Isotopic invisibility of protozoan trophic steps in marine food webs


"CSIRO ScienceImage 7595 Dunaliella" by CSIRO. Licensed under CC BY 3.0 via Wikimedia Commons

TitleIsotopic invisibility of protozoan trophic steps in marine food webs
Publication TypeJournal Article
Year of Publication2014
AuthorsGutierrez-Rodriguez A., Decima M, Popp BN, Landry MR
JournalLimnology and Oceanography
Date Published2014/09
Type of ArticleArticle
ISBN Number0024-3590
Accession NumberWOS:000345462100011
Keywordsamino-acids; atlantic; compound-specific delta-n-15; ecosystem; growth; mass-spectrometry; phytoplankton; sea; stable nitrogen isotope; Zooplankton

According to modern oceanographic perspectives that emphasize microbial pathways, phagotrophic protists comprise one to several levels of intermediate consumers between phytoplankton and larger metazooplankton (copepods and krill). However, recent attempts to quantify pelagic trophic structure in the open ocean using nitrogen stable isotope techniques have brought into question whether such measurements adequately account for protistan trophic steps. Here, we use a two-stage chemostat system, with Dunaliella tertiolecta and Oxyrrhis marina as a predator-prey model, to address this question experimentally. To investigate N-15 trophic discrimination under different conditions of nitrogen availability and recycling, Oxyrrhis was fed in the light and in the dark on phytoplankton provided with high and low nutrient ratios of N : P. We used both bulk and amino acids-compound specific isotopic analysis (AA CSIA) to distinguish trophic fractionation from changes in the delta N-15 values of phytoplankton (isotopic baseline). Results demonstrate that protistan consumers are not, in fact, significantly enriched in N-15 relative to their prey, a marked departure from the general findings for metazoan consumers. In addition, we show that changes in the isotopic baseline propagate rapidly through the protistan food chain, highlighting the need to account for this variability at ecologically relevant time scales. If protistan trophic steps are largely invisible or significantly underestimated using nitrogen isotope measurements, research that utilize such measurements in ecological, fisheries, and climate change studies may miss a large part of the ocean's variability in food-web structure and ecosystem function.


"Over the past 3 decades, oceanographic research highlighting the importance of microbial organisms and processes (Pomeroy 1974; Azam et al. 1983) has profoundly altered our understanding of community ecology, food-web dynamics, and biogeochemical transformations in the oceans. Prior to this microbial revolution, perceptions were blinded by the inability of existing methods to visualize the vast number of bacterial cells that reside in seawater (Azam et al. 1983). Despite progress in many areas, the results presented here suggest that contemporary methods similarly continue to obscure the full realization of microbial effects on food-web structure and energy transfer."

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