麻豆淫院

Trees get most of the love, but diatoms, a group of photosynthetic microalgae, produce 20% of Earth's oxygen and are the foundation of aquatic food webs. The prevalence and diversity of diatoms have made them highly successful, suggesting the evolutionary history of diatoms is worth understanding as an important piece of the larger puzzle of life on Earth.

A new study led by researchers from the U of A found that diatoms evolved slowly for the first 100 million years of their existence. Then, 170 million years ago, they reached an inflection point characterized by a burst of rapid speciation orders of magnitude faster than anything that had preceded it. This included changes to their shape, size and mode of reproduction, as well as repeated movements from oceans into freshwater systems, a typically difficult barrier for aquatic species to cross.

With an estimated 100,000 species, diatoms are now one of the most diverse groups of microalgae. They are small enough that dozens could fit on the head of a pin and are found almost anywhere there is water and sunlight.

The paper, in the Proceedings of the National Academy of Sciences, was led by first author Andrew Alverson, a professor of biological sciences at the U of A. The paper represents nearly a decade of intense analysis overseen by Alverson. Eight of the paper's 15 authors are or were affiliated with the U of A at the time the research was conducted.

The bulk of the time was spent combining fossil information about diatoms with the newly sequenced transcriptomes (the genes expressed by an organism) from 181 different diatoms to reconstruct the pattern, timing and genomic context of major evolutionary transitions. In all, the team sequenced thousands of genes to reconstruct the family genealogy of diatoms, which has not been done at this scale before.

Alverson noted that one characteristic of this evolutionary burst was, evolutionarily speaking, a sudden increase in genetic duplication, the equivalent of getting not one set of chromosomes from each parent, as humans do, but two sets.

"Genome duplications have been associated with these kinds of diversification events," Alverson explained. "It creates lots of fodder for evolution because now you've duplicated all the genetic material. In other groups, like flowering plants, their history is peppered with these genome duplication events that are associated with bursts of diversity."

Gaining an understanding of how diatoms evolved also helps fill in the picture of how other biological processes on Earth evolved.

"Now that we know the timescale of diatom evolution," Alverson explained, "we can superimpose a lot of things on that, and one of those things is ocean history. There's a lot of data about how the ocean has changed over millennia, and diatoms are major players in ocean ecology and the biogeochemistry of nitrogen, silicon and phosphorus cycling.

"Now we can take what we know about changes in the ocean and overlay this history of diatoms, we can start to make correlations when silicon, which is 25% of Earth's crust, started to drop precipitously in the oceans as diatoms increased. Simultaneously, we see atmospheric oxygen levels going up, so you can start to overlay this timeline on Earth and ocean history and draw some inferences about how diatoms are involved."

Now that they've identified this inflection point, a clear break from the past, the next mystery to answer is: why? What happened to prompt this evolutionary burst of activity? Were there atmospheric or environmental changes? Did other organisms die off, vacating a niche for diatoms to inhabit?

While Alverson has some guesses, but no certain answers yet, he will continue to make the case for better understanding the evolution of diatoms.