Table 1.
Details of planktonic foraminiferal specimens collected.
Fig 1.
Relative abundance of taxonomically assigned 16S rRNA gene sequences from six individual foraminifer specimens.
Two N. dutertrei (DUT55 and DUT59), three N. incompta specimens (INC25, INC27 and INC28) and one G. bulloides specimen (BUL34; [59]) are shown for comparison. Sequences are assigned to operational taxonomic units (OTUs) and have been grouped in the figure at higher levels of taxonomic classification for visual clarity (see key). Assignation of lower taxonomic ranks are discussed in the text, such as the OTUs grouped within the Gammaproteobacteria, in the order Vibrionales.
Fig 2.
Fluorescence micrograph of a DAPI-stained decalcified N. dutertrei cell.
(a) Diffuse autofluorescence can be observed throughout the cytoplasm. The observed ~2 μm diameter structures prevalent throughout the cell are consistent with the presence of picoeukaryotic algae. The white arrowhead denotes an example of the bright spots, 5–10μm in size that may be food vacuoles containing condensed prey items. The white rectangle denotes the area magnified in (b) where the black arrowhead highlights one of the ~2 μm diameter putative picoeukaryotes.
Fig 3.
Fluorescence micrographs of decalcified N. incompta and G. bulloides cells.
(a) High levels of background autofluorescence can be observed throughout the cytoplasm of the DAPI-stained decalcified N. incompta cell. However, in contrast to N. dutertrei (Fig 2), there are no abundant algal nuclei observable above background signals. In addition, in contrast to decalcified G. bulloides, where cyanobacterial cells can easily be detected (b) no bacterial cells can be observed in N. incompta.
Fig 4.
Transmission electron micrographs of pelagophyte cells inside N. dutertrei.
(a) Intact picoeukaryotic algae can be observed (Al) in close proximity to the cell periphery. The inner organic layer (IOL) is also clearly visible. A probable food vacuole can also be seen (Fv). (b) An individual algal cell with a clearly visible horse-shoe shaped chloroplast (Chl) with girdle lamella (GL) and discernible nucleus (N). (c) Two intact algal cells with obvious chloroplasts and nuclei can be seen above an algal cell being digested (DC). The nucleus and chloroplast of this cell are still observable, but the cell membrane appears no longer to be fully intact.
Fig 5.
Transmission electron micrographs of N. incompta.
(a) Low-magnification region of N. incompta showing the nucleus (N) and a fibrillar body (fb). (b) Higher magnification image shows a fibrillar body (Fb) and probable food vacuoles (black arrowheads). (c) A very small number of algal cells in early degradation (Al) were observed in some N. incompta individuals.
Fig 6.
Transmission electron micrographs of G. bulloides.
(a) The black arrowheads depict cyanobacteria of the genus Synechococcus [59] within the G. bulloides cytoplasm. (b) Higher magnification of a Synechococcus endobiont, with visible characteristic carboxysomes (white arrowhead).
Fig 7.
Proposed trophic interactions of N. dutertrei and N. incompta.
The illustrated trophic interactions are based on 16S rRNA gene metabarcoding performed in this study and laboratory observations reported in the literature. Black arrows indicate direction of heterotrophic carbon flow. We propose that N. dutertrei Type Ic is a mixotrophic feeder. Pelagophyte algae provide energy for growth through photosynthesis, either via direct consumption of the algae and/or by cross-membrane transport of photosynthates. In addition, N. dutertrei feeds on other protists and phytoplankton, either from the water column or from inside the POM feeding cyst. For example, within the feeding cyst N. dutertrei consumes Cercozoa that graze on the POM associated bacteria. In contrast, we propose that N. incompta Type II is a heterotrophic feeder. As zooplankton moult their chitin carapaces or die, their chitin becomes incorporated into the POM. Here chitinoclastic bacteria (eg. orders Vibrionales and Alteromonadales) break down the chitin to utilise the C and N source. N. incompta feeds on these orders of bacteria within its POM feeding cyst. N. incompta also feeds minimally on phytoplankton, but whether from the water column and/or from the feeding cyst is unknown (dashed black arrow).