Everyone is familiar with the concept of the Amazonian rainforest as the lungs of the world, but what is less well known is that 50% of the world's photosynthesis is carried out by algae1 -- simple aquatic plants found in all freshwater and marine systems. By analogy with higher plants, which evolved from the green algal lineage, it is assumed that algae are autotrophic, requiring merely light and a mixture of inorganic compounds to grow. It is perhaps surprising therefore to discover that more than half of all algal species require exogenous cobalamin for growth. Recent work has established that the source of the vitamin is from closely associated bacteria, which appear to live in symbiosis with the algal cells. At the same time, reports of other interactions between algae and bacteria are appearing, prompting a rethink on the concept of organisms existing as separate entities, and having profound implications for our understanding of oceanic ecosystems.

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