Occupying virtually every water-containing environment, essential to marine food chains, and implicated in the spread of toxic blooms, diatoms—algae with a siliceous cell wall—are of intense practical interest. They also fascinate scientists. Biologists study diatoms to better understand the evolution of sexual behavior. Bioengineers hope to breed diatoms that could contribute to molecular ecology and biofuels applications. And drug developers think that diatom structures could inspire the design of novel drug-delivery vehicles.
Although diatoms could be used to advance many exciting applications, scientists have been perplexed by two persistent mysteries: Do diatoms have sex? and Can diatoms be induced to have sex?
The answers, in short, are yes and yes. So say scientists based at Oregon State University. These scientists, led by microbiologist Kimberly Halsey, Ph.D., have found that Thalassiosira pseudonana, a species of diatom thought to be asexual, can in fact reproduce sexually. Moreover, the scientists determined that T. pseudonana and several other single-celled algae may be “put in the mood” by ammonium.
Details of the work appeared July 7 in the journal PLOS One, in an article entitled “Morphological and Transcriptomic Evidence for Ammonium Induction of Sexual Reproduction in Thalassiosira pseudonana and Other Centric Diatoms.” Besides presenting evidence of diatoms’ sexual behavior (and how it may be induced), this article suggests how diatoms’ sexual behavior may arise from diatom genetics.
“We show that ammonium reliably induces the key sexual morphologies, including oogonia, auxospores, and spermatogonia, in two strains of T. pseudonana, T. weissflogii, and Cyclotella cryptica,” wrote the article’s authors. “RNA sequencing revealed 1,274 genes whose expression patterns changed when T. pseudonana was induced into sexual reproduction by ammonium.”
The “centric” T. pseudonana is a common model organism for researchers; it's one of two diatoms, the other being the “pennate” diatom Phaeodactylum tricornutum, to have had its genome sequenced. Centric diatoms are radially symmetrical—think of them as shaped like a soup can, Halsey said—and pennate diatoms are bilaterally symmetrica, elongated in the manner of a pea pod.
Diatoms hold great potential as a bioenergy source and also for biosensing. In addition, their intricate, silica cell walls offer promising nanotechnology applications.
“Diatoms are amazing; their silica frustules [cell walls] are beautiful and exquisite,” noted Dr. Halsey. “Now that we can control their sexual pathway, that should open the door to being able to make crosses between different diatoms with different characteristics. We should be able to breed them just like we do with corn or rice or strawberries to select for traits that are really desirable.”
“Everybody said T. pseudonana was asexual, because they'd never seen anything else,” Dr. Halsey pointed out. “The general thinking was that it just lost the ability or need to go through sex.” Other scientists, Dr. Halsey continued, had showed T. pseudonana retained genes necessary for meiosis, a type of genetic replication specific to sexual reproduction, and concluded the diatom wasn't using those genes.
“But we started seeing very different morphologies,” changes in cell structure, Dr. Halsey pointed out, in this case related to sexual activity. “We also saw genes expressed that are involved in flagellar structures and assembly, which would only happen with sperm cells.”
Graduate student Eric Moore, lead author on the research, was astonished to learn “these single-celled organisms can differentiate into male and female cells, completely changing their morphologies.”
“In fact, I was convinced my cultures were contaminated before I realized what was actually going on,” he said.
Previous work by other researchers studying different types of centric diatoms showed that growth stress—interruptions of light, changes in salinity, shifts in nutrients—can sometimes, but not reliably, cause cells to become sexual.
“Lab efforts to induce sex in centric diatoms have ranged from sweet talk to torture,” Dr. Halsey remarked.
But manual microscopic analysis by Dr. Halsey's team found that ammonium, a common compound that's a metabolic waste product of animals, reliably caused two strains of T. pseudonana and two other centric diatoms to change their cell structures, making eggs and sperm. Ammonium caused the diatoms to get ready for sex when at least one other cell growth factor—such as light, phosphorus or silica—was scarce.
In addition, RNA sequencing showed more than 1200 diatom genes that changed in activity when ammonium lit the algae's sexual fires. Dr. Halsey suggests that in nature, the protists that graze on the diatom blooms excrete the ammonium that triggers the diatoms' sexualization.
“The specific collection of environmental factors that make diatoms have sex aren't yet known,” she admitted. “But identifying ammonium as a sexuality inducer potentially opens the door to new avenues of research into breeding and genetic modification to control important traits.”