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Urea metabolism by the marine cryptomonad, Rhodomonas Lens Johnson, Sherran Ruth

Abstract

The effect of urea on a marine cryptomonad, Rhodomonas lens, was investigated. Cells from a nitrate enriched, natural sea water medium were acclimated to synthetic sea water with either nitrate (control medium) or urea (experimental medium) as the sole nitrogen source. Cells acclimated readily to the control medium; acclimation to the experimental medium was erratic. Control and experimental cells were taken from media on which good growth habitually occurred. Tested optimal growth conditions occurred at 22°C and a 16:8 hour, light:dark cycle with fluorescent light at approximately 150 foot-candles. Growth was followed turbidimetrically and by direct cell count. Control cultures exhibited the typical lag, logarithmic and stationary phases. The stationary phase merged into a gradual decline phase. Culture colour proceeded from a red-pink during early phases to a red-orange, then green during stationary-decline phase. Initially, ultrastructure was typical of cryptomonads: cells exhibited a bi-lobed chloroplast having thylakoids, often arranged in bands of two, with a loculus size of 110-260 Å. Occasional lipid droplets were present within the cytoplasm. Stationary-decline phase showed an accumulation of a lipid-like material. Thylakoids were arranged singly; loculus size being decreased to 70-150 Å. Experimental cultures at concentrations less than 1.25 mM urea exhibited similar growth to control cultures. At urea concentrations equal to or greater than 1.25 mM urea, cells during lag and early logarithmic phases showed a lipid-like accumulation; during later logarithmic phase, lipid droplets were observed. Stationary phase was short and cells proceeded directly into a rapid decline phase. Concomitantly, an increase in pH occurred and cultures became a creamy white in colour. To test electrolytic and pH effects, NaOH and NH₄OH were added to a healthy control culture. Increased pH with NaOH had no gross effect on the cultures: increased pH with NH₄OH caused a lytic effect similar to lysis observed during decline phase of experimental cultures. Observations on control cells during stationary-decline phases suggest a lack of available nitrogen, probably due to depletion: observations on experimental cells during lag and early logarithmic phases are similar, thus suggesting a lack of nitrogen. As sufficient urea is present, it is suggested that the uptake and/or metabolism of urea in experimental cells is inefficient, causing nitrogen stress. Increase in pH prior to decline phase, and the results of electrolytic experiments suggest that urea is converted into ammonia and excess ammonia is released into the medium. The increasing ammonia concentration becomes toxic and accelerating cell lysis thus results in a decline phase.

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