Carnivorous plants of the deep

The discovery of living phytoplankton at 4000 m depth during the Malaspina expedition raised new questions about phytoplankton ecology. Guo et al. (2018) found some answers by literally digging deeper: they sampled pico- and nanophytoplankton (PN) from the Mariana Trench up to a depth of 8320 m. PN were identified using the V1-V3 region of the 18S rRNA gene (for eukaryotes) and the plastid 23S rRNA gene (mainly for bacteria). Overall PN diversity was virtually constant between 4 and 8320 m. Dominant OTUs across all depths belonged to the phylum Dinoflagellata and family Prochloraceae. Interestingly, dominant eukaryotic PN at 8320 m belonged to the classes Chrysophyceae and Eustigmatophyceae, which are known to be capable of heterotrophy.

This study significantly increased our knowledge on deep sea plant communities. Nonetheless, it remains to be seen whether the main metabolic pathway of deep PN is heterotrophy. Other explanations for deep PN may be that they survive as resting cysts or sink more quickly than previously thought. However, the latter is unlikely because Guo et al. (2018) showed that there are characteristic deep PN communities. Either way, future research in this field will have profound implications for our understanding of the oceanic carbon cycle!

Guo, R., Liang, Y., Xin, Y., Wang, L., Cao, C., Xie, R., Zhang, C., Tian, J. & Zhang, Y. (2018). Insight into the pico- and nano-phytoplankton communities in the deepest biosphere, the Mariana Trench. Frontiers in Microbiology, 9, 2289.