Molecular approach (PCR-DGGE) to diet analysis in young Antarctic krill Euphausia superba

Abstract

Antarctic krill Euphausia superba Dana comprise a key component of the Southern Ocean food web, yet despite decades of research, questions concerning the regional, seasonal and ontogenetic differences in their diet remain. All current methods used to characterize krill diet have limitations for identifying the full complement of the diet. Using DNA as a marker molecule, our goal in this study has been to evaluate the efficacy of a PCR-DGGE (denaturing gradient gel electrophoresis) approach targeting the 18S rDNA gene to discriminate among diet constituents in gut and fecal pellet samples from young Antarctic krill relative to their feeding environment -the seawater and sea ice microbial community. We conducted 2 laboratory-based feeding experiments with known food items and 3 field samplings of both the krill and their feeding environment. Sequenced PCR-DGGE phylotypes from laboratory trials clearly distinguished diatom and copepod prey, while in situ feeding analyses revealed that a broad diversity of taxa were ingested, including diatoms (Bacillariophyta, the most prevalent group detected), dinoflagellates, cryptomonads, prasinophytes, ciliates, cercozoans, choanoflagellates, turbellarians and (possibly) sponge larvae. Band image analyses allowed environmental and diet phylotypes to be matched. On average, 32% of those from the environment were present in the diet; conversely, of the phylotypes detected in the diet, an average of 59% were in common with the environment. Changes in environmental phylotypes among sampling dates were reflected by similar changes in the krill diet as potential prey diversity (richness) decreased during a phytoplankton bloom.KEY WORDS: Antarctic krill · Euphausia superba · Diet · DGGE · 18S rDNA · Sea ice microbial community · Omnivory Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 319: [155][156][157][158][159][160][161][162][163][164][165] 2006 undigested hard parts such as copepod mandibles, diatom frustules, or tintinnid loricae (Hopkins & Torres 1989), gut fluorescence detects only algal pigments (Ross et al. 2004), and monoclonal antibodies target specific species of interest (Haberman et al. 2002). Biomarker methods analyzing fatty acid and stable isotope composition of krill have been found to target a wider variety of diet components, but are limited in the taxonomic resolution they provide (Frazer 1996, Alonzo et al. 2003. PCR-based molecular methods for detecting DNA offer promising new avenues for diet analysis (Symondson 2002), and are currently being used in marine systems (Jarman et al. 2002, Nejstgaard et al. 2003. A distinct advantage of using a molecule such as DNA to detect diet diversity is that all potential prey items have DNA, and thus detection of a dietary item will depend more on sensitivity to the molecular signal than on such group-specific biases as those mentioned above. Although a number of studies have utilized some form of DNA detection of prey in predator diets (e.g. Zaidi et al. 1999, R...

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Microbial dynamics in autotrophic and heterotrophic seawater mesocosms. II. Bacterioplankton community structure and hydrolytic enzyme activities

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