Developing Molecular Tools to study Trophic Interactions in zooplankton and their implementation in a vent system -Norwegian Sea Hydrothermal Vents
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Understanding hydrothermal plumes in the Arctic and Nordic Seas is an important objective for ecosystem assessments of these habitats. Very little is known about what role the hydrothermal plumes play in the Nordic Seas. Recent visual observation and echo-sound imaging suggest that this habitat is densely inhabited by larger zooplankton compared to the surroundings. In this thesis I focused particularly on the diet of a pelagic predator that is commonly found in plumes of the Nordic Seas, the amphipod Themisto abyssorum (Paper III). The diet of T. abyssorum in the deep-sea and at hydrothermal vents was previously unknown and new tools were needed to access this information. Therefore, we developed a molecular assay to help us find and identify predator-prey interactions that are difficult to detect using classical microscopy methods (Paper I and II). We also conducted a genetic survey of the pelagic eukaryote microorganism community at the Jan Mayen Vent Field, Loki’s Castle and the Håkon Mosby Mud Volcano (Paper IV) to understand the vent impact on these communities.
Molecular tools have become important in predator-prey interaction studies where dissection can be inadequate. The most common approach is to use specific primers that target known prey taxa. However, to study the complete range of prey items consumed we needed to work with universal primers. We therefore developed an application for the Denaturing High-Performance Liquid Chromatograph (DHPLC) for separating amplicons in a mixture, in order to study the total prey DNA from stomach contents. This technique was developed as part of a study of predator-prey interactions of the copepod Limnocalanus macrurus from the Bothnian Bay in the Northern part of the Baltic Sea (Paper I and II). The DHPLC assay was efficient, unbiased and could be used for any predator-prey interaction (Paper II). More than 30 taxa (at different taxonomic levels) were identified from the samples suggesting highly carnivorous feeding preferences. The assay development in Paper I and II confirmed that it was possible to explore stomach DNA using universal primers without significant bias from predator DNA.
Until now, dissection and microscopy have been the most common tools used to study trophic interactions of T. abyssorum. However, sole use of such tools may have caused an underestimation of prey diversity, particularly among those prey items not visible in the microscope. Themisto spp. are very useful as target species in deep-sea studies with long haul times because they have long gut passage time, so prey items are not digested as quickly as in some other species. The novel technique for excluding predator amplicons developed in Paper II was used to analyse the gut content of T. abyssorum. Specimens from three localities; The Jan Mayen Vent Fields, The Loki’s Castle Vent Field and The Håkon Mosby Mud Volcano, were analysed using a 3-500 bp long 18S rRNA sequence. The number of Operational Taxonomical Units (OTUs) reported in Paper III was highest in T. abyssorum guts from Loki’s Castle while the Jan Mayen samples had the lowest numbers of prey. The most abundant prey was calanoid copepods, supporting previous findings. However, the molecular approach has also revealed soft-bodied prey and possibly detritus. In summary, our study showed much higher diversity of prey than previous studies and the wide range of taxa found also indicates that T. abyssorum is likely highly omnivorous.
In Paper III, our data indicates that the localities were quite different from each other in terms of prey diversity and we therefore hypothesised that biodiversity of hydrothermal vents might be more closely linked to the general water masses of the Nordic Seas than to the hydrothermal activity alone. A molecular diversity assay was used to describe the eukaryote community of the hydrothermal vents in the Norwegian Sea (Paper IV). Water samples from five localities and from three different depths were collected. The five localities were Jan Mayen Vent Fields with a reference station, Loki’s Castle with a reference station and the Håkon Mosby Mud Volcano. The results obtained from Paper IV support the hypothesis from Paper III, where water masses gave the strongest effect followed by depth, and thirdly a differentiation between Loki’s Castle and the reference station indicated an effect of the hydrothermal vent.
Papers I, II and III illustrate the power of molecular markers in dietary studies, generating a more in-depth understanding of predator-prey interactions. In addition, both the stomach content analysis in Paper III and the genetic survey of the water column in Paper IV suggests that the plankton taxa composition was not vent specific. The results illustrate the need to go deeper and further into a complete inventory of the plankton community surrounding vents and seeps. Furthermore, several technical challenges regarding sampling for stomach content analysis are yet to be solved and an even better understanding of predator-prey interactions depends on that development.