If it can be cultured, we can kill it: The first glimpse of a globally abundant microbial predator

The diplonemids (Euglenozoa:Excavata) are a group of heterotrophic, biflagellated protozoa. To date, only two genera have been formally described and are both characterised as either benthic phagotrophs or opportunistic parasites. The diplonemids shot to fame in the field of marine microbiology following the publication of de Vargas et al (2015). Using data from Tara Oceans, the authors discovered a vast diversity and abundance of this group orders of magnitude higher than classical plankton groups, such as the foraminifera. The diversity and abundance of these protists could make them among the most abundant global heterotrophs, yet these diverse lineages have never been seen – our understanding of them is limited to 18S rRNA sequences. Therefore, Gawryluk and colleagues (2016) conducted further investigation into these enigmatic organisms.

The authors used single cell genomics (SCG) and light microscopy to characterise ten marine diplonemids isolated from the Californian coast. SCG sequencing revealed the genomes of the isolates to exhibit a high abundance of non-canonical introns (lacking the conventional GT-AG splice sites) in nuclear genes. Analysis suggests that these introns may represent novel, mobile active elements spliced at the transcriptional level that, while present across the euglenozoa, rapidly diversified in the marine, ‘planktonic’ diplonemids. Obscure genomic peculiarities such as this stress the importance of establishing stable diplonemid cultures for further genomic and transcriptional exploration and may prove a hindrance in further large-scale, automated metagenomic studies.

As well as this, the authors were able to infer ecological insights about the group from the annotated SCG data. Fragments of prasinophyte algal contigs were found in some isolates suggesting that certain eukaryotic taxa may serve as prey for the diplonemids. Simple light microscopy further corroborated this, showing the presence of large, green bodies in the food vacuoles of diplonemid cells. The fusion of traditional microscopy and next generation sequencing by the authors was a major strength of this work, and the light micrographs captured of the isolates were the first ever of what may be the most abundant predators on Earth – a thrilling achievement splashed across scientific news outlets.

Overall, this paper is a triumph in making the first leap into characterising a potentially vastly important and recondite taxon. It may be easy to criticise the small sample size as a methodological weakness, but this must be weighed against the novelty and difficulty of studying such a group. This work typifies the high-impact short communication – a brief description of an exciting and upcoming subject by big names in eukaryotic microbiology.  Only by overcoming hurdles associated with culturing marine organisms and taxon-specific HTS artefacts, can deeper understanding be gained.

In future work, I would be fascinated to see established cultures of diplonemids subjected to phagotrophy and chemotaxis assays, to gain a real-world understanding of their trophodynamics. This could have serious implications to understanding global carbon flow. It would also be a shame not to see this hybrid approach of microscopy and SCG not exploited to study other enigmatic marine eukaryote groups, such as MALV II, as morphology and microanatomy can define the physiology of eukaryotic cells. Our understanding of marine diplonemids has only just begun.

Reviewed Paper: Gawryluk, R. M., del Campo, J., Okamoto, N., Strassert, J. F., Lukeš, J., Richards, T. A., ... & Keeling, P. J. (2016). Morphological identification and single-cell genomics of marine diplonemids. Current Biology, 26(22), 3053-3059. http://www.sciencedirect.com/science/article/pii/S0960982216310624

Also: De Vargas, C., Audic, S., Henry, N., Decelle, J., Mahé, F., Logares, R., ... & Carmichael, M. (2015). Eukaryotic plankton diversity in the sunlit ocean. Science, 348(6237), 1261605 http://science.sciencemag.org/content/348/6237/1261605