The industrial-scale production of biofuels from cultivated microalgae has gained considerable interest in the last several decades. While the climate benefits of microalgae cultivation that result from the capture of atmospheric CO2 are known, the counteracting effect from the potential emission of other greenhouse gases has not been well quantified. Here, we report the results of a study conducted at an industrial pilot facility in Hawaii to determine the air−water fluxes of N2O and CH4 from open raceway ponds used to grow the marine diatom Staurosira sp. as a feedstock for biofuel. Dissolved O2, CH4, and N2O concentrations were measured over a 24 h cycle. During this time, four SF6 tracer release experiments were conducted to quantify gas transfer velocities in the ponds, and these were then used to calculate air−water fluxes. Our results show that
pond waters were consistently supersaturated with CH4 (up to 725%) resulting in an average emission of 19.9 ± 5.6 μmol CH4 m−2 d−1. Upon NO3 depletion, the pond shifted from being a source to being a sink of N2O, with an overall net uptake during the experimental period of 3.4 ± 19 3.5 μmol N2O m−2 d−1. The air−water fluxes of N2O and CH4 expressed as CO2 equivalents of global warming potential were 2 orders of magnitude smaller than the overall CO2 uptake by the microalgae.
Ferrón, S; Ho, D; Johnson, Z; Huntley, M. Air-water fluxes of N2O and CH4 during microalgae cultivation (Staurosira sp.) in an open raceway pond. Environmental Science & Technology DOI: 10.1021/es302396j
