Cyclical changes in biomass productivity and amino acid content of freshwater macroalgae following nitrogen manipulation

The effective supply of nitrogen to algal cultures is an important aspect of intensive cultivation and critical if the biomass is to be used as a source of protein. In this study two complementary experiments examine how variation in the supply of nitrogen to cultures influenced the biomass productivity and protein content of the freshwater macroalga, Oedogonium. The first examined how robust Oedogonium is to the intermittent supply of nitrogen (supplied weekly, every second week or every third week) by quantifying its biomass productivity, photosynthetic capacity and internal nitrogen content through time. Biomass productivity over a 12-week period was highest (10.6 g DW m− 2 day− 1) when nitrogen was supplied weekly and lowest (8.1 g DW m− 2 day− 1) when nitrogen was supplied every third week. The second experiment examined the recovery of nitrogen and amino acids in the biomass following periods of nitrogen-depletion. Prolonged periods (2 weeks) without nitrogen reduced the internal nitrogen and amino acid content of the biomass by up to 80%. However, in all treatments the internal nitrogen content recovered within 24-48 h and the amino acid content had recovered within 72 h following the resupply of external nitrogen. These results demonstrate that nitrogen should be supplied in a relatively constant manner to maximize the growth rates of Oedogonium; however, the protein in nitrogen-deplete cultures can be rapidly rejuvenated by the addition of nitrogen in the days prior to harvest.