Delayed Diversification of Eukaryotes Not Due to Lack of Zinc,Study Finds

The delayed diversification and proliferation of eukaryotes, creatures that possess a membrane-bound nucleus, was not due to low availability of zinc, a new study finds.

Eukaryotes are organisms with more complex structures than prokaryotes like bacteria. Fossil records have shown that the formation, structure and growth of these creatures were limited some 800 million years ago. It was hypothesized that the delay in diversification of eukaryotes was due to low levels of the trace metal zinc in seawater during that time.

Zinc is essential for a diverse range of basic cellular processes. Zinc-binding proteins are located in the cell nucleus and are involved in the regulation of gene transcription. Eukaryotes increasingly evolved with zinc-binding protein structures during the last third of their evolutionary history about 600 million years ago, while the organisms arose some 2 billion years ago. The delay was attributed to the low bioavailability of this trace metal.

But a research team from the University of California - Riverside (UCR), has shown in its new study that the popular hypothesis is false. They analyzed marine black shale samples from countries like North America, Africa, Australia, Asia and Europe. The age of the samples ranged from 2.7 billion years to 580 million years old. They found that the shales indicated high levels of zinc availability during the Proterozoic period between 2.5 billion and 542 million years ago.

"We argue that the concentration of zinc in ancient marine black shales is directly related to the concentrations of zinc in seawater and show that zinc is abundant in these rocks throughout Earth's history," study author Clint Scott, a former UC Riverside graduate student, said in a statement. "We found no evidence for zinc biolimitation in seawater."

Previous hypothesis on availability of zinc was based on a theory that oceans during the Proterozoic period were sulfidic. This could mean that zinc precipitated rapidly in the oceans, leaving only a scarce amount of the metal available.

But a study paper published in 2011 by researchers from UCR suggested that the Proterozoic oceans were broadly ferruginous - low in oxygen content but rich in iron oxides. The paper argued that the sulfidic nature of the oceans was restricted during that time.

The research team involved in the new study explains that "the ferruginous deep oceans combined with large hydrothermal fluxes of zinc via volcanic activity on the seafloor and maintained high levels of dissolved zinc throughout the oceans." This suggests that a stable reservoir of zinc has been available for the last 2.7 billion years.

The study concludes that the delay in diversification of eukaryotes in the oceans was not due to low levels of zinc, but due to some other unknown factors.

The findings of the study are published in the journal Nature Geoscience.