Aureoumbra lagunensis

Aureoumbra lagunensis is a unicellular planktonic marine microalga that belongs in the genus Aureoumbra under the class Pelagophyceae. It is similar in morphology and pigments to Aureococcus anophagefferens and Pelagococcus subviridis. The cell shape is spherical to subspherical and is 2.5 to 5.0 µm in diameter. It is golden-coloured and is encapsulated with extracellular polysaccharide layers and has a single chloroplast structure with pigments. A. lagunensis thrives in a warm hypersaline environment as the greatest cell density has been found in water with salinity higher than 40 PSU and with temperatures between 25–30 °C (77–86 °F). Generally, the density of A. lagunensis is in the order of 106 cells mL−1, being higher in the summer months with lower abundance during the winter. A. lagunensis causes harmful algal blooms and was the dominant microalga in the 7 year long brown tide (1990 - 1997) that happened in Laguna Madre, Texas. Its dominance over other coexisting phytoplankton is in part associated with its encapsulating mucus layer of exopolymer secretions (EPS) and its ability to revert from vegetative and resting cell forms. The A. lagunensis blooms cause a substantial increase in light attenuation, which in turn contributes to marine biodiversity loss, particularly phytoplankton communities and benthic invertebrates. A. lagunensis thrives in low-light conditions, where it can maintain high growth rates at 150 umol photons m−2 s−1, thus an increase in light attenuation causes a positive feedback which further perpetuates blooms. A. lagunensis nutrient uptake is unlike other common microalgae. It uptakes inorganic nitrogen in the form of ammonium (NH4+) and nitrite (NO2−) and organic nitrogen in the form of urea, but does not utilize nitrate (NO3−). It uses environment dissolved organic phosphorus as the sole source to regenerate phosphate for growth. The name Aureoumbra lagunensis is Latin derived, where 'Aureo' comes from the word Aureus meaning golden, pertaining to the colour of A. lagunensis populations; umbra refers to the reduction in light penetration that occurs when blooms occur, and lagunensis refers to the lagoon environment, namely Laguna Madre, Texas, from which this organism was first isolated. A. lagunensis causes harmful algal blooms, most notably in Laguna Madre, Texas from 1990 to 1997, where it was first isolated. For over 20 years, A. lagunensis was confined to this bay system in Texas. More recently, small background concentrations of the species have been found in coastal bay regions along the rest of the Mexican Gulf coast, with larger populations found in Florida, Texas, and Mexico. In 2012 and 2013, A. lagunensis also caused brown tides in the Indian River Lagoon and Mosquito Lagoon, in Florida, USA. A. lagunensis brown tides have expanded as far as the Caribbean Sea into Guantánamo Bay, Cuba in 2013. A. lagunensis cause a bloom in Laguna Madre, Texas for seven years where it maintained densities from 0.5 x 106 cells mL−1 to 5 x 106 cells mL−1. The hypersaline conditions of Laguna Madre (45 – 75 PSU) is thought to be the cause of the bloom which allowed A. lagunensis to thrive, as they achieve maximal growth at 70 PSU and can continue to grow in environments of 10 – 90 PSU. The development of these hypersaline conditions was due to a period of drought which caused a loss of benthic and planktonic grazers. This effectively reduced the grazing pressure on phytoplankton like A. lagunensis. In December 1990 a large fish die-off occurred upon severe freezing which served to release nutrients like ammonium to further support the phytoplankton population. By October 1997, The Texas Brown Tide was disrupted, after seven years of uninterrupted growth, due to heavy rainfall. The first brown tide in Florida began during the summer of 2012 and collapsed a few months thereafter, and the second brown tide began in the spring of 2013 and collapsed in mid-summer 2013. During the brown tides in Florida, A. lagunensis consisted of 98% of the phytoplankton community with abundances greater than 2 x 106 cells/mL. Similar to the causes of the Texas brown tides, the Florida brown tides were proliferated by high salinity, low grazing pressure, high dissolved organic nitrogen and low inorganic nutrients, and competitive advantage over other phytoplankton. However, these brown tides were short-lived compared to that in Texas, lasting no more than a few months.