ized Nitrogen Metabo ism in a Freshwater n kter, Chrysochromu a bre viturrita John D. Wehr,' Lewis M. Brown, and Kathryn O'Grady Department of Plant Sciences, The University of Western Ontario, bondon, Ont. NQA 5687 Wehr, j. D., b. M. Brown, and #. O'Grady. 1987. Highly specialized nitrogen metabolism in a freshwater phytoplankter, Chrpocbrrsmu%ina brevitussita. Can. j. Fish. Aquat. Sci. 44: 736- 742. A field and laboratory culture study was carried out on the nitrogen metabolism of isolates sf the freshwater phytoplankter Chsysocbrrornulim breviturrita Nich. (Pryrnnesisphyceae). These were isdated from two different softwater lakes, one believed to be influenced by acidic precipitation (Cinder Lake) and another wkich was experimentally acidified with HrSB, (Lake 302-South). The alga was able to utilize only NH4' as an inorganic N source. A range sf irradiances and molybdenum concentrations failed to induce NOr- utilization. Among 17 organic N compounds including amino acids, purines, and other amines, only urea plus Ni2' as a cofactor would serve as the sole N source for this species, Nonetheless, growth rates in media supplied with urea were significantly less than with NH4'. Field data from Lake 302-S indicate that a predominanceof NH4' versus NO,- as the major inorganic N species may have favored the development of a Chrysochromudipra-dominated community during August 1984. A detailed depth profile also indicated that a rnetalimnetic peak (>20 x 1 o6 ce!Is/L) of this alga coincided with a distinct NH4' depletion, which occurred at no other time during the year. Experiments with isolates of C. brevitussita and a Nadanochdsris sp. (Chlorophyceae) (-1 Frn in diameter) from this community indicated that the former alga possessed a highly specialized N metabolism much like the Cinder Lake isolate. The Nawwoch%oris sp. from the same envirsnrnerpt grew on NO3- and NH4' equally well. It is suggested that the specialized NH4' utilization by C. breviturrita may itself influence the pH regime of poorly buffered waters through selective N H, ' uptake and H' generation. Les auteurs ont & d i e sur le terrain et en Baboratoire le metabolisme de I'azote chez des isolats de !'organisme phytoplanctoniqkse d'eau douce Chrysochromulina breviturrita Nich. (pryrnnesiophyc6es).bes organisn~es snt ete preIev6-s dans deux lacs d'eau douce diffkrents. Le premier, Ie lac Cinder, semble subir les effets des pr6cipitations acides. Le second, /e lac 302-sud, a ete experirnentalement acidifie au HaSO4. Les algues ne puvaient utiliser que le NH,' cornme source d'azote insrganique. L'utilisation d'une garnrne vari6e d'irradia- tiows et de diverses concentrations de molybdene n'a pas permis d'induire I'utilisatisn du NO,-. Des 17 cornposks anstes organiques utilises, notarnment des acides arnines, des purines et d'autres amines, seule I'uree (avec du Ni" comme co-facteur) 6tait utilisbe comme source d'azote par cette esgce. kes taux de croissance obtenus en milieux enriehis 2 I'uree ktaient cependant significativement infkrieurs A ceux obtenus en milieux enrichis au NH,'. Les donnees obtenues sur le terrain au lac 3892-5 indiquaient que la prkdominance du NH4'par rapport au NO3- comme principale substance azstee insrganique avait pu favoriser Ifexpansion, en aoQt 1984, d'une communaut6 dsrninke par Bes Chrg/sssAromulina. Un profis des profondeurs detail l6 indiquait aussi I'existence d'uw pic metalimnetique (>20 x 1 (I6 cellules/ L) de cette algue qui csi'ncidait avec une nette carence de NH,', phenomene qui ne s'est produit A aucun autre moment de I'annee. Des essais portant sur des isolats de C. brevitusrita et de Nannocbrlosis sp. (chlorophyce+es) d'envison 1 pm de diametre prelevbs de cette cornmu- mute ont montre que la premiere algue possedait un rnktabolisme fortement spkcialise de I'azote qui ressemblait beaucoup i celui not6 pour I'isolat du lac Cinder. Les Nannoshloris sp., qui provenaient du m6me environne- ment, utilisaient aussi bien te NO3- que le NH,'. Les auteurs $mettent i'hypothese que Iputi8isatisn specialisee du NH,' par C. brevitursita peut en elle-m6me influer sur le regime du pH des eaux ma1 tamponnees de par une assimilation selective du NH4' et la production de H'. Received August 8, I986 Accepted November 24, 1986 (J8898) 'hile phosphorus is widely recognized as the nutrient wkich most commonly regtslates algal productivity in Bakes (Schindler 1977; Wetzel 1983), nitrogen may influence algal biomass in some systems and may also affect species composidoaa (Kalff 197 1; Liao and Lean 1978). Phytoplankton growth in Castle Lake, a mesooligotrsphic lake in California, is limited at least during part of the year by N availability and regeneration '~utkasr to whom correspondence should be addressed. Present address: Department of Biological Sciences, Larkin Hall, Fordham University, Bronx, NY 10458, USA. (Axler et al . 1 982). Several forms sf N are bnsaaally present and are recycled in freshwaters. The cycling of ammonium (NH4+) is particularly rapid (a few hours), wkich indicates a close coupling between regeneration and assimilation (Brezonik 1992; AxIer et aI. 1981). In oligotrophic, granitic basins, NH4+ inputs are frequently derived in l u g e part from atn~ospheric sources such as gaseous forms and precipitation (Likens et al. 1977). In suck systems, which are also influenced by acidic precipitation, inputs sf NH,' are retained much more strongly than nitrate (NO3-) (Galloway et al. 1983). This could be significant, particu%arly as NH4+ represents on average the second most abundant 736 Can. 9. Fish. Aqmt. Sci., Vof. 44, 1987 Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Renmin University of China on 06/03/13 For personal use only.