Changes in metabolism, growth and nutrient uptake of Ulva fasciata (Chlorophyta) in response to nitrogen source

Abstract

The form of inorganic nitrogen (N) determines the biofiltration performance of Ulva, particularly by inhibition of nitrate assimilation in the presence of ammonia. In the current study, Ulva fasciata from a biofilter of fishpond effluent was examined for its biomass production, photosynthetic activity, nutrient uptake and activity of nitrate reductase when supplied with either ammonia or nitrate as the sole nitrogen source. The effects of prior acclimation to either nitrogen source were also studied. Cultivation of Ulva with ammonia resulted in a rapid uptake of this nutrient, whereas photosynthesis, uptake of orthophosphate and C:N ratio were greater in cultures with nitrate. The transfer of Ulva from ammonia to nitrate nutrition increased carbohydrate content in the biomass. Longer culture in nitrate did not influence photosynthetic yields, growth, or N and P uptake rate by the algae, but increased both lipid and P content in biomass. Once transferred from ammonia-rich to nitrate-rich seawater, nitrate uptake by Ulva commenced immediately, although the activity of the enzyme nitrate reductase was first detected only 28 h later. Culture of Ulva in different nitrogen forms, ammonia and nitrate, may determine application of the resulting biomass in different industries, e.g., feeding, bioethanol, biofiltration, etc., considering the rapid production of protein-rich Ulva in the presence of ammonia, while yield is lower in the presence of nitrate but the biomass contains more carbohydrates and removes P from the effluent more rapidly.