How Tiny Ocean Organisms Trade Sugar for Nutrients to Survive Barren Seas

In the nutrient-poor regions of the open ocean, tiny single-celled organisms called Collodaria have thrived for millions of years by hosting hundreds to thousands of microscopic algae inside their colonies. A research team at the University of Jena in Germany has now uncovered the metabolic exchange behind this partnership, reporting the findings in the journal Nature Communications.

The close association between Collodaria and their algae, known as a "holobiont," works like a coordinated shared household: the algae supply energy-rich sugars produced through photosynthesis, while the Collodaria provide protection and pass on nutrients they draw directly from seawater. Because Collodaria are fragile and cannot be kept in laboratory culture, the team led by first author Vera Nikitashina and 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 track the exchange of substances between host and algae.

The labelling experiments were carried out in the Mediterranean Sea, with detailed chemical analyses performed later in Jena. "For the first time, we were able to identify the nature of the chemical compounds exchanged between the partners," Pohnert said. "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."

Because Collodaria are widespread and play a role in global biogeochemical cycles, the researchers say the findings can help improve ecological models of marine productivity and carbon storage, and predict how marine plankton respond to climate change.