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Diatoms Provide an Attractive Home for Marine Bacteria

Microscopy image showing stained algal and bacterial cells.
This image taken by a confocal laser scanning microscope shows algal cells (stained blue and red) colonized by bacteria (green). Credit: University of Oldenburg.
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From the perspective of marine bacteria, unicellular algae are an attractive and amazingly diverse habitat. A research team led by microbiologist Prof. Dr. Meinhard Simon from the University of Oldenburg has now made it visible for the first time. The team presents the results in the current issue of the journal "Journal of Phycology" and thus sheds light on the complex interrelationship between algae and bacteria, which is of fundamental importance for material cycles and food webs in the sea.

Diatoms are an important part of the so-called phytoplankton, mostly microscopic, unicellular algae in the oceans. Surrounded by a solid silicate shell, the diatoms produce about a fifth of the oxygen in the atmosphere and convert carbon dioxide from the air into biomass during photosynthesis. Overall, they bind more CO 2 than the tropical rainforests, which is why they are of great importance for the carbon cycle and the climate. "Diatoms live in close association with bacteria, but little is known about many aspects of this interrelationship," reports Simon. He and his team have now examined these microscopic interactions, which are enormously important for the ecology and biochemistry of the oceans, in more detail.

The Oldenburg team around Simon, Dr. Sara Billerbeck and the doctoral student Tran Quoc Den selected a globally distributed diatom called Thalassiosira rotula and examined its colonization by bacteria in detail using various microscopic methods. Also taking part in the study was Dr. Thomas Neu from the Helmholtz Center for Environmental Research in Magdeburg.

With the help of fluorescent dyes, the researchers demonstrated that the surface of the alga shows small-scale biochemical differences. They used so-called lectins - complex biochemical compounds that bind very specifically to certain protein-carbohydrate complexes on the surface of the algae - to mark the different areas, stain them and make them visible with a special microscope like a three-dimensional image. They also discovered that different types of bacteria are specialized in colonizing the different areas. The bacteria discovered belonged in particular to the Roseobacter group and to the Flavobacteria.

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“The cell surface of diatoms has a surprisingly varied structure. We would not have expected how finely tuned the usage by different types of bacteria is to this structure," explains Simon. The team also found that certain types of bacteria prefer to be found on the fine hairs on the surface of the algae. "The colonization patterns reflect the very different metabolic properties of different types of bacteria and their ability to colonize surfaces," says Simon, summarizing the findings.

The results are also interesting because they provide an insight into the direct environment of the diatoms. This microcosm, called the “phycosphere”, is rich in organic substances that the algal cells excrete. "Previous studies have shown that the algae use this to attract certain bacteria, which produce substances that are essential for them, such as vitamins - very similar to how plants attract bees with their flowers," explains Simon. Building on the new results, future studies could now examine the function of the different types of bacteria in the phycosphere in more detail.     

The current study was created as part of the Transregio Collaborative Research Center (SFB) "Roseobacter" funded by the German Research Foundation (DFG). In it, more than 60 researchers from Oldenburg, Braunschweig, Göttingen and Bonn have examined the bacteria of the Roseobacter group over the past 13 years. These occur in all marine habitats – from the tropics to the polar seas, from the water surface to the deep sea. Among other things, the researchers discovered many new strains, described the interactions between representatives of this group and other microorganisms, their distribution and functional biogeography in the world's oceans for the first time. In total, more than 250 published scientific articles are based on research in the context of the CRC.

Reference: Den TQ, Neu TR, Sultana S, Giebel HA, Simon M, Billerbeck S. Distinct glycoconjugate cell surface structures make the pelagic diatom Thalassiosira rotula an attractive habitat for bacteria. J Phycol. 2023;59(2):309-322. doi:10.1111/jpy.13308

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