Dolichospermum (formerly Anabaena) is a cyanobacteria genus that is commonly found in freshwater phytoplankton assemblages. In nutrient-rich lakes it can form dense blooms. Superficially, Dolichospermum and Anabaena look similar, but Dolichospermum can form gas vesicles, making it planktonic. Anabaena never forms gas vesicles and is usually associated with benthic environments. There are many common species of Dolichospermum, and more than one species may be present simultaneously in a toxic cyanobacteria bloom.
Individual Dolichospermum cells are spherical, oval, or barrel-shaped, and tiny (width = 5-20 μm; for comparison, a strand of spider silk is about 5 μm wide). Under magnification, Dolichospermum cells are dark brown and appear granular or mottled due to gas vesicles in the cells. (Anabaena, which does not form gas vesicles, will have cells that are pale blue-green or blue-gray in color.) The cells are joined together end-to-end to form long, unbranched filaments that are surrounded by clear, often transparent mucilage. Depending on the species, the filaments can be straight, bent, coiled, or irregularly twisted, and may be solitary or aggregated into tangled clumps.
In addition to ordinary (vegetative) cells, the filaments may contain pale blue heterocytes (also called heterocysts) and large, granular, thick-walled akinetes. Heterocytes are specialized cells that convert dissolved nitrogen gas into ammonium that can be used for cell growth. Akinetes are resting cells that are resistant to cold temperatures and other unfavorable environmental conditions, and can overwinter in lake sediments. Heterocytes, and especially akinetes, are used to identify Dolichospermum (and Anabaena) to species.
Dolichospermum blooms often form during warm, calm weather in lakes and ponds with relatively high nutrient concentrations (nitrogen or phosphorus) or low nitrogen to phosphorus ratios (N:P<15).
Because Dolichospermum is capable of converting dissolved nitrogen gas ammonium, it can dominate blooms when inorganic nitrogen (ammonium, nitrate, and nitrite) is limiting to other types of algae.
Dolichospermum blooms usually contain other types of Cyanobacteria, especially Aphanizomenon , Gloeotrichia, Microcystis, and Woronichinia .
Although Dolichospermum filaments are rarely consumed by zooplankton, the cells are may be parasitized by viruses, bacteria, or fungi, and the tangled clumps of Dolichospermum may be colonized by stalked, filter-feeding zooplankton.
Identifying which cyanobacteria species are producing toxins is more difficult that it sounds. Historically, cyanobacteria taxa were described as "potentially" toxic based on whether they were collected in a toxic bloom. With the advancement of culturing techniques and genetic analysis, toxicity information is becoming more exact. But this is an ongoing process, so the toxicity information on these pages should be considered a work in progress.
Dolichospermum cells may produce microcystins (liver toxin), cylindrospermopsin (liver toxin), anatoxins (nerve toxin), saxitoxins (nerve toxin - paralytic shellfish toxin group), lipopolysaccharides (skin irritants), and BMAA (beta-Methylamino-L-alanine; nerve toxin). These toxins are released into the ambient environment when the cell wall is disrupted (cell lysis).
Higher water temperatures and light appear to be associated with increased toxin production.
Not all Dolichospermum blooms result in the release of toxins.
Many species in the genus Anabaena have been moved to Dolichospermum. Revisions for some of the common species associated with toxic blooms are as follows:
Dolichospermum planctonicum (formerly Anabaena planctonica or Anabaena solitaria f. planctonica).<\li>
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This guide was prepared by Dr. Robin Matthews, former Director of the Institute for Watershed Studies (http://www.wwu.edu/iws/) and professor emeritus at Western Washington University. In addition to this guide she has also written two ebooks (more on the way) on phytoplankton identification (see the "algae books" link on http://www.wwu.edu/iws/) and an online key to the cyanobacteria (http://www.snoringcat.net/cyanobacteria_key/index.html).