Preferential nitrogen uptake in the duckweed Spirodela sp. (Lemnaceae), and the effects of a potential N-fixing symbiosis on nutrient acquisition
Jackie M. Hill1, Zuma B.2, Martin P. Hill1, Kaehler S.3
1Dept. Zoology & Entomology, Rhodes University; 2Institute of Environmental Biotechnology, Rhodes University; 3IsoEnvironmental CC, South African Institute for Aquatic Biodiversity, Grahamstown
Excessive nitrogen loading is a primary driver behind the global deterioration of aquatic ecosystems and the establishment of invasive weeds. Recently, incubation and transplantation of the duckweed Spirodela sp. has been suggested as a potential early-warning tool for the identification of incipient eutrophication using stable isotope analysis (SIA). δ15N values of Spirodela can distinguish between
different elemental N sources and reflect nitrogen concentration gradients. Current research however, suggests that Spirodela may display preferential N assimilation and may have a symbiotic relationship with N-fixing cyanobacteria, allowing the assimilation of atmospheric nitrogen and complicating attempts to trace N-loading in aquatic systems. This study aimed to investigate preferential nitrogen uptake of Spirodela over a range of concentrations as well as to investigate the effect of an N-fixing symbiosis on Spirodela nitrogen uptake and assimilation through the application of enriched isotopes. Spirodela plants exposed to both NH4 and NO3 showed a preference for NH4 over NO3 in all medium
– high concentrations (8.5, 20.0 and 60.0 mg N/L), but did not differentiate between nitrogen forms at
low concentrations (0.5 mg N/L). Uptake rates were similar for NH4 and NO3 in treatments where only one form of nitrogen was available. Neither the application of antibiotics and/or fungicide had significant effects on the uptake rates of NH4 or NO3 by Spirodela sp. Preferential assimilation of NH4 may complicate the applicability of Spirodela SI values as an early indicator of inorganic eutrophication. However, similar uptake rates of plants treated with antibiotics and/or fungicide indicate that any symbiotic relationship with an N-fixing counterpart has minimal contribution to the N- pool assimilated by the duckweed.