Tracking the past of the ocean’s tiny organisms

Spotlight on Research is the research blog I author for Hokkaido University, highlighting different topics being studied at the University each month. These posts are published on the Hokkaido University website.


While many people head to the ocean to spot Japan’s impressive marine life, graduate student Norico Yamada had a more unusual goal when she joined the research vessel, ‘Toyoshio Maru’ at the end of May. Norico was after samples of sea slime and she’s been collecting these from around the world. 

As part of Hokkaido University’s Laboratory of Biodiversity II, Norico’s research focusses on the evolution of plankton: tiny organisms that are a major food source for many ocean animals.

Plankton, Norico explains, are a type of ‘Eukaryote’, one of the three major super groups into which all forms of life on Earth can be divided. To make it into the Eukaryote group, the organism’s cells must contain DNA bound up in a nucleus. In fact, the esoteric group name is just an allusion to the presence of the nucleus, since ‘krayon’ comes from the Greek meaning ‘nut’. 

The Eukaryote group is so large, it contains most of what we think of as ‘life’. Humans belong to a branch of Eukaryotes called ‘Opistokonta’, a category we share with all forms of fungus. This leads to the disconcerting realisation that to a biodiversity expert like Norico, the difference between yourself and a mushroom is small. 

Of the five other branches of Eukaryote, one contains all forms of land plants and the sea-dwelling green and red algae. Named ‘Archaeoplastida’, these organisms all photosynthesise, meaning that they can convert sunlight into food. To do this, their cells contain ‘chloroplasts’ which capture the sunlight’s energy and change it into nutrients for the plant.

This seems very logical until we reach Norico’s plankton. These particular organisms also photosynthesise, but they do not belong to the Archaeoplastida group. Instead, they are part of a third group called ‘SAR’ whose members originally did not have this ability, but many acquired it later in their evolution. So how did Norico’s plankton gain the chloroplasts in their cells to photosynthesise? 

Norico explains that chloroplasts were initially only found a single-cell bacteria named ‘cyanobacteria’. Several billion years ago, these cyanobacteria used to be engulfed by other cells to live inside their outer wall. These two cells would initially exist independently in a mutually beneficial relationship: the engulfing cell provided nutrients and protection for the cyanobacteria which in turn, provided the ability to use sunlight as an energy source. Over time, DNA from the cyanobacteria became incorporated in the engulfing cell’s nucleus to make a single larger organism that could photosynthesise. The result was the group of Archaeoplastidas.

To form the photosynthesising plankton in the SAR group, the above process must be repeated at a later point in history. This time, the cell being engulfed is not a simple cyanobacteria but an Archaeoplastida such as red algae. The cell that engulfs the red algae was already part of the SAR group, but then gains the Archaeoplastida ability to photosynthesise. 

To understand this process in more detail, Norico has been studying a SAR group organism that seems to have had a relatively recent merger. Dubbed ‘dinotom’, the name of this plankton combines its most recent heritage of a photosynthesising ‘diatom’ plankton being engulfed by a ‘dinoflagellate’ plankton. Mixing the names ‘dinoflagellate’ and ‘diatom’ gives you the new name ‘dinotom’. This merger of the two algae is so recent that the different components of the two cells can still be seen inside the dinotom, although they cannot be separated to live independently.

From samples collected from the seas around Japan, South Africa and the USA, Norico identified multiple species of dinotoms. Every dinotom was the product of a recent merger between a diatom and dinoflagellate, but the species of diatom engulfed varied to give different types of dinotoms. As an analogy, imagine if it were possible to merge a rodent and an insect to form a ‘rodsect’. One species of rodsect might come from a gerbil and a bee, while another might be from a gerbil and a beetle. 

Norico identified the species by looking at the dinotom’s cell surface which is covered by a series of plates that act as a protective armour. By comparing the position and shape of the plates, Norico could match the dinotoms in her sample to those species already known. However, when she examined the cells closer, she found some surprises. 

During her South Africa expedition, Norico had collected samples from two different locations: Marina Beach and Kommetjie. Both places are on the coast, but separated by approximately 1,500 km. An examination of the plates suggested that Norico had found the same species of dinotom in both locations, but when she examined the genes, she discovered an important difference. The engulfed diatom that had provided the cells with the ability to photosynthesise were not completely identical. In our ‘rodsect’ analogy, Norico had effectively found two gerbil-beetle hybrids, but one was a water beetle and the other a stag beetle. Norico therefore concluded that the Marina Beach dinotom was an entirely separate species from the Kommetjie dinotom.

This discovery did not stop there. Repeating the same procedure with a dinotom species collected in Japan and the USA, Norico found again that the engulfed diatom was different. In total, she found six species of dinotoms in her sample, three of which were entirely new. 

From this discovery, Norico concluded that the engulfing process to acquire the ability to photosynthesis likely happens many times over the evolution of an organism. This means that the variety of species that can mix is much greater, since the merger does not happen at a single point in history. Multiple merging events also means that the organism can sometimes gain, lose and then re-gain this ability during its evolution. 

Next year, Norico will graduate from Hokkaido and travel to Osaka prefecture to begin her postdoctoral work at Kwansei Gakuin University, where she hopes to uncover still more secrets of these minute lifeforms that provide so much of the ocean’s diversity.