Controls on the production of organohalogens by phytoplankton: Effect of nitrate concentration and grazing

Abstract

Using a number of laboratory culture experiments we have been able to show that trace gas release by phytoplankton is affected by changes in ambient nitrate conditions. Nutrient concentrations are fundamentally important to phytoplankton community structure and primary production, so it is important to understand if changes in nitrate levels are likely to affect metabolic processes. Recent human activities have added a large quantity of atmospheric nitrogen to the open ocean and this, in turn, will affect both carbon dioxide drawdown and nitrous oxide release. In contrast, ocean prediction models tell us that large parts of the ocean are going to become more oligotrophic. How these changes will affect other climatically active gases, e.g., organohalogens, is unclear. In this study we look at the effect of varying the nitrate to phosphate ratio on the release of three methyl halides. Some attention is also given to the effects of grazing as a means of gas release. Assuming that a 16:1 nitrate:phosphate ratio is the norm, we show that increasing the nitrate level above 16 μmol L−1 can lead to up to an 85% increase in trace gas production per unit cell while lowering the concentration can result in up to a 65% reduction. This is the first time that such a nutrient effect has been observed. Grazing by the microzooplankton Oxyrrhis marina also increases release, but whether this is due to enhanced metabolic processes caused by stress or the breaking down of cells due to grazing is unclear.