Phosphate May Be Fully Reduced to Phosphine Gas in Anoxic Seawater

Thermodynamic models suggest that phosphite was the dominant form of phosphorus in the Archean ocean, as compared to the most oxidized form of phosphorus, phosphate, which is most prevalent today. Reduction of phosphate to phosphite by ferrous iron is a rapid reaction that may have been widespread in the Archean, likely due to the absence of oxygen. Under these conditions, it is believed that the most reduced form of phosphorus, phosphine gas (PH₃), was abundant in the Archean atmosphere. Millions of years after the Great Oxygenation Event, PH₃ exists today as a trace atmospheric gas. Various freshwater systems have been found to emit PH₃, but the biogeochemical cycling of this in the ocean remains largely unexplored. In attempt to link past and present, we collected water from a brackish pond at multiple depths across a known oxic/anoxic transition where ferrous iron is present. After maintaining an anoxic atmosphere during sampling, we analyzed the samples using gas chromatography. Samples from the anoxic region contained excess amounts of hydrogen sulfide (H₂S); because PH₃ and H₂S have similar chromatographic retention times, H₂S was removed using sodium hydroxide. Subsequent analysis showed that PH₃ was present in the anoxic water, but not in the oxygenated or transition zone waters. Additionally, concentrations of dissolved inorganic phosphate (DIP) and total particulate phosphorus (TPP) were determined using phosphomolybdic blue colorimetry. Results show that all measured forms of phosphorus were most abundant in the anoxic, brackish waters of the sampling site. The detection of PH₃ in these environmental conditions indicates that PH₃ may be present in the anoxic regions of the current ocean and supports the hypothesis that PH₃ was a component of the Archean atmosphere. The lack of PH₃ in the oxygenated water provides a potential link to today’s atmosphere, overall furthering our understanding of phosphorus redox cycling.