1. Why do we study this topic?

Phytoplankton produce nearly half of the global oxygen through photosynthesis. These unicellular microalgae occur in the oceans where species proliferate in so-called algal blooms until the resources in the water are used up and the population gives way for succeeding species.

       Figure 1 NASA reconstruction of the ocean chlorophyll a concentration which is indicative of photosynthetic activity of phytoplankton.

 The performance of unicellular species has thus important implications for global biogeochemical cycling. The decline of an algal bloom in nature is usually due to the exhaust of the nutrients and when limiting elements are again available, the bloom will reoccur. In the ocean this re-fertilization can be triggered by currents that transport the cells to favorable environments. We have chosen ecologically important diatom, Coscinodiscus radiatus (a bloom-forming dominant species in the North Sea of Germany) to simulate this natural limitation / re-fertilization process in the laboratory, and studied the physiological and metabolic changes of individual cells and the role of associated symbiotic bacteria during recovery. 

2. How did we discover the age-related vesicle production from diatom?

The species we chose for our experiments was a diatom with large cells, in the range of 50-200 microns in diameter. The algae provided us with the opportunity to observe the dynamics of even single sorted cells under a variety of conditions using simple optical microscopy. Spending a lot of time observing the cellular development of algae led us to discover an interesting phenomenon.

Figure 2 Young cells (left) and old cells (right) of diatom Coscinodiscus radiatus.

Young cells are lightly pigmented and divide readily, while old cells that have been reared for a couple of days under nutrient limitation start to accumulate more pigments and finally die (See Figure 2). As we placed the older cells in fresh medium, we observed that these cells began to produce bubbles (extracellular vesicles), and that the vesicle-producing cells did not go to death. Instead, they rejuvenated and restarted cell division.

Figure 3 A vesicle producing old cell. The process precedes re-initiation of cell division.

 We concluded that we had discovered a previously unknown rejuvenation mechanism and decided to study this phenomenon in depth. Our paper describes the physiology, chemistry and function of the vesicles in detail, and it adds the observation that vesicle production can be even controlled by associated bacteria. This highlights the complex organization of the ocean microbiome.

3. Addressing challenges in a cooperative manner

We are a group of chemical ecologists that is devoted to the study an ecologically important, but non-model, organism. It is challenging to conduct research on this species when the genetic background is unclear and genetic manipulation is not established. However, the research group to which we belong, the Cluster of Excellence “Balance of the Microverse”, is a great interdisciplinary research team that can support even the most challenging tasks in microbiology, microscopy, and ecology. It includes colleagues from different disciplines with a strong spirit of collaboration. By discussing the topic together, they contribute their ideas that often result in joint experiments. It was quickly decided to use laser confocal observation, metabolomics, fluorescence activated cell sorting (FACS), lipidomics and other protocols for the identification of extracellular vesicles.

The vesicles were collected by Dr. Thomas Sommermann, a postdoctoral fellow in charge of FACS. The process was not trivial, since the structures need the osmotic strength of seawater to survive. To overcome this technical problem, Thomas switched the sheath fluid for the whole FACS system from general PBS to atypical seawater. After several rounds of attempts, the unruptured vesicles were finally collected and a gating strategy was established for subsequent vesicle collection and metabolic analyses.

4. More follow-up works are needed

Although we have shown that vesicle production is a means to remove harmful metabolites for regulating senescence in diatoms, it has to be established to what extend this phenomenon has an ecological function in the oceans. As a next step, Prof. Pohnert's group plans to go to the North Sea to carry out mesocosm experiments to observe existence and function of vesicles in real ocean.