Posted in | News | Climate Change | Ecosystems

Low Iron Levels and Warming Ocean Waters Impair Growth of Microalgae Living in Coral Cells

According to a new research headed by scientists from The Pennsylvania State University (Penn State), corals’ ability to react to climate change may partly depend on the already limited amount of iron available in their habitat.

Coral cells contain dense populations of microalgae that provide them with nutrients (pictured here under a microscope). When faced with hot water temperatures and low iron levels, these symbiotic algae grow poorly and take up trace metals in different ways. Image Credit: Todd LaJeunesse.

The research shows that the mixture of low iron levels and hot water temperatures compromises the algae living within the coral cells, indicating that low iron levels—which could reduce with warming ocean waters—could worsen the effects of climate change on corals.

Corals are the foundation for one of the most important ecosystems in the world. They support significant amounts of biodiversity, protect our shorelines from storms, provide habitat for our fisheries, and boost our economies with their opportunities for tourism.

Todd LaJeunesse, Professor of Biology, The Pennsylvania State University

LaJeunesse continued, “Climate change affects not only the coral, but also their symbiotic microalgae and the partnership between them. In this study, we explored two aspects of climate change—warming waters and altered amounts of trace metals like iron—on the algae.”

Earlier, the team had found that the photosynthetic microalgae residing within the coral cells—which offer up to 90% of the coral’s daily nutritional requirements via photosynthesis—have very high iron demands.

In this study, we found that limiting the available iron lowered the heat tolerances of two species of microalgae, which potentially could have cascading effects on the coral and on the reef ecosystem.

Hannah Reich, Study Author and Graduate Student in Biology, The Pennsylvania State University

In their research, which was published online in the Journal of Phycology on September 30th, 2020, the team explored the impacts of high water temperatures and reduced iron availability on the growth of two microalgae species cultured in the laboratory—one species normally located in tropical waters and the other from more temperate regions.

At limited iron levels and high temperatures, both species did not grow well compared to normal iron levels and moderate temperatures.

High temperatures increase metabolic demands, which forces the microalgae to work harder function properly. It also increases dependence on processes that require iron, like photosynthesis and assimilating other nutrients. We found that under high temperatures, the microalgae needed more than five times as much iron to reach typical, exponential growth rates.

Hannah Reich, Study Author and Graduate Student in Biology, The Pennsylvania State University

Limited availability of iron at extreme temperatures also compromised the photosynthetic capacity of the algae, decreasing their efficiency, which the scientists believe adds to the restricted growth under these circumstances. Furthermore, warmer temperatures impacted the relative quantities of trace metals within the algae, termed as their metal profiles.

These alterations could indicate differences in metal usage, likely affecting the biological functions in which they are used,” added Reich. “Notably, with limited iron, the more tropical species grew better and had less compromised photosynthetic ability at high temperatures and a larger reserve of many trace metals.”

Our results also highlight that trace metal profiles could be a metric with which to assess heat sensitivity or tolerance among symbiont species,” added LaJeunesse. “Moreover, access to higher concentrations of trace metals may improve a coral’s tolerance of thermal stress.”

Going forward, the team plans to investigate how the requirements for trace metals differ in various conditions in the field, and to study the effects of warming waters and limited iron availability on microalgae residing within a host.

While it is important to understand how access to iron supplies can impact the ability of corals to respond to climate change stressors, there is still a dire need to reduce carbon dioxide emissions to combat the climate crisis,” concluded Reich.

Besides Reich, who is currently a postdoctoral scientist at the University of Rhode Island, and LaJeunesse, the research group comprises Wan-Chen Tu, Irene Rodriguez, Yalan Chou, and Tung-Yuan Ho at Academia Sinica and Elise Keister and Dustin Kemp from the University of Alabama at Birmingham.

This research is documented as the Bold Award feature paper by the journal, who invites the winner of the Harold C Bold Award from the Phycological Society of America yearly meeting to submit a paper. Reich received a number of other exceptional presentation awards at national and regional scientific meetings for this research.

The study was feasible through the National Science Foundation East Asia and Pacific Summer Institute for U.S. Graduate Students (EAPSI) program. The program enabled Reich to travel to and work with partners at Academia Sinica in Taiwan, who had the required cleanroom facilities and know-how in working with microalgae to consistently and precisely quantify trace metals.

The study received additional funding from the National Science Foundation, the Ministry of Science and Technology of Taiwan, the NASA Pennsylvania Space Grant Consortium, Academia Sinica, and the National Oceanic and Atmospheric Administration.

Journal Reference:

Reich, H. G., et al. (2020) Iron availability modulates the response of endosymbiotic dinoflagellates to heat stress. Journal of


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