Amanda Maxwell

Jun 23rd 2021

In a Flash: How Bioluminescent Organisms Signal Ocean Acidification

Ocean acidification is on the rise as our seas soak up rising atmospheric carbon dioxide levels. Although the increase in the last 200 years over the industrial age has been modest, the change is noticeable. A recent presentation at the annual Society for Integrative and Comparative Biology conference showed that when the ocean is more acid, bioluminescence is brighter.

The presentation highlighted work by researchers from the University of Hawaii, who mimicked the anticipated rise in ocean acidity to investigate its effect on secretory bioluminescence, as Science News reports. After extracting the bioluminescence chemicals from various marine organisms, the light reaction generated in the increased acidity was up to 15% brighter, according to the presentation abstract. This suggests ocean acidification could have a major effect on light generation in a host of bioluminescent ocean organisms and seriously affect the marine sensory environment.

Ocean Life and Bioluminescence

Bioluminescence is a way of life for millions of sea creatures who generate their own light sources to feed, mate and escape predators in the gloomy depths. Some organisms, such as the dinoflagellates that create red tides, make the light internally. But for others, the process is external or secretory. Because it requires a chemical reaction, bioluminescence is often governed by surrounding acidity. For secretory bioluminescence, it’s the pH of the surrounding seawater that is important.

Acidity is measured by pH, a logarithmic scale of hydrogen ion activity in an aqueous solution. Neutral solutions, such as water, measure at around pH 7, whereas acidic lemon juice is closer to pH 2. Preindustrial age oceans measured around pH 8.2; now they are pH 8.1, according to the Environmental Protection Agency. The National Ocean and Atmospheric Administration (NOAA) predicts further ocean acidification will reach pH 7.67 by 2100.

Carbon Dioxide Drives Ocean Acidification

Human activity such as burning fossil fuels and deforestation continues to release more CO2 into the atmosphere, forcing climate change. Our oceans play an important role in soaking up some of the excess gas, which produces chemical reactions that reduce seawater PH, according to the NOAA. With more CO2 available in the atmosphere, acidity increases.

Researchers predict that the rising acidity will affect animal behavior. Some species use bioluminescence to signal to a potential mate or to lure food, as Science News points out. Having your own light source is incredibly valuable in the depths of the ocean where sunlight doesn’t penetrate. There are also some that use a bioluminescent burst to escape from predators, releasing a glow bomb to distract and confuse or by setting a beacon that brings other larger predators to the buffet.

However, not only will animals that rely on bioluminescence have to adapt swiftly, but they may also see changes in where they live and what they eat.

Coral bleaching has been covered extensively as a prime example of warmer waters due to climate change. Acidification also makes it difficult for coral beds to survive. The NOAA notes that fewer carbonate ions are available to bond with calcium for skeleton growth and repair. This also impacts shell building in a wide range of marine species, including oysters and pteropods. Pteropods, also known as sea butterflies, are sea snails that form a valuable foundation of the food web for animals ranging from krill to whales. In acidic conditions, the pteropod shells simply dissolve.

Beyond the Oceans

Species loss and brighter oceans are two possible consequences, but acidic seas will impact terrestrial life, too. NOAA research predicts that with rising acidity, the ocean’s buffer capacity could be 34% less by 2100. As a result, much less CO2 will be absorbed and more will remain in the atmosphere to drive climate change.

Acidification could also impact another marine ecosystem that’s important for trapping carbon. The biological carbon pump (BCP) relies partly on phytoplankton at the surface that take in CO2. Woods Hole Oceanographic Institute describes how the phytoplankton convert the gas into organic carbon during photosynthesis. This is then sequestered in the ocean depths either as waste from the zooplankton that eat these tiny organisms or as the bodies of the creatures themselves. Researchers speculate that turning off the BCP could double atmospheric CO2.

In Nature Climate Change, commenting on their original paper, the authors note that their 10-year study shows that ocean acidification does affect the BCP, as well as marine nutrient cycling. Though the results are highly variable and still not clear, the researchers note that the effects of ocean acidification on the smallest organisms have potential to impact the carbon cycle worldwide.