It's not easy being green (but it used to be) Free

By Rachel Berkowitz

One of the big questions regarding a mass-extinction event is how quickly the remaining ecosystem recovers. Phytoplankton—photosynthesizing microorganisms that live in the upper layers of seawater—play a pivotal role in that recovery because they form the base of the marine food web.

During the Triassic–Jurassic extinction event 201.6 million years ago, seawater chemistry changed to favor green algae, which is a type of photosynthetic eukaryote, over red algae, which had previously thrived. Although red algae eventually recovered to become the oceans' dominant primary producers, for the first 10 million years of the Jurassic, green algae and another type of photosynthetic eukaryote, green sulfur bacteria, created an evolutionary bottleneck during which a significant number of other species were either reduced or became extinct.

Green sulfur bacteria require both light and free hydrogen sulfide for their metabolism. One brown-colored strain produces the pigment isorenieratane, which is used during photosynthesis. Fossilized evidence of that pigment in black shale formations therefore indicates the presence of green sulfur bacteria and thus high concentrations of free H₂S in shallow oceans. Sulfidic conditions are further evidenced by pyrite-rich sediments, which form via the reaction of H₂S with iron.

In a paper published recently in Nature Geoscience, Bas van de Schootbrugge of Goethe University Frankfurt and his colleagues report their analysis of black shale cores from Germany and Luxemburg.[1] Whereas shales from the Late Triassic contain no detectable pigment fossil, the German cores from the Early Jurassic contain pigment at depths of 72–100 meters. The lack of other fossils above those layers suggests that the sulfidic conditions were unfavorable to other marine life forms. The Luxemburg core contains several peaks of pigment concentration.

Unlike red algae, green sulfur bacteria can thrive in anoxic (oxygen-depleted) water. “We think that at the former coastal site in northern Germany, nitrogen isotopes preserve an additional fingerprint of anoxic waters,” says van de Schootbrugge. His team's analysis of nitrogen isotopes showed that the Triassic–Jurassic boundary coincided with a shift in the predominant nitrogen-bearing ion in the oceans, from nitrate to ammonium. The shift would have favored the green algae, which have been shown in experiments to do better when ammonium concentrations are elevated.

Ocean oxygenation eventually increased, possibly due to a period of cooling, and led to an increase in circulation that favored dinoflagellates, the main type of red algae. In their dormant phase, dinoflagellates form cysts and sink to the ocean bottom. When conditions are good, as is the case now, dinoflagellates leave their cysts. But when the ocean bottom was anoxic, as was the case in the Early Jurassic, they remained inside.

The Triassic–Jurassic boundary corresponded to a period of high atmospheric carbon dioxide concentrations caused by volcanism and forest fires. Eruption of the Central Atlantic magmatic province, which may have doubled or quadrupled atmospheric CO₂, is consistent with increased anoxic and sulfidic conditions. A similar large igneous province event at the Permian–Triassic boundary 252 million years ago ejected CO₂ into the atmosphere and is thought to have led to ocean anoxia and a similar extinction.

“It seems that there were multiple episodes of long-lasting volcanic activity that led to repeated phases of anoxia in the oceans during the Early Jurassic,” says van de Schootbrugge. The so-called Toarcian oceanic anoxic event about 183 million years ago prompted blooms of green algae in an anoxic ocean.

The apparent repeated “poisoning” of the shallow seas with H₂S allowed green algae to thrive during the Early Jurassic but delayed the recovery of other, more nutrient-rich species of algae, presumably because green algae, green sulfur bacteria, and other photosynthetic cyanobacteria did not constitute a nutritious diet for higher organisms. How exactly different algae provide different quality food is one of the next questions that geologists and biologists will try to answer about the evolution of modern phytoplankton.

Reference

• 1.  S. Richoz et al., Nat. Geosci. 5, 662 (2012).