High production of nitrous oxide (N₂O), methane (CH₄) and dimethylsulphoniopropionate (DMSP) in a massive marine phytoplankton culture

Abstract. The production of large amounts of algal biomass for different purposes such as aquaculture or biofuels, may cause impacts on the marine environment. One such impact is the production of radiatively active trace gases and aerosols with climate cooling (dimethyl sulfide DMS and its precursor DMSP) and warming (N₂O and CH₄) effects. Total and dissolved DMSP, N₂O and CH₄, together with other environmental variables were monitored daily for 46 days within a massive microalgae monoculture of Nannochloris (Chlorophyceae) in an open pond system. The growth of this green microalgae was stimulated by the addition of N- and P-rich salts, resulting in exponential growth (growth phase) during the first 17 days observed by cell abundance (1 × 10⁶ to 4.4 × 10⁶ cell mL⁻¹) and Chl-a levels (from 1.4 to 96 mg Chl-a m⁻³) followed by a decrease in both Chl-a and cell abundance (senescence phase). Total DMSP (from 6.3 to 142 μmol m⁻³), dissolved DMSP i.e. 5.8 to 137 μmol m⁻³ and N₂O (from 8 to 600 μmol m⁻³) abruptly peaked during the senescence phase, whereas CH₄ steadily increased between 2 and 10 μmol m⁻³ during the growth phase. Different ratios between tracers and Chl-a during both phases reveal different biochemical processes involved in the cycling of these gases and tracers. Our results show that despite the consumption of large quantities of CO₂ by the massive algal culture, a minor amount of DMS and huge amounts of greenhouse gases were produced, in particular N₂O, which has a greater radiative effect per molecule than CO₂. These findings have important implications for biogeochemical studies and for environmental management of aquaculture activities.