Incorporation of nitrogen from N₂ fixation into amino acids of zooplankton

Abstract

Eurytemora affinis (Copepoda) were fed 15 N-labeled Rhodomonas salina (Cryptophyta) or 15 N-labeled Nodularia spumigena (Cyanobacteria) in excess under controlled laboratory conditions. Zooplankton collected from the Baltic Sea were fed natural phytoplankton amended with 15 N-labeled N. spumigena. We quantified the direct incorporation of 15 N tracer from N 2 -fixing N. spumigena (diazotroph nitrogen) and ammonium-utilizing R. salina into the amino acid nitrogen (AA-N) of zooplankton using complementary gas chromatographycombustion-isotope ratio mass spectrometry, gas chromatography-mass spectrometry, and elemental analysisisotope ratio mass spectrometry approaches. Specific and mass-specific TN and AA-N incorporation rates of the 15 N tracers were calculated for zooplankton. Highest incorporation of 15 N was found in field zooplankton relying on N. spumigena and in E. affinis relying on R. salina. Lowest incorporation was found in E. affinis relying on N. spumigena. Decreasing specific and mass-specific rates during field experiments possibly were due to food shortage, whereas decreasing rates in E. affinis grazing on R. salina were more likely due to satiation. Specific and mass-specific rates were consistently low in E. affinis when exposed to N. spumigena, suggesting that these animals were reluctant to feed on N. spumigena. Essential isoleucine received most of the diazotroph nitrogen in field zooplankton, while nonessential amino acids received most 15 N tracer in E. affinis. N. spumigena was clearly an important amino acid nitrogen source for Baltic Sea zooplankton.

Incorporation of diazotrophic fixed N2 by mesozooplankton — Case studies in the southern Baltic Sea

“…Burns and Xu, 1990;Kozlowsky-Suzuki et al, 2003). A high incorporation rate of diazotroph nitrogen into amino acid nitrogen was also observed by Loick-Wilde et al (2012). Although grazing on cyanobacteria proves to be of more relevance than previously accepted, the majority of diazotrophic N is expected to remain in the particulate fraction, is remineralized or is lost by sedimentation.…”

“…Maximum rates of our study even exceed rates published in the former study. Still, it has to be noted that rates might have been over estimated during our studies, because we did not purge the guts of mesozooplankton species to remove non-digested 15 N 2 labelled food, as Loick-Wilde et al (2012) did. Food particles ingested by zooplankton might not have been incorporated into biomass, but either lost by defecation or after assimilation excreted as dissolved nitrogen (e.g.…”

“…Mean values presented here are in the same range as published by Loick-Wilde et al (2012), who used the same stable isotope tracer addition approach, determining values of 3 μg N 100 μg −1 PON h −1 . Maximum rates of our study even exceed rates published in the former study.…”

Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N<sub>2</sub> fixation activity and diversity in the western tropical South Pacific Ocean

“…Regarding higher trophic levels, the low 15 N isotopic signature (δ 15 N) of zooplankton reveals that N 2 fixation can significantly contribute to zooplankton N requirements in high N 2 fixation areas (Aberle et al, 2010;Landrum et al, 2011;Loick-Wilde et al, 2012;Mompeán et al, 2013;Montoya et al, 2002;Sommer et al, 2006;Wannicke et al, 2013;Hunt et al, 2016). Few studies report active grazing of Trichodesmium by some specific copepods (Micro-and Macrosetella sp., O'Neil et al, 1996;O'Neil and Roman, 1992).…”

Contribution and pathways of diazotroph-derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

“…The δ 15 N values of zooplankton reflect the balance between these contrasting N sources, the relative contributions of which can be estimated using a two-part mixing model (Montoya et al, 2002). This modelling approach has been used to demonstrate a significant contribution of diazotroph-derived N (DDN) to particulate matter and zooplankton biomass (Aberle et al, 2010;Landrum et al, 2011;Loick-Wilde et al, 2012;Mompean et al, 2013;Montoya et al, 2002;Sommer et al, 2006;Wannicke et al, 2013), and transfer of DDN beyond zooplankton to micronekton (Hunt et al, 2015). However, despite this measured contribution of DDN, the predominant pathways of DDN into marine food webs are still in question (Wannicke et al, 2013).…”