Newly Identified Branch of Marine Eukaryotes on the Tree of Life

We’re only just now starting to get a grasp on the sheer amount of global biological diversity, most of which has been very difficult to observe with conventional observational means.  Changes in technology and sampling strategies have resulting in the acquisition of information regarding many previously undocumented forms of biological life.  Along with microorganisms associated with plant roots – the strict focus on my research interests – phytoplankton represent a large group of organisms that we still know little about.  For selfish reasons I was interested in this study because I wanted to see how these authors addressed ways of learning more about a previously unknown lineage of ocean phytoplankton.  As evidenced by next generation sequencing efforts, there are many unknown and undescribed fungi in soils and there is a huge amount of commonality of the diversity of microbial life in soils and oceans.

Published in the Proceedings of the National Academy of Sciences, a study entitled “Newly identified and diverse plastid-bearing branch on the eukaryotic tree of life”, by Kim et al, describes a recently identified and previously uncultured marine and freshwater microalgal lineage of Eukaryotic organisms.  The researchers title this group of phytoplankton the rappemonads, from the initial paper (authored by Rappé et al 1998) that reported unknown DNA sequences from this lineage.  The researchers designed nucleotide primers and fluorescent probes from initial DNA sequences (from the Rappé et al study) and used these molecular diagnostics to observe marine and freshwater samples for their presence or absence of these unknown organisms.

Phylogenetic analysis of environmental nucleotide sequences revealed that rappemonads are related to both haptophyte and cryptophyte algae but constitute a diverse and independent lineage.  To resolve the phylogenetic position of the rappemonads the authors designed specific nucleotide primers spanning the 18S-ITS1-5.8S-ITS2-28S rRNA genes and sequenced this gene cluster.  The authors used maximum likelihood algorithms to construct a phylogeny, which resolved the rappemonads between the haptophyte and cryptophyte algae.  It should be made clear that there is low branch support (at around 50) for some of these clades, so more data is needed for strict resolution of the red plastid algae.

Probes for fluorescent in situ hybridization were developed to observe rappemonads.  Rappemonads were described to be relatively large in size – approximately 6 µm in diameter versus the smaller picophytoplankton (2 to 3 µm) – significantly larger than open-ocean phytoplankton.  Rappemonads appear to contain two to four plastids and are putatively photosynthetic.

Using quantitative PCR methods, the authors identified high concentrations of rappemonads in late-winter blooms along the surface waters at a site in the Sargasso Sea.  Rappemonads were rare or absent in stratified summertime conditions, when concentrations of chlorophyll containing microorganisms are at their highest in deep waters.  Rappemonads were frequently found in North Pacific anticyclonic eddy samples, which are characterized by colder more nutrient-rich waters that have been brought to the sea surface.  When considering water characteristics (such as depth, salinity, phosphate, nitrate, and nitrite), there were no statistical significance between samples containing rappemonads and those where they were absent.  In addition, rappemonads were found in both marine and freshwater conditions, bringing into question when and where one may find these organisms and which would warrant further study.