Comparative effects of ammonium, nitrate and urea on growth and photosynthetic efficiency of three bloom-forming cyanobacteria

1. Urea-based fertilisers have grown in popularity over the past half-century and are now the dominant nitrogen (N) form applied to agricultural landscapes. The widespread use of urea fertilisers has favoured its export to local waterways, and urea pollution may be contributing to the propagation and maintenance of cyanobacteria harmful algal blooms in fresh waters. The relative success of cyanobacteria in response to urea and inorganic N additions was studied to understand whether the recent changes in the magnitude and composition of N loading have created a scenario that now favours the dominance of cyanobacteria in fresh waters. 2. Growth and photosynthetic efficiency of three bloom-forming freshwater cyanobacteria (Microcystis, Dolichospermum and Synechococcus) grown on nitrate (NO 3), ammonium (NHthorn4) and urea (CO(NH2) 2) as the sole N form were monitored. We hypothesised that N substrates that require the lowest energetic investment or offer the highest energetic return would favour optimal growth and photosynthetic performance. We predicted that urea would result in higher cellular growth and pigment production relative to NHthorn4 or NO 3, as urea provides twice the amount N and an additional carbon (C) source making it more energetically efficient. 3. Cyanobacteria biomass was not significantly enhanced on urea relative to inorganic N forms. Growth on urea was matched by NO 3 for all species, whereas growth on NHthorn4 was halved compared urea or NO 3. However, cyanobacteria cells had higher pigment concentrations when grown on urea relative to inorganic N sources. These findings suggest that the additional nutrient building blocks supplied from the hydrolysis of urea were not directed towards active growth, but rather accumulated in secondary pools to increase production of Nrich compounds, such as pigments. 4. Although urea did not influence cyanobacteria quantity (i. e. biomass) compared to inorganic N sources, it produced higher quality cells by enhancing pigment synthesis and potentially giving cyanobacteria a competitive advantage in lightlimiting conditions. Furthermore, when supplied in excess, cyanobacteria rapidly consumed urea in excess of their biosynthetic requirements suggesting a form of urea gluttony. These results demonstrate the importance of N speciation on cyanobacteria physiological responses and reinforce the emerging links between urea and cyanobacteria harmful algal blooms in inland waters.