Oxygen isotopes of phosphate (d18Op) are used to trace the cycling of this essential nutrient in terrestrial ecosystems, in lakes, and in the oceans, and to reconstruct past environments on Earth and on Mars. Many advances have been made in recent years in refining the use of this tracer, through a better understanding of the control that different enzymes have on d18Op. However, the isotopic effects of two major processes are still not well studied. First, there is evidence that phosphate uptake by bacteria is associated with fractionation that enriches the isotopic composition of the remaining phosphate in the environment. However, this fractionation is still disputed, and it is not known how the fractionation factor varies between organisms and under different conditions. We will present evidence for the uptake fractionation in various organisms including cyanobacteria (Synechocystis) and higher plants (Lemna minor, Lemna gibba, Egeria densa). The experiments were set up with organisms exposed to different levels of phosphate-deficiency stress, which activated different transport systems, and enables estimation of the associated fractionation. Second, it is well known that some of the water in a cell is the result of the cellular respiration processes (metabolic water). It has been heretofore assumed that in small organisms, like bacteria, in which the ratio of surface area to volume is large, the exchange of water with the environment is fast enough to flush out this metabolic water. However, recent studies have reported that metabolic water has a large effect on the isotopic composition of bacteria cell water, and on the phosphate equilibrated with it. This finding has an important implication for the use of d18Op, and if also confirmed for the hard tissues of other organisms, for the reconstruction of past changes in the ocean and in the climate. Here we show results from an experiment with bacteria (Pseudomonas), which directly explores this question. Together these two research directions will close the last major gaps in our understanding of the factors controlling d18Op in the environment, and thus make it a better tool for studying natural variability in the past, and current phosphate cycling on land and in the oceans.

The effects of phosphate uptake and metabolic water on the phosphate oxygen isotopes

ACS Fall 2021

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