History of Karenia brevis and Red Tides. Karenia brevis is a marine dinoflagellate, commonly found in the Gulf of Mexico and is responsible for red tides (now more commonly called harmful algal blooms) from west Florida to Texas. The characteristics of Karenia brevis (e.g. fish kills, toxic shellfish, and human respiratory irritation) have been seen in the Gulf of Mexico off the coasts of Florida, Texas and Mexico in numerous occurrences as far back as the nineteenth century (Steidinger, 2009). The organism was recorded as a Gymnodinium species by F.G. Walton Smith and P.S. Galstoff following a bloom from 1946-1947 bloom (Woodcock, 1948). Karenia brevis was classified by Charles C. Davis Gymnodinium brevis, which he studied because of the noticeable killing of marine life (1948). Decades later, Hansen and Moestrup were the first to characterize the genus Karenia (Daugbjerg, et al., 2000) and five of the fifteen known species were identified and co-occurring in the Gulf of Mexico (Steidinger et al., 2008). Karenia brevis is the only documented dinoflagellate in the world whose blooms are known to have concurrent fish kills, neurotoxic shellfish poisoning, and human respiratory irritation making the study and understanding of this marine dinoflagellate an important research topic (Steidinger, 2009).
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Karenia Brevis is a unicellular, photosynthetic marine dinoflagellate, which means it utilizes energy from the sun and nutrients from the surrounding water to make an energy-like glucose. The cell itself is unarmored (or naked) and small to medium-sized (18-45 μm wide). Karenia brevis is composed of 10-20 peripheral chloroplasts that contain chlorophyll. The cell is eukaryotic meaning its nucleus has a nuclear envelope and contains typical eurkaryotic organelles, including mitochondria and golgi (Steidinger, et. al 1978).
Effects of an Estuarine Environment on Karenia brevis
As mentioned above, Karenia brevis is photosynthetic, so its living environment is important to its life cycle. Several factors play key roles in the dynamics of a successful bloom of Karenia brevis which include optimum levels of salinity, temperature, dissolved oxygen, and nutrients (any compound an organism takes from its environment except oxygen, carbon dioxide, and water). It has been shown that temperature and salinity ranges in which Karenia brevis can survive were 9-33 °C and 17 to 40 and the most favorable conditions were 20-28 °C and 31-37 (Finucane and Dragovich, 1959; Rounsefell and Dragovich, 1966; Dragovich and Kelly, 1966). The temperature range plays an important role in the metabolism of Karenia brevis because warmer water is usually accompanied by a deeper penetration of light, giving more area for the organism to grow and reproduce, as well as more access to nutrients deeper in the water column. Salinity measures the amount of dissolved inorganic solids in the water. This gives us an idea as to how much dissolved nutrients are available to the organism.
One of the important concepts in understanding the bloom of Karenia brevis is the spring bloom. Ideal growing conditions for a bloom consist of a water column that is strongly mixed with the nutrients present in the water throughout the column. During the summer, the column becomes highly stratified. This is result of the surface waters, where light is readily available, being depleted of nutrients during primary production and respiration producing nutrients below the light level. In contrast, the winter time experiences water temperatures outside the preferred living conditions of the organism. During these times the nutrient storage in the water builds up. This result in the spring and fall operating as ideal times when the temperature is warm enough and the water column is mixed enough to support high production of Karenia brevis. Generally due to the greater amount of available nutrients, spring experiences the greatest production and the term, spring bloom, refers to this production (Iriarte, 2004). It is generally accepted that a concentration of 106 Karenia brevis cells per liter of water qualifies as a bloom (Wilson, 1966).
The synchronized timing of ideal water temperature and high concentration of nutrients place an essential role in Karenia brevis' photosynthesis-respiration cycle. This photosynthesis-respiration cycle is vital to the production of energy for all photosynthetic organisms. Photosynthesis is simply the utilization of water, nutrients (such as nitrogen and phosphorus), and sunlight to make oxygen (usually given off) and organic matter (kept in the form of proteins, fats, carbohydrates or nucleic acids). Respiration is done by microbial organisms that break down the organic matter produced in photosynthesis (even if it has moved through a food chain into heterotrophs) to give off nutrients, water, and carbon dioxide.
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Karenia brevis has the capacity to adapt and be photosynthetically resourceful in varying wavelengths of light through its flexible pigment systems (Kusek et al., 1999). This ability to adapt to the amount of light it receives allows Karenia brevis to thrive under certain conditions. Under exactly what conditions lead to cause a bloom, there are many hypotheses (Vargo et al., 2008), but some factors have been observed to be consistent throughout many different blooms.
Nutrient Effects and Availability
In addition to salinity, temperature, and light, the role of nutrients such as phosphorus and nitrogen are essential to the growth of Karenia brevis. As mentioned earlier, nutrients can be any compound an organism takes from its environment except oxygen, carbon dioxide, and water. In the early years of research on requirements of Karenia brevis blooms focused on phosphorus until Dragovich et al. (1963) concluded that high concentrations of phosphorus were not required to support Karenia brevis blooms. Few Karenia brevis cells were found in waters with total phosphorus of less than 0.2 μM (micro molar) or greater than 4 μM. This was confirmation that phosphorus, though key in the maintenance and support of a bloom of Karenia brevis, is a non-limiting factor to the dinoflagellate (Vargo et al., 2008). Though Karenia brevis is adapted for growth in environments with low phosphorus content it does not mean phosphorus does not play a role in dynamics of a bloom. The calculated cellular yield per unit of phosphorus was found that between 2 and 9x106 cells of Karenia brevis can be produced per millimole of available phosphorus. (Vargo and Howard-Shamblott, 1990). From this it has been inferred that unidentified sources of phosphorus are required to support any growth in biomass during one of the phytoplankton's blooms that can occur for months at a time. To maintain this high amount of Karenia brevis, the phosphorus supply would have to be replenished from rapid microbial processes or from high input from an external source of phosphorus to the water column (Vargo et al., 2008).
The understanding of the sources of many nutrients is still unknown but there are areas that have been narrowed down. The availability of phosphorus to a water body is largely dependent on the speciation of phosphorus. The most useful form is phosphate (PO43-) and how readily phosphate occurs depends on the form of phosphate that enters the water body (Bianchi, 2007). Atmospheric input of phosphorus is mostly insignificant and is rarely factored, and most phosphorus enters estuarine and ocean environments from river flow (Bianchi, 2007). It is in the estuarine environment that most phosphorus species are broken down into usable forms of phosphate (Bianchi, 2007).
The nutrient believed to be the limiting factor for Karenia brevis is nitrogen, rather than phosphorus. (Hecky and Kilham, 1988). However, due to previous understanding of the two, there is less information on the metabolism of nitrogen. It is inferred by Vargo et al. (2008) that a typical bloom of Karenia brevis requires approximately 0.48 μM Wilson, 1966 Wilson, W.B., 1966. The suitability of sea water for the survival and growth of Gymnodinium breve Davis; and some effects of phosphorus and nitrogen on its growth. Florida State University Professional Paper Series No. 7, Florida State Board of Conservation, 42pp.of phosphorus, would then require approximately 8.6 μM nitrogen to maintain the concentrations of nitrogen to phosphorus in the cells, which is 17.7:1 (Shanley and Vargo, 1993), higher than the Redfield-Richards ratio of 16:1, which outlines the average nitrogen to phosphorus ratio in phytoplanton (Bianchi, 2007).
Similarly to phosphorus, nitrogen has to be fixed into ionic forms such as ammonium (NH4+), nitrate (NO3-), or nitrite (NO22-) before it can be readily used for primary production. Though no one is yet to quantify the amount of nitrogen required for a bloom, it has been inferred that 106 cells would require 120 milligrams of nitrogen (~8.6 μmoles) based on the cellular N:P ratio (Wilson, 1966 cited by Vargo, 2009). However it was determined by Vargo et al. that there are insufficient concentrations of nitrogen and phosphorus present in waters off the western coast of Florida to allow for a Karenia brevis bloom (2008). So where then do these nutrients come from?
The supply of nutrients available to support Karenia brevis blooms have been questioned since the organism was recognized as the source of these harmful blooms. Nutrient additions can lead to eutrophication, a process that causes a rise of the nutrient rich deep waters to allow the nutrients to be available in more shallow depths (Jorgensen and Richardson, 1996). Inputs of nitrogen have been hypothesized to come from numerous sources such as water runoff, the atmosphere, and even cycling through the water (Paerl et al., 2002).
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Since Karenia brevis is a nitrogen-limited organism, the anthropogenic nitrogen inputs into watersheds often end up in exporting into coastal water bodies (Valiela et al., 1990). These high concentrations of nitrogen associated with watershed systems running into estuaries have been known to cause elevated levels of primary production in harmful algal blooms, similar to Karenia brevis (Burkholder et al., 1992).
In addition to watersheds, another source of nitrogen to the estuaries is from plants. The phytoplankton themselves can even contribute to the nitrogen content. Growing phytoplankton communities give off dissolved free amino acids that are broken down into dissolved nitrogen by bacteria (Berman and Bronk, 2003). Also plant detritus leaching, terrestrial runoff, soil leaching and sediment can add to the nitrogen content (Berman and Bronk, 2003).
Another theory is that the nitrogen is recycled within the estuary. Nixon et. al show that up to 65% of nitrogen in estuaries is retained and recycled rather than it moving on to the open ocean (1996). This nitrogen recycling occurs both in the sediments and in the water column and involves (but not limited to) nitrogen fixation, nitrification, denitrificaiton as the nitrogen is moved into different ions by bacteria and other microbial organisms (Wollast, 1993).
The Role of Organic Matter
Organic matter in estuaries is usually divided in estuaries based on size. Organic matter that is greater than 0.45 μm is considered particulate. Organic matter that is less than 0.45 μm is considered dissolved (Bianchi, 2007). The dissolved organic matter is the one that plays the biggest role in primary production of Karenia brevis which is usually less than 0.45 μm in size itself. Dissolved organic matter (DOM) is vitally important to the life of autotrophic organisms. The consumption and transformation of organic matter by microbial communities transfers the DOM into usable nutrients that can be used in primary production (Wetzel, 1995).
It was mentioned earlier that rarely enough nutrients are available for a bloom to begin. And it was determined that inputs of nutrients to a water body can lead to high enough levels of nutrients to support the initial bloom. But what keeps these blooms going for months at a time? However it has been speculated that the fish decay theory could only provide the necessary phosphorus but not the nitrogen, though floating and decay fish could provide the source of nutrients to Karenia brevis (Vargo et al., 2008). The most likely sources of nutrients to support a Karenia brevis bloom, other than existing concentrations of nutrients, come from the estuarian flux in the water column and the excretion of zooplankton (Vargo et al., 2008). However, a newly proposed theory is that the decay of fish killed during the bloom may result in increased amount of organic matter. This new supply of organic matter can then be broken down by microbial organisms to continuously replenish the nutrient concentrations (Walsh et al., 2009). This could therefore mean than that the results from the preceding bloom can give rise to additional growth of the existing bloom of Karenia brevis. The effect that organic matter has when added to the nutrient pool is what this study will examine.