Ocean plankton reveal sugar-for-nutrients partnership that sustains life in barren seas — Phys.org

RESOLVING THE LEDGER

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Add as preferred source In the vast, nutrient-poor regions of the world's oceans, there is a constant struggle for survival. Yet tiny organisms known as Collodaria have thrived there for millions of years. These Collodaria host microscopically small algae that are essential for survival.

A research team at the University of Jena has uncovered the metabolic secret behind the symbiosis and provides entirely new insights into marine ecosystems. The findings are published in the journal Nature Communications .

Collodaria are marine single-celled organisms, with some species forming large colonies in the sunlit upper ocean. Within their colonies, Collodaria host hundreds to thousands of microscopic algae. This close biological association, known as a "holobiont," functions like a perfectly coordinated shared household.

The study shows that the algae supply energy-rich sugars through photosynthesis. In return, the Collodaria provide protection to their microscopic tenants and supply essential nutrients that they obtain directly from seawater. Both partners benefit from this collaboration.

Until now, scientific hypotheses about how algae support their Collodaria hosts have varied widely. Because Collodaria are very fragile organisms that cannot currently be maintained in laboratory cultures, the Jena team adopted an innovative field-based approach to elucidate the processes.

First author Dr. Vera Nikitashina and Dr. Georg Pohnert, professor of analytical chemistry at Friedrich Schiller University Jena, collected fresh colonies directly from the open ocean and used stable-isotope tracing to reveal the exact exchange of substances between host and algae. This method allows researchers to track the incorporation of supplied labeled compounds into the metabolism of the holobiont, revealing which pathways are active.

The labeling experiments were conducted onsite in the Mediterranean Sea, while the detailed chemical analyses were performed later in the laboratory in Jena.

"It is fascinating to see how precisely this biological partnership is organized," Pohnert says. "In these regions of the ocean, the nutrients are very scarce, and for organisms that cannot photosynthesize, survival is very challenging. Yet Collodaria that are dependent on nutrients from their environment are highly abundant. Now we can understand how they overcome limited nutrition—they rely on their microscopic photosynthetic partners to do the work."

"For the first time, we were able to identify the nature of the chemical compounds exchanged between the partners," he adds. "The algae provide sugars produced through photosynthesis, and the host acquires protective compounds from the surrounding water. This highly efficient division of labor on a microscopic scale is the key to why these organisms can thrive even in the most nutrient-poor regions of the ocean."

The discovery provides an important building block for modern climate research and has significant implications for global environmental protection management. Because these tiny organisms are so widespread and play a crucial role in global biogeochemical cycles, the new data will enable scientists to predict much more accurately how marine plankton responds to climate change.

"Our results fundamentally refine our understanding of marine food webs," Nikitashina emphasizes. "With this knowledge, we can significantly improve ecological models of marine productivity and carbon storage. This will help us better understand and predict the direct impacts of anthropogenic activities on marine ecosystems."

Vera Nikitashina et al, Metabolic partitioning between photosynthesis and osmotrophy in Collodaria photosymbiotic holobionts, Nature Communications (2026). DOI: 10.1038/s41467-026-76549-6

Journal information: Nature Communications

Provided by Friedrich Schiller University of Jena

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Ocean plankton reveal sugar-for-nutrients partnership that sustains life in barren seas — Phys.org | AIPROPX