Karenia brevis
A Microbial Biorealm page on the genus Karenia brevis
Classification
Higher order taxa
Eukaryota; Alveolata; Dinophyceae; Gymnodiniales; Gymnodiniaceae; Karenia4
Species
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NCBI: Taxonomy |
Karenia brevis
Description and Significance
Karenia brevis is an aquatic marine organism in the phylum Dinoflagellate and super group Alveolates. These flagellated Protists also referred to as algae, are of microscopic proportion usually between 20 and 40 mm in size. They are unicellular, flagellated, photosynthetic organisms with cellulose plates (theca) that surround the cell as the outer surface. The plates are secreted by Alveoli (membrane bound vesicles just below the cell membrane)- hence their super group name- and create the outer boundary for the cell. Located on the cellulose plates are two grooves called the transverse and longitudinal groove where K. brevis’s two flagellum are located and attached. One flagellum wraps around the body of the cell in the transverse groove, while the other extends from the body of the cell on the longitudinal groove. Using their flagella for locomotion, they are able to have some source of propelling movement in the water column. Both the theca and flagella are visible in Figure 1. These dinoflagellates are usually found in abundant masses near coastal waters in warmer conditions. Though they are found in several other places in the vast ocean water, this area is of particular concern. K. brevis has an active involvement in harmful algal blooms or “red tides” off the coasts of many places around the world. This is a problem due to the potent neurotoxins called brevetoxin’s that these cells create. When there is an abundance or bloom of these organisms’ resources become limited. There is more competition for space and sunlight, as these organisms die from lack of resources they release their neurotoxins. The neutoxin’s cause all sorts of environmental and economic problems such as massive fish kills, fisheries crashing, paralytic shellfish poisoning, etc. This is a dangerous time for humans to eat seafood and can cause some major health problems2.
Genome structure
K. brevis has a large haploid genome consisting of about 1 x 1011 bp. It consists of permanently condensed chromatin that lack nucleosomes. The condensed chromosomes have a characteristic banding pattern with stacked disks that form a continuous left-handed twist along the longitudinal axis. The disks end in less tightly packed loops of DNA that contain actively transcribed DNA7. Due to its large size, the K. brevis genome has not yet been sequenced.
Cell structure and metabolism
Karenia brevis is an unarmored dinoflagellate. It uses its two flagellas to move more easly through the water. K. brevis is about 20-40 μm in size. The nucleus is round and commonly found in the lower left quadrant of the cell. Cholorplasts are present within the cell which makes the cell a yellow-green color. K. brevis is known to be a mixotrophic (which means it can gain energy from a combination of sources). It gains its energy by utilizing organic molecules such as, nitrogen and phosphorus. It also can ingest other photosynthetic prokaryotes known as Synechococcus. Dinoflagellates have high cellular respiration rates as well3.
Ecology
Karenia brevis is a dinoflagellate which is found in the Gulf of Mexico, along the coasts of Texas, Louisiana, Florida, and North Carolina. They are photosynthetic and perform much of the area's primary production. Because they require light, they cannot live at depths below 200 feet. Karenia brevis has a temperature range between 4 and 33 degrees Celsius. However, their optimal range is 22-28 degrees Celsius. In addition, this organism can live in a salinity of between 25-45 ppt. While they are not symbiotic organisms, they do provide a great deal of oxygen to the environment with one estimate stating they perform around 20% of the primary production in the West Florida Shelf during blooms or red tides, as is shown in Figure 2. While researchers are unsure of the conditions necessary for these red tides, several hypotheses revolve around the species' requirements for metals2.
Pathology
Algal blooms can occur when there is a change in chemical levels in the water. A chemical change in the water can take place for multiple reasons. The most common is when chemicals are dumped in the water from local run-off zones. These chemicals can come from certain fertilizers used for agricultural growth near a coastal run-off zone. If chemical levels such as nitrogen increases past normal levels, the algae will use this for nitrogen fixation and reproduce rapidly. This is what we call the “algal bloom”, and these blooms can deplete the oxygen in the water and create a shade from the sun, preventing organisms that need sunlight from obtaining it. K. Brevis produces harmful algal blooms that cause red tides. When red tides occur, toxins are let out in the oceans and may kill or harm marine animals, as well as cause several human illnesses that can arise from eating seafood that have retained levels of these toxins2.
K. brevis produce neurotoxins when there is a bloom. The toxins are called brevetoxins and the brevetoxin specific to K. brevis is labeled PbTx-2. These lipid soluble brevetoxins adversely affect human health as well at ecological ecosytems. These toxins will activate voltage-gated sodium channels in the body directly harming the nervous system of an organism even at small concentrations. This can result in neurological symptoms in the affected organisms. The most common way for humans to be exposed to these toxins is by the consumption of contaminated shellfish. Though these toxins do not affect the shellfish, the brevetoxins will exist in the tissues of the shellfish. A human eating shellfish too close to a red tide can get an illness called Neurotoxic Shellfish Poisoning. Ecological health effects include massive mortality rates for invertebrates, fish, birds and even some marine mammals. This could be due either by direct exposure to the toxins themselves, or from the brevetoxins in the food web1.
For marine organisms these toxins can cause disorientation, losing their ability to hunt or navigate the oceans, and can also cause them to not be able to swim properly., putting them in a paralyzed position causing death. For humans the effects of ingesting these toxins are severe and also include paralysis1.
Current Research
Immune Response to Aerosolized Brevetoxins
The harmful brevetoxins produced by Karenia brevis during red tide blooms cause health concerns when they are ingested from eating contaminated shellfish, or inhaled when the toxins become aerosolized. The formation of aerosolized toxins occurs through lysis of the K. brevis cells by wave action in the tides. PbTx-2 is the most prevalent brevetoxin variety in marine aerosol and is linked to the deaths of many marine mammals. Humans, as well as marine mammals, are a hight-risk group to brevetoxin inhalation. Exposure to the aerosolized toxins result in eye and throat irritation, nasal congestion, cough, wheezing, shortness of breath, and further complications in individuals with chronic inflammatory lung conditions. While a link between symptoms and toxin exposure has been established, the exact causative mechanism behind the pathology has not been concluded5.
When an inhaled pathogen enters the lungs the innate immune response is the immediate response and results in anatomic, physiologic, and inflammatory mechanisms. The primary cells of this response are macrophages which are involved in maintaining inflammatory reaction and recruiting additional immune cells. In order to establish whether brevetoxins stimulate injury due to an immune response or from a direct cytotoxic effect on cells, Sas and Baatz used an alveolar macrophage cell line (MH-S) to test for cell growth, cytokine secretion, phagocystosis, and gene regulation following exposure to brevetoxin-2. The results of the study show that, not surprisingly, PbTx-2 is responsible for initiating an inflammatory response in MH-S cells in vitro. Data show that following PbTx-2 exposure, macrophage phagocytosis was enhanced, inflammatory-mediating cytokine secretions were altered, but there was little change in gene expression. Collectively these results conclude that PbTx-2 initiates inflammatory immune response mechanisms in lung alveolar macrophages. Interestingly, Sas and Baatz found that PbTx-2 did not significantly alter MH-S cell growth rates which supports the notion that the brevetoxin does not directly induce cytotoxic effects on alveolar cells. The researchers do state that with increased PbTx-2 concentrations, cell viability was reduced, however, these toxin concentration were extremely elevated and were deemed unlikely to occur in the environment based on previously published air concentrations and normal lung volume and inspiration rates. While this study does only specifically focus on one lung cell type, it does stake a claim that macrophages, and thus inflammation response, are directly affected by aerosolized brevetoxins and further research on inhaled biotoxins may provide insight into immune response to such antigens and the body's ability to recover from exposure5.
Food-web Disruption During Karenia brevis Red Tides.
Zooplankton feed on phytoplankton and thus control their growth. Some phytoplankton, however, can create harmful algal blooms (HABs) that make them less edible to zooplankton and alter the balance of the ecosystem. The release of these HAB species can lead to a positive feedback interaction that supports the bloom formation and proliferation while simultaneously starving the grazing species of the ecosystem8.
The red tides produced by the dinoflagellate Karenia brevis are routinely present along the western coast of Florida. The blooms are usually monospecific and become highly toxic due to the release of brevetoxins. While there are multiple brevetoxins, the most potent varieties, PbTx-1, PbTx-2, and PbTx-3, are all produced by K. brevis. It has been noted that during the K. brevis blooms, many grazing species select against consuming K. brevis and will even choose to survive on lower ingestion and reproductive rates. Researchers attribute this change in diet to the brevetoxin levels within the blooms. In order to test what effect the brevetoxins were having on the ecosystem Waggett et al. tested the consumption and reproduction rates of grazers with diets consisting of either highly toxic, mildly toxic, or non-toxic brevetoxins. Results showed that the grazing population with exposure to the highly toxic K. brevis brevetoxins had lower consumption rates, reduced egg production, and individuals that consumed the toxins showed lower survival rates than the individuals that chose to starve instead of consume the K. brevis8.
It was concluded from these results that the alteration in the grazers diet is specifically due to the brevetoxins in the bloom that were not only nutritionally insufficient but also increased consumer mortality rates. The results from the experiments with the mildly toxic and non-toxic brevetoxin diets also showed reduced consumption and insufficient nutritional value which led to a decrease in egg production. This was attributed to the fact that K. brevis lacks the ability to produce cholesterol which many grazing populations require from their food source. This result shows that it is not only the toxicity of the bloom that alters the balance of the ecosystem, but the proliferation of the K. brevis species with little outside competition from other phytoplankton species that causes grazer mortality rates to increase. This study provides evidence that Karenia brevis has evolved mechanisms to reduce grazing pressure and promote their own survival during blooms, which alters food web dynamics in the immediate ecosystem and leads to further wide-spread effects8.
Utilizing Competing Phytoplankton to Decrease Karenia brevis Bloom Toxicity.
Red tides in the Gulf of Mexico occur during blooms of Karenia brevis which produce brevetoxins. The production of the brevetoxins has a wide-spread effect ecologically, and is known to be harmful to organisms ranging from marine inverterates, fish, and seabirds, to manatees and dolphins. A current study by Redshaw et al. proposed that by lowering brevetoxins through the presence of competitive phytoplankton, the harmful effects of the toxins on marine invertebrates was reduced. It was found that a range of competitor phytoplankton species were able to reduce the concentrations of PbTx-1 and PbTx-2, the most toxic and abundant varieties of brevetoxins. While it is hard to currently predict the level of toxicity a bloom will create, it may be possible to use competative phytoplankton as a biocontrol agent to reduce the toxic effects of the brevetoxins. Redshaw et al. suggest that populations of competing phytoplankton or proteins derived from them should be utilized as a method to control bloom toxicity and reduce ecosystem-wide deleterious impacts6.
Cool Factor
An interesting fact about Karenia brevis is the fact that they are not passive particles that drift with the current, but rather are microbes that travel using vertical migration. These microbes travel to areas of the sea in order to optimize the carbon fixation from photosynthesis. During the day the microbe stays near the top of the surface to obtain the nutrients from the sun. At night K. brevis travel to the bottom of the ocean where dissolved nutrients have fallen. The most interesting part of this, is that they can travel at speeds up to 1 m/h which tend to be driven my phototaxis and geotaxis. That means K. brevis can travel up to 100,000 x its body length per hour7.
References
1. Cohen, J., Tester, P., & Forward, R. (2007). Sublethal effects of the toxic dinoflagellate karenia brevis on marine copepod behavior. Journal of Plankton Research, 29(3), 301-315.
2. Harmful algal blooms. (2012, January 13). Retrieved from http://www.cdc.gov/nceh/hsb/hab/default.htm
3. Hitchcock, Gary L. "Net Community Production and Dark Community Respiration in a Karenia Brevis (Davis) Bloom in West Florida Coastal Waters, USA." Harmful Algae 9.4 (2010): 351-58
4. Ncbi. (n.d.). Retrieved from http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi
5. Redshaw, C.H., et al., Tracking losses of brevetoxins on exposure to phytoplankton competitors: Ecological impacts. Harmful Algae (2011), doi:10.1016/j.hal.2001.09.007
6. Sas, K.M., and Baatz, J.E., Brevetoxin-2 induces an inflammatory response in an alveolar macrophage cell line. International Journal of Hygiene and Environmental Health (2010), doi:10.1016/j.ijeh.2010.06.007
7. Van Dolah, F.M., et al., The Florida red tide dinoflagellate Karenia brevis: New insights into cellular and molecular processes underlying bloom dynamics. Harmful Algae (2009), doi:10.1016/j.hal.2008.11.004
8. Waggett, R.J., et al., Toxicity and nutritional inadequacy of Karenia brevis: synergistic mechanisms disrupt top-down grazer control. Marine Ecology Progress Series (2012), doi:10.3354/meps09401
9. Karenia brevis (Davis) G. Hansen et Moestrup 2000 (Gymnodinium breve Davis, 1948; Ptychodiscus brevis (Davis) Steidinger 1979)
10. Red tide general collection. (2012, February 4). Retrieved from http://serc.carleton.edu/microbelife/topics/redtide/general.html.
Edited by student of Iris Keren
