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Algal Blooms Responsible for Mass Extinctions?
When you think of mass extinctions, you think of a catastrophic event: meteorites slamming into the Earth, hurling hunks of rocks and clouds of suffocating dust; volcanic eruptions that block out the sun with the massive amounts of ash expelled into the air. However, you don't think of algae, the seemingly innocuous seaweed, being able to significantly contribute to a mass extinction. However, recent research is showing that algae may have played a significant role in all five mass extinctions.
Algae blooms occur fairly regularly in the ocean. When large numbers of algae come together, they discolor the surrounding waters and outcompete, poison or asphyxiate any other life forms dwelling there. A particularly nasty form of alga, Heterosigma akashiwo, caused the death of 260 tons of farmed Atlantic Salmon in 2007. Algae come in a number of different forms: dinoflagellates, the producers of red tides, cyanobacteria (though reclassified as bacteria, it is sometimes still considered algae because it causes an algal bloom), and euglenids, the cause of green algal blooms.
Geologist James Castle from Clemson University and John Rodgers, an environmental toxicologist, wanted to see if algal blooms were as harmful millions of years ago as they are today. An investigation of current research showed that rock formations had increased populations of cyanobacteria called "microbial mats," during four out of the five mass extinctions. A comparison of the structures of modern cyanobacteria with ancient cyanobacteria found that the species has not changed very much over the years.
"Since they've changed very little in their structure, and they make toxins today, we propose that they did so in the past," states Castle.
However, algal blooms were probably not the sole agents responsible for mass extinctions. Instead, they provided a "kill mechanism," a way for environmental change to contribute to increased deaths.
"The toxins tend to become stronger and released when there's something that stresses the algae, [such as] a change in salinity of the water, a change in temperature," explains Castle. "There could have been an impact or volcanic eruption, and that may have stressed the algae, [and] by changing the conditions, they released toxins that kill the organism."
However, some scientists don't agree with Castle's and Rodger's hypothesis, claiming that the microbial mats were formed after the extinctions had already occurred.
"In all cases, the microbial sediments post-date the extinction, so it seems unlikely that they were the actual cause," said Matthew E. Clapham, a marine paleoecologist at the University of California, Santa Cruz. "Although many extinctions do share common causes, I don't know that it's really as good of an idea to try and explain all things by just one cause like this."
Though it is possible that algal bloom may have played a role in prolonging recovery from extinctions, or added additional stress to species during an extinction period, it's unlikely that they were the major cause of an extinction.
"Because cyanobacteria have been around for most of the history of life on Earth, it's virtually certain that there were times in the past when there were local die-offs due to cyannobacterial toxins," states Clapham. "But to demonstrate that it was a global event, or something significant enough, I'm not sure that's likely."
Frederick Rich, a geologist at Georgia Southern University poses the question: "Who knows what the causes were, or might be, for a species' extinction? It is all, and is absolutely, and without equivocation, speculation to decide how something or someone died."
Thanks for the lead, Labyrinth!
Source: Live Science
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