Abstract
Uncovering the complexities of trophic and metabolic interactions among microorganisms is essential for the understanding of marine biogeochemical cycling and modelling climate-driven ecosystem shifts. High-throughput DNA sequencing methods provide valuable tools for examining these complex interactions, although this remains challenging, as many microorganisms are difficult to isolate, identify and culture. We use two species of planktonic foraminifera from the climatically susceptible, palaeoceanographically important genus Neogloboquadrina, as ideal test microorganisms for the application of 16S rRNA gene metabarcoding. Neogloboquadrina dutertrei and Neogloboquadrina incompta were collected from the California Current and subjected to either 16S rRNA gene metabarcoding, fluorescence microscopy, or transmission electron microscopy (TEM) to investigate their species-specific trophic interactions and potential symbiotic associations. 53–99% of 16S rRNA gene sequences recovered from two specimens of N. dutertrei were assigned to a single operational taxonomic unit (OTU) from a chloroplast of the phylum Stramenopile. TEM observations confirmed the presence of numerous intact coccoid algae within the host cell, consistent with algal symbionts. Based on sequence data and observed ultrastructure, we taxonomically assign the putative algal symbionts to Pelagophyceae and not Chrysophyceae, as previously reported in this species. In addition, our data shows that N. dutertrei feeds on protists within particulate organic matter (POM), but not on bacteria as a major food source. In total contrast, of OTUs recovered from three N. incompta specimens, 83–95% were assigned to bacterial classes Alteromonadales and Vibrionales of the order Gammaproteobacteria. TEM demonstrates that these bacteria are a food source, not putative symbionts. Contrary to the current view that non-spinose foraminifera are predominantly herbivorous, neither N. dutertrei nor N. incompta contained significant numbers of phytoplankton OTUs. We present an alternative view of their trophic interactions and discuss these results within the context of modelling global planktonic foraminiferal abundances in response to high-latitude climate change.
Highlights
Food web networks represent a range of pathways in ecological communities including predator-prey interactions, symbiotic associations, nutrient uptake and remineralisation that enable characterization of the transfer of nutrients and energy between species and trophic levels
The partial small subunit (SSU) rRNA gene sequences amplified from specimens DUT55 and DUT49 identified them as N. dutertrei Type Ic
Type I has been found throughout the North and South Atlantic, while only Type II has been identified within the northeast Pacific waters of the California Current [21, 57, 58], within our study area
Summary
Food web networks represent a range of pathways in ecological communities including predator-prey interactions, symbiotic associations, nutrient uptake and remineralisation that enable characterization of the transfer of nutrients and energy between species and trophic levels. This provides a basis for understanding large-scale ecosystem processes such as community structure and biogeochemical cycles. The planktonic foraminifera are comparatively easy to collect (plankton net tows and scuba diving) and isolate using stereo microscopy, due to their relatively large size (50–1000μm) They are identifiable to the morphospecies and small subunit (SSU) ribosomal (r)RNA gene bar code level [19]. They represent an ideal organism for testing high-throughput DNA sequencing methodologies in the protists
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