Zooplankton Predation
Marine phytoplankton fix ca. 30 - 50 *10^9 tons of carbon annually, accounting for as much as 40% of global primary production. Much of this carbon is remineralized in the euphotic zone upon cell death related to, in large part, zooplankton predation and viral lysis. Zooplankton also play a central role in transferring energy to higher trophic levels. Marine phytoplankton and their predators therefore represent a key link in global carbon turnover and foodweb dynamics.
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Historically, the identification of mesozooplankton prey has been achieved via gut content analysis for morphologically identifiable parts or HPLC analysis for diganostic pigments. Identification of phytoplankton by morphological analysis requires extensive taxonomic expertise, is time consuming, and cannot provide evidence for those species lacking hard parts. It is also typically plagued by large quantitative uncertainty. Pigment analysis of zooplankton gut contents or fecal pellets is more robust, but is limited by the fact that it can only classify algae into large groups via diagnostic pigments. Pigments degrade rapidly during digestion, resulting in underestimations, and many algal groups lack specific marker pigments.
More recently, DNA based approaches have been applied to study zooplankton feeding. Generally, it is expected that molecular detection provides greater species resolution than traditional approaches . Polymerase chain reaction (PCR) products can also be examined by cloning and sequence analysis, and molecular detection may provide information when non-pigmented prey such as ciliates or dinoflagellates are being utilized as prey.
In this context, we have been examining the rbcL sequence diversity of mixed phytoplankton assemblages in seawater and copepod guts. rbcL encodes the catalytic subunit of RubisC/O, the key enzyme in the Calvin cycle, which is the primary carbon fixation pathway in all major marine phytoplankton groups. rbcL is not found in the genomes of metazoans and copepod DNA therefore does not interfere with the analysis as it might if the 18S rRNA gene were targeted.
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Figure 1. Consensus tree of Form ID rbcL sequences obtained by cloning and sequencing of rbcL PCR products from ambient seawater and whole copepods. Sequences were translated to amino acids before bootstrapped (5000 replicates) Neighbor-Joining analysis using the Tajima-Nei distance method. Closest relatives to unknowns were obtained by blastX of DNA sequences against the NCBI NR database and species names are shown on the tree. OTU names begin with the sampling station (F0,F1, F2A, and F5), followed by the form for which rbcL primers were specific (form ID or A/B), and end with either ‘GS’ for ‘Gut Sequence’ or ‘SW’ for Sea Water’ sequences. Sequences detected in whole copepod DNA are also shaded in grey. |
References:
Boling, W.B., G.Sinclair and B. Wawrik. Molecular Detection of Calanoid Prey Species by Gut Content Analysis. In review (Marine Biology).
Nejstgaard, J. C., M. E. Frischer, C. L. Raule, R. Gruebel, K. E. Kohlberg, and P. G. Verity. 2003. Molecular detection of algal prey in copepod guts and fecal pellets. Limnol Oceanogr Methods 1:29-38.
Nejstgaard, J. C., M. E. Frischer, P. Simonelli, C. Troedsson, M. Brakel, F. Adiyaman, A. F. Sazhin, and L. F. Artigas. 2008. Quantitative PCR to estimate copepod feeding. Mar Biol 153:565-577.
Nejstgaard, J. C., L. J. Naustvoll, and A. Sazhin. 2001. Correcting for underestimation of microzooplankton grazing in bottle incubation experiments with mesozooplankton. Mar Ecol Prog Ser 221:59-75.