abstract

Non-scalable components occupy an increasing fraction of the biomass of cells as their size decreases, thus decreasing the fraction of the biomass available for other scalable, essential activities. The non-scalable components include the genome, and membranes such as the plasrnalemma and the outer membrane of cyanobacteria and the plastid envelope membranes of chlorophytes. The predicted influences of the increasing fraction of non-scalable components in small cells (0.5 μm radius spherical cells relative to 5 μm radius cells) are threefold. One prediction is a decreased maximum specific growth rate (biomass increase per unit biomass per unit time) due to a decreased fraction of biomass occupied by scalable catalysts. This in turn gives a lower catalytic activity per unit biomass leading to a lower material or energy conversion rate per unit biomass. A second prediction is a reduced fraction of the biomass occupied by light-harvesting material, thus partly or entirely removing the photon-harvesting advantage of smaller cells and diminishing specific growth rate at low incident photon flux densities. A third prediction is that the different C:N:P ratio in non-scalable components compared to ‘average’ biomass means a variation in C:N:P ratio at low cell sizes, with a lower C:N ratio in smaller cells and a higher C:P ratio in small cyanobacterial cells, but a lower C:P ratio in smaller chlorophyte cells. The difference in C:P ratio predictions is a function of the higher DNA content of chlorophyte than cyanobacterial cells. Comparisons of these predictions with the observed effects on cell properties and behaviour of variations in cell size in the range 0.3–2 μm radius for cyanobacteria and 0.5–9 μm radius for chlorophytes yields the following results. The general trend for an increase in maximum specific growth rate with decreasing cell size (a finding with little theoretical explanation) appears to be reversed as cell size decreases below ∼0.9 μm radius for cyanobacteria and 2 μm radius for chlorophytes, in at least semiquantitative agreement with predictions. For the prediction of a decreased photon absorptance in the smaller cells (compared with predictions ignoring non-scalable components), the available data do not permit a conclusion to be reached as to whether such a decreased photon absorptance, with its consequences for growth at low incident photon flux densities, occurs. Similarly, further work is needed to determine if the predictions of changed C:P and C:N ratios are borne out by observations.