Broad-scale comparison of photosynthetic rates across phototrophic organisms.

We tested the existence of general patterns in the photosynthetic metabolism of oxygen-evolving organisms, based on a compilation of data for 315 species ranging from cyanobacteria to tree leaves. We used thickness and chlorophyll a concentration of the photosynthetic structure (cell, thallus, leaf) to scale differences in photosynthetic metabolism among plants, because of the demonstrated importance of these plant traits in regulating light absorption properties and photosynthetic rates of particular plant groups. We examined only the properties of the photosynthetic structure because this is the plant unit responsible for the photosynthetic process and thus is closely related to plant productivity, whereas there is a lack of general quantitative descriptors of the whole organism useful for such broad-scale comparisons, and few studies report net photosynthetic rates of whole organisms, including respiration rates of all non-photosynthetic structures. The results demonstrated that descriptors of plant metabolism such as maximum net photosynthesis, initial slope of the photosynthesis-irradiance (PI) curve and dark respiration display strong positive interrelationships. The metabolic rates declined with increasing thickness of the photosynthetic structures and more steeply for photosynthesis than respiration. Photosynthetic rates also changed with increment of volume of the photosynthetic structure resembling patterns that have been previously described for animal metabolism related to body weight. The strong relationship of metabolic rate and chlorophyll a concentration to the thickness of photosynthetic tissue reflects broad-scale patterns and not the adaptive response of individual or closely-related species of similar tissue thickness to varying environmental conditions. Thickness of the photosynthetic structures, therefore, plays an important role in the environmental control of plant performance and, consequently, it might have been an important driver of plant evolution, setting thresholds to the metabolism and productivity of phototrophic organisms.