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

Abstract

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. Growth and photosynthetic efficiency of three bloom‐forming freshwater cyanobacteria (Microcystis, Dolichospermum and Synechococcus) grown on nitrate (NO3−), ammonium (NH4+) 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 NH4+ or NO3−, as urea provides twice the amount N and an additional carbon (C) source making it more energetically efficient. Cyanobacteria biomass was not significantly enhanced on urea relative to inorganic N forms. Growth on urea was matched by NO3− for all species, whereas growth on NH4+ was halved compared urea or NO3−. 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 N‐rich compounds, such as pigments. 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 light‐limiting 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.

Differential Drawdown of Ammonium, Nitrate, and Urea by Freshwater Chlorophytes and Cyanobacteria1

“…As with Erratt et al. (), all three N substrates were assayed to check for shifts among N pools. For all experiments, no significant changes among N pools were observed at the concentrations selected.…”

“…, Erratt et al. ). In order for urea to affect phytoplankton community composition—by way of actively promoting the growth of harmful taxa—two pre‐requisites must co‐exist: 1) the physiological ability of phytoplankton to exploit urea; and 2) a source of anthropogenically derived urea entering the environment (Kudela et al.…”

“…, Erratt et al. ). However, the underlying physiological mechanisms behind why these two taxonomic groups differ remain inconclusive.…”

“…N concentrations in the culture medium were monitored via a suite of colorimetric microplate techniques, as previously described in Erratt et al. (). Measurements were recorded every 2 d, beginning from the initial inoculation (day 0) and followed until cells reached stationary growth (day 10).…”

Selection of Bioassay for Ecological Research of Water, Polluted by Mineral Nitrogen Forms

“…Вместе с фосфатами минеральные соли азота становятся пусковым механизмом эвтрофикации водоемов. Поступление в воды минеральных азотных удобрений приводит к размножению наиболее негативной для водоемов флоры -цианобактерий, выделяющих специфические токсины [14]. Также при исследовании водных объектов Ямало-Ненецкого автономного округа было показано, что в воде озер, не имеющих прямого поступления в них сточных и фильтрационных вод, снижается уровень рН вследствие кислотных осадков, формирующихся под влиянием выбросов азота и серы [1].…”