Coral reefs, like humans, require a certain amount of iron to stay healthy. While iron is an important component of human blood, in corals it supports the heath of a reef’s photosynthetic co-stars: microscopic algae.

Most tropical corals have a mutually-beneficial relationship with zooxanthellae, a type of microscopic algae. Zooxanthellae live within the coral’s tissue, giving corals their magnificent color.

During the daytime, these little zooxanthellae provide their coral hosts with up to 90% of their daily nutritional needs in the form of photosynthetic byproducts. In return, the coral provides the algae with shelter and nutrients.

Previously, research conducted on the Indo-Pacific ‘smooth cauliflower’ coral (S. pistillata) found iron-deficient seawater exacerbated the coral’s susceptibility to underwater heatwaves. When scientists artificially reduced iron levels in the surrounding seawater, the smooth cauliflower coral bleached more readily than corals kept in seawater with normal iron levels. Bleaching is a hallmark of severe coral stress and can indicate the death of a coral is near.

Building on this important finding, scientist Dr. Hannah Reich and her collaborators recently discovered zooxanthellae require a greater amount of iron to achieve maximum growth than their free-swimming algae counterparts.

“We found that species [of microscopic algae] with similar ecological niches — either found in similar habitats or with shared ecological abilities — had similar metal profiles,” explains Dr. Todd LaJeunesse, a co-author of this published research. “If the microalgae are using trace metals in different amounts or in different ways, limitation of iron could have cascading effects on vital functions, like photosynthesis and whether they are able to take in other nutrients for survival.”

The team’s discovery further clarifies the importance of iron for the health of coral reefs.