Image from Wikimedia Commons
Humans have burned enormous amounts of fossil fuels since the Industrial Revolution, pumping carbon dioxide into the atmosphere. The ocean absorbs vast amounts of heat as a result of increased concentrations of greenhouse gases. The steady absorption of atmospheric heat leads to rising ocean temperatures. Data from the US National Oceanic and Atmospheric Administration shows that the average global sea surface temperature has increased approximately 0.13 degrees Celsius per decade over the past 100 years. Increasing ocean temperatures directly affect marine species and ecosystems. Coral bleaching is one phenomenon mainly caused by these changes to the oceanic equilibrium. Algae called zooxanthellae live in the tissues of corals. The algae undergo photosynthesis to supply essential nutrients to the corals. In return, corals provide algae with a protected environment and the compounds they need for photosynthesis. Reefs are threatened by rising ocean temperatures. Corals will expel the algae that lives in their tissues when the water is too warm. This causes the coral to lose its color along with many of the benefits that come from symbiosis.
The symbiotic relationship between algae and coral began more than 210 million years ago, according to a new study by researchers from Princeton University. Evidence of symbiosis was found in fossilized coral dating back to the late Triassic period. During this time period, the corals inhabited nutrient-poor environments where symbiosis aided in reef development and survival. Algae belonging to the dinoflagellate group live inside the corals’ tissues. The algae use photosynthesis to produce nutrients, many of which they pass on to the corals’ cells. In return, the corals produce waste in the form of ammonium, which the algae consume. This effective recycling process provides reefs with nutrients. Corals that host algae can also emit calcium carbonate, which forms the hard exterior of the coral, up to 10 times faster than non-symbiotic corals.
Image from Wikimedia Commons
Since dinoflagellates do not fossilize, researchers looked for three key elements in the coral fossils that indicate the past presence of algae: fossil microstructures, nitrogen levels, and levels of carbon and oxygen. A microstructural analysis revealed regularly spaced patterns of growth consistent with symbiotic corals’s reliance on photosynthesis. The next element, nitrogen levels, derive from the ammonium the corals excrete. The nitrogen atoms are trapped in the fossil’s calcium-carbonate matrix. They come in two forms and vary by how many neutrons they have. Symbiotic corals contain a lower ratio of 15N to 14N compared to non-symbiotic corals. The researchers found strong evidence that the fossilized coral were symbiotic and lived in a nutrient-poor environment. They were able to connect the environmental conditions from 200 million years ago to the evolution of corals.
This study uncovered the benefits of the relationship between algae and coral. Rising ocean acidity and temperatures subject coral to new stressors they have not yet adapted to. Some coral can recover from bleaching if conditions return to normal. However, warmer temperatures and other factors like poor water quality can leave the living coral in a weakened state. It becomes more vulnerable to disease and mortality. A transition from fossil fuels to renewable energy is necessary to reduce carbon pollution, slow the rate of rising ocean temperatures, and protect coral reefs.