A team of researchers has utilized nitrogen isotope analysis to reveal that corals from the Eifel and Sauerland regions, dating back 385 million years, shared a relationship with symbiotic organisms. This discovery marks the earliest known instance of photosymbiosis in corals. Understanding this relationship could shed light on how ancient coral reefs reached such impressive sizes in environments that lacked nutrients.
A team led by scientists from the Max Planck Institute for Chemistry in Mainz has conducted nitrogen isotope analysis, proving that corals from the Eifel and Sauerland areas, which are 385 million years old, had symbiotic partners. This is a significant finding as it is the earliest record of photosymbiosis in corals. This relationship may help explain the immense growth of ancient coral reefs in nutrient-scarce conditions.
Coral reefs are among the most diverse ecosystems on the planet, often likened to the rainforests of the ocean. Modern reef-building corals originated during the Triassic Period, approximately 250 million years ago. They frequently engage in a symbiotic relationship with tiny organisms, mainly algae, that perform photosynthesis. This type of symbiosis is highly advantageous in waters with low nutrients, as it allows corals to recycle limited resources.
Geological records indicate that corals existed in the Devonian period, which is over 385 million years ago, including those found in the Eifel and Sauerland regions of Germany. Fossils of the now-extinct coral orders Tabulata (often called “honeycomb corals”) and Rugosa (“horn corals”) suggest that the Rhenish Massif was once submerged in a tropical sea during the Middle Devonian period, where large reefs thrived. However, it has remained uncertain whether these ancient corals had symbiotic relationships with photosymbionts during that time.
A research team from the Max Planck Institute for Chemistry, Goethe University Frankfurt, and the Senckenberg Research Institute and Natural History Museum Frankfurt has now demonstrated that some extinct corals from the Middle Devonian were indeed symbiotic. This discovery provides geochemical evidence supporting the existence of photosymbiosis in corals from this ancient era.
Investigating Symbiotic vs Non-Symbiotic Corals
The researchers identified this symbiosis by comparing nitrogen isotope ratios in the organic material of modern symbiotic corals with those of non-symbiotic corals. Nitrogen isotopes, particularly the ratio of “heavy” nitrogen (15N) to “light” nitrogen (14N), are valuable for assessing different positions within the food chain. By analyzing these isotopes, researchers can determine whether an organism followed a plant-based or meat-based diet. The higher an organism is in the food chain, the elevated its nitrogen isotope values tend to be, as lighter nitrogen is metabolized and excreted more swiftly compared to heavier nitrogen.
Through their examination of current corals, the researchers noted a clear distinction: corals that derive their energy mainly from photosynthetic algae exhibit lower nitrogen isotope values, whereas those that capture plankton for sustenance show higher values.
According to Jonathan Jung from the Max Planck Institute for Chemistry: “The consistent nitrogen isotope differences align with our expectations, indicating the typical progression in the food chain. This reveals that symbiotic corals occupy a level below non-symbiotic corals.” Jung is the lead author of this study published in the journal Nature.
Analyzing Fossil Samples from Various Regions
Co-author and initiating researcher Simon Felix Zoppe from the Goethe University Frankfurt explains: “With this knowledge, we could explore the ecological niche occupied by corals during the Devonian.” To achieve this, the researchers studied freshly collected fossil corals from the Sauerland, in addition to museum specimens from the Eifel, Western Sahara, and Morocco, sourced from the Senckenberg Research Institute and Natural History Museum Frankfurt.
One challenge was that the organic material necessary for analysis in fossils is notably scarce. However, a team led by Alfredo MartÃnez-GarcÃa from the Max Planck Institute devised a new analytical technique that only needs a few milligrams of finely ground fossil coral.
The findings from these fossils also indicate significant variations in nitrogen isotopes among different coral species. Typically, colony-forming corals from the Tabulata and Rugosa orders exhibited much lower nitrogen isotope values than the largely solitary corals from the Rugosa order. This led the scientists to infer that certain coral species were already living in a photosymbiotic state during the Middle Devonian period. “Photosymbiosis helps explain how ancient reefs were not only highly productive but also gigantic in size, despite their low nutrient environment,” states Alfredo MartÃnez-GarcÃa.
This study sets the stage for further exploration of nutrient cycles during the Paleozoic era, which encompasses the Devonian. The findings may help clarify how the mass extinction of corals and other reef species towards the close of the Devonian correlates with ocean nutrient levels. “Additionally, this newfound understanding can enhance our knowledge of early reef food chains,” adds Eberhard Schindler from the Senckenberg Research Institute and Natural History Museum Frankfurt. Going forward, the researchers aim to delve deeper into the geological past.
The Devonian Period
The Devonian is a geological period within the Paleozoic Era, beginning around 419 million years ago and concluding about 359 million years ago. During this time, Earth’s tectonic plates were highly active, leading to the convergence of the Laurussia and Gondwana supercontinents. Massive coral reefs developed in the Middle Devonian, many of which are preserved today as fossilized structures across Europe, North America, North Africa, Australia, Siberia, and China.
Understanding Nitrogen Isotope Analysis
A higher ratio of nitrogen isotopes 15N to 14N in a sample indicates an animal’s elevated position in the food chain. Animal metabolism produces nitrogenous waste such as ammonium or urea. The excretion of these waste products leads to an increased ratio of “heavy” nitrogen (15N) to “light” nitrogen (14N) relative to the organism’s food sources.
