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During Algal Bloom, Dinoflagellates Increase Expression of Metabolic, Other Genes

NEW YORK (GenomeWeb) – During an algal bloom, dinoflagellates ramp up the expression of certain genes, according to a new study.

Throughout blooms — which are increasing in frequency and intensity and have been linked to eutrophication — algae grow out of control, which can harm fish, marine mammals, and people. Algal blooms cost an estimated $82 billion a year in damages.

Researchers from the University of North Carolina at Chapel Hill collected samples from an algal bloom that occurred in an estuary as well as from other regions of that estuary that were not experiencing a bloom. As they reported in The ISME Journal: Multidisciplinary Journal of Microbial Ecology, the UNC researchers found that dinoflagellates increased production of genes belonging to a number of metabolic and other pathways during a bloom.

These increased expression levels, the researchers noted, could serve as a marker of the onset of a bloom.

"This technique has given us one of the most detailed looks to date into the strategy algae use to grow uncontrollably, leading to devastating consequences in our coastal communities," senior author Adrian Marchetti from UNC said in a statement. "It is also one of the first efforts to get the algae to tell us what's going on in their natural environment."

In the fall of 2012, Marchetti and his colleagues discovered that part of the Neuse River Estuary in North Carolina was experiencing an algal bloom. They collected samples from the bloom site as well as from two other nearby spots that were not experiencing the bloom for metatranscriptomic analysis.

By extracting and sequencing mRNA from the samples, the researchers generated 112 million paired-end raw reads from the bloom site and 182 million and 122 million from the two non-bloom sites. They assembled the reads and functionally and taxonomically annotated them.

In the bloom samples, some 42 percent of the sequence reads could be traced to dinoflagellates, while only 22 percent and 17 percent of the reads from the other sites were from dinoflagellates, respectively. This, the researchers noted, suggests that dinoflagellates were driving this bloom.

Through a differential expression analysis of dinoflagellate-related genes, the researchers homed in on genes that were expressed at different levels in bloom and non-bloom samples. From this, they found that genes involved in metabolic pathways were particularly enriched among bloom samples.

For instance, they noted high concentrations of ATP-binding proteins and genes involved in cell wall metabolism, which they said reflects high rates of cell division. At the same time, they reported high levels of components of the C4/CAM carbon fixation process and of genes involved in nitrogen metabolism.

They further noted an over-representation of transcripts linked to the phosphate starvation response system and the iron complex transport system.

Transcripts involved in polysaccharide transportation were also enriched in bloom samples, Marchetti and his colleagues reported. They noted that with higher metabolic and growth rates, dinoflagellates would have an increased demand for polysaccharides that make up their cell membranes.

In addition, they noted that glycosaminoglycan (GAG) gene transcripts were over-represented in bloom samples. While GAGs likely have multiple roles, the researchers noted that they are involved in cell adhesion and make the cell surface sticky.

At the same time, the researchers found that dinoflagellates have increased expression of genes involved in nitrogen, phosphorus, and iron acquisition and B vitamin biosynthesis. However, they noted a decreasing supply of nitrogen, suggesting that the dinoflagellates were beginning to be limited by the nitrogen levels.

Based on these findings and other findings, the researchers postulated that during a bloom, the dinoflagellates produce excess polysaccharides and vitamins to lure bacteria in and attach to them so they can acquire nutrients bacteria produce, like nitrogen.

Such gene expression changes that occur during a bloom could be forecast when one is about to take place, the researchers said.

"There isn't a way to prevent these blooms just yet, but the ability to predict them opens up that possibility, should future technologies or strategies arise that allow researchers and state officials to do so," first author Weida Gong, a graduate student in Marchetti's lab, said in a statement.