Rare Marine Amoeba Rediscovered From Coastal Seawater in Japan, Prompts Scientists To Propose New Taxonomic Classification

Amoebas are fascinating microorganisms that can inhibit different types of environments, including marine ecosystems. In a recent study, a team of marine biologists embarked on a research expedition to study the biodiversity of marine amoebas. During their exploration, they stumbled upon a strange amoeba species that closely resembled the long-lost Rhabdamoeba marina.

Unusual Marine Amoeba

Rhabdamoeba marina is a tiny marine amoeba that was first discovered and described in England in 1921. For over a century, only two reported cases of R. marina sightings have been reported.

This microorganism has amoeboid cells, which are characterized by near immobility. They can produce flagellated cells with two rearward-extending flagella through budding under particular conditions of prey scarcity. Despite these distinct characteristics, the taxonomic classification of Rhabdamoeba marina has remained unverified with an uncertain phylogenetic position.

It is generally accepted that heterotrophic protozoa, especially flagellates, constitute a significant group regulating bacterial abundance in marine ecosystems. However, it was also shown that amoeba can be numerically significant on surfaces and in the water column attached to flocs and benthic habitats.

For centuries, the systems of classifying organisms, including amoebas, were based on the similarities in their observable characteristics and morphology. From this classification, there is no coherent group of organisms that can be referred to as amoeba, according to Sutherland Mciver from the University of Edinburgh. Instead, an amoeba is identified as any protozoan cell that moves by crawling.

Molecular phylogenetics has changed the system of taxonomic classification for eukaryotes. In this method, scientists compare the similarities and differences in particular DNA sequences within organisms, allowing them to discern how closely related they are.

Historically, amoebas were grouped in a single taxonomic category called Sarcodina, distinguished by their use of pseudopodia. However, this system of classification did not capture the evolutionary relationships between the various types of amoebas.

Rediscovery of Rare Species

Researchers at the University of Tsukuba have successfully established a culture strain of Rhabdamoeba marina from coastal seawater obtained from Tottori Prefecture in Japan. The team, led by Takashi Shiratori and Ken-Ichiro Ishida, performed a molecular phylogenetic analysis using the small subunit ribosomal RNA gene sequence.

By analyzing the genetic sequence of this strain, they discovered that this marine amoeba does not align with its previously assumed taxonomic classification. Instead, it was found to be closely related to chlorarachnid algae within the phylum Cercozoa. Because of this, the research team proposed that Rhabdamoeba marina branched as a basal lineage of Chlorarachnea. This group of marine photosynthetic algae also belongs to the phylum Cercozoa within the supergroup Rhizaria.

The study not only allowed experts to unveil the gene sequence of the scarcely encountered amoeba but also helped clarify its phylogenetic position. More importantly, it highlights the importance of observing environmental samples to rediscover unicellular organisms that lack genetic data. These efforts are all indispensable for understanding microbial diversity.