Taming the Oceans Smallest Predators

The application of culture independent approaches have revolutionised marine microbiology since their appearance during the 90’s, allowing us to study the diversity and function of non-culturable organisms. In fact some linages are known only from environmental sequences and have never been cultured. However using such 'omics' methods only provides us with half the picture. So our knowledge of many ecologically important organisms’ physiology, ultrastructure and other genomic information is poor. This is not only an issue for marine bacteria, but also for marine eukaryotes such as heterotrophic flagellates. In the paper by Campo et al., previously uncultured protists were successfully cultured using oligotrophic conditions.

Samples of seawater were collected and flagellates either isolated using a single cell dilution method or flow cytometry. Cells which grew well were eventually scaled up into full sized cultures.  The cells were fed on bacteria originating from the same sample sites. These species are more likely to be grazed by protists in nature than the large bacteria normally used as food. This allowed protists which are more representative of those found in the sea to be grown. Interestingly, serial dilutions and unsupplemented seawater, have previously been used to successfully culture marine bacteria (Munn 2011). Could other lessons learned from bacteria be used to further improve culture methods for protists?

The 18S rDNA genes of the cultured organisms were sequenced and used for phylogenetic analysis. This revealed a number of previously uncultured strains. The researchers then focused on the most novel organism, later named Minorisa minuta, which had previously only been identified from environmental sequences. Use of electron microscopy revealed the cells to be tiny, a mere 1.0-2.1µm in length, with little morphological features other than a single flagellum (see picture). The 18S rDNA sequence was then used to design a probe for detecting cells of M. minuta using a modified version of FISH. This revealed it to be highly abundant from the Atlantic, Pacific, Indian, Southern Oceans and Mediterranean. Especially in coastal areas, where it can make up to 5% of heterotrophic flagellates. Such discoveries have implications for its importance in carbon cycling and control of bacterial populations. Follow-up experiments can now be carried out on behaviour to determine its importance in controlling coastal ecosystems.

Previous to this study M. minuta was just another environmental sequence. Culturing it allowed additional techniques to be brought to bear which caused a step change in our understanding of the microscopic predator. Interestingly the authors were not aiming to isolate any particular organism, but a group related in their ecology. Perhaps in future studies researchers could aim to culture the creatures behind sequences of specific interest. The key point about this study is that pure culture is not dead. And when used innovatively it can provide a wealth of information on the microbes, and the ecology of our oceans.

SEM image of Minorisa minuta (Campo et al. 2013)

Campo, J., Not, F., Forn, I., Sieracki, E. & Massana R. (2013). Taming the smallest predators of the oceans. International Society of Microbial Ecology, 7, 351-358.

Munn, C.B. (2011). Marine Microbiology: Ecology and Applications, 2^nd ed. New York: Garland Science.