Science1 publisher3 min readPublished
First complete genomes of Gloeotrichia show no characterized toxin clusters, and the geosmin genes in every sample
Bigelow Laboratory and Oregon State assembled genomes of a puffball-forming cyanobacterium from lakes in five states. Every sample carried the geosmin genes, and none carried the pathways for microcystin.
The Scientist · Science desk
What happened
- Researchers at Bigelow Laboratory for Ocean Sciences and Oregon State University assembled the first complete genomes of the bloom-forming cyanobacterium Gloeotrichia echinulata and published the comparison in Harmful Algae.
- The assemblies contained no evidence of the machinery required to produce the toxins commonly associated with cyanobacteria.
- The comparison drew on complete and partial genomes from lakes in Maine, New Hampshire, California, Oregon and Washington.
- The Northeast samples were sequenced with Oxford Nanopore, which reads long DNA strands and shows how genes are arranged as well as which ones are present.
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Why it matters
- decision A lake or utility manager confirming a Gloeotrichia bloom is now choosing between a treatment response for taste and odor and a toxin advisory, and the geosmin genes are the only biosynthetic capacity these genomes support.
- constraint A clean genome for one species cannot clear a bloom, because the authors say Gloeotrichia grows alongside toxigenic species, so species-level monitoring stays on the budget for recreational and drinking-water lakes.
- contradiction An earlier study credited this species with microcystin production; anyone citing either result now has to specify which strains and which lakes they mean.
- precedent If the genetic uniformity across coasts holds up in wider sampling, a manager could treat a Gloeotrichia identification as carrying predictable properties without sequencing their own lake.
A genome says what a cell can build. The team went looking for the gene clusters behind the well-characterized cyanotoxins and did not find them [3]. That is a different sort of negative from a water sample that came back clean on the day it was drawn. Robin Sleith, the study's co-lead author, drew the boundary himself. "It may be producing yet-to-be-characterized toxins or molecules we don't understand," he said. "But for all the toxins that science has done the hard work of characterizing, and that have the most acute human health hazards, there were no obvious pathways." [6]
Five states across two coasts [19] is a thin sample of an organism with a wide distribution in the Northeast, where it blooms very visibly in popular recreational lakes [18]. The paper does not say how many genomes that adds up to, describing the material as several complete and partial genomes [20]. For an absence claim, long-read assembly is the part that keeps a missing cluster from being an artifact of a fragmented draft [12].
An earlier result points the other way: at least one prior study suggested this species can produce microcystin, among the most prevalent cyanotoxins and a cause of serious liver damage [9]. Sleith arrived here because he could not find microcystin genes in a sample from New Hampshire's Lake Winnipesaukee, and that led him to Theo Dreher, an emeritus professor at Oregon State who was working on toxin production on the West Coast [10]. The authors name two conditions under which both findings could stand: toxin-producing strains they did not sample, and the toxigenic species that bloom alongside Gloeotrichia [14].
Geosmin is the finding with an operating cost attached: every sample carried the genes for it, and geosmin affects the taste of water and can warrant additional treatment [4]. The genes establish capacity in the populations sequenced, though how much geosmin a given bloom sends through an intake, and in which week, is beyond what a sequence can tell you.
The same genomes also say something about habitat: all the samples had genes that could let the organism grow on top of lake sediment, which the authors offer as insight into how it thrives in low-nutrient lakes [13]. It blooms in clear water, in thick mats of visible green or white puffballs [16]. "Prime swimming season here in New England is August, which also happens to be when they're most abundant," said Peter Countway, a senior research scientist at Bigelow and one of the study's co-authors [8].
Sleith puts the monitoring case on the other organisms in the water. "There's a whole consortium of cyanobacteria," he said. "Even if you don't have to worry about Gloeotrichia, it's valuable to understand who the other players are and what all the risks are. Ongoing monitoring with these molecular approaches can help answer those questions." [15]
What to watch
- Whether sampling outside these five states turns up a Gloeotrichia strain that does carry a toxin cluster.
- Whether anyone puts chemistry behind the uncharacterized molecules Sleith left room for.
- Whether New England water systems begin logging geosmin events against Gloeotrichia abundance.