Red tide

Red tides occur naturally off coasts all over the world. Marine dinoflagellates produce ichthyotoxins, but not all red tides are harmful.[4][5] Where red tides occur, dead fish wash up on shore for up to two weeks after a red tide has been through the area. In addition to killing fish, the toxic algae contaminate shellfish. Some mollusks are not susceptible to the toxin, and store it in their fatty tissues. Shellfish consume the organisms responsible for red tide and concentrate saxitoxin (produced from these organisms) in their tissues. Saxitoxin blocks sodium channels and ingestion can cause paralysis within 30 minutes.[6] Other animals that eat the shellfish are susceptible to the neurotoxin, leading to neurotoxic shellfish poisoning[7] and sometimes even death. Most mollusks and clams filter feed, which results in higher concentrations of the toxin than just drinking the water.[1] Scaup, for example, are diving ducks whose diet mainly consists of mollusks. When scaup eat the filter-feeding shellfish that are concentrated with high levels of the red tide toxin, their population becomes a prime target for poisoning. However, even birds that do not eat mollusks can be affected by simply eating dead fish on the beach or drinking the water.[8] The toxins released by the blooms can kill marine animals including dolphins, sea turtles, birds, and manatees.[4][9] Fish such as Atlantic herring, American pollock, winter flounder, Atlantic salmon, and cod were dosed orally with these toxins in an experiment. Within minutes of receiving doses of the toxin, fish started to exhibit a loss of equilibrium and began to swim in an irregular, jerking pattern followed by paralysis and shallow, arrhythmic breathing and eventually death after about an hour.[10] Scientists concluded that the toxic red tide had negative effects on fish that were exposed to it.

Humans are affected by the red tide species by ingesting improperly harvested shellfish, breathing in aerosolized brevetoxins (i.e. PbTx or Ptychodiscus toxins) and in some cases skin contact.[3] The brevetoxins bind to voltage-gated sodium channels, important structures of cell membranes. Binding results in persistent activation of nerve cells, which interferes with neural transmission leading to health problems. These toxins are created within the unicellular organism, or as a metabolic product.[11] The two major types of brevetoxin compounds have similar but distinct backbone structures. PbTx-2 is the primary intracellular brevetoxin produced by K. brevis blooms. However, over time, the PbTx-2 brevetoxin can be converted to PbTx-3 through metabolic changes.[11] Researchers found that PbTx-2 has been the primary intracellular brevetoxin that converts over time into PbTx-3.[12]

In most cases like in the U.S., the seafood consumed by humans is tested regularly for toxins by the USDA to ensure safe consumption. However, improper harvesting of shellfish can cause paralytic shellfish poisoning and neurotoxic shellfish poisoning in humans.[7][6] Some symptoms include drowsiness, diarrhea, nausea, loss of motor control, tingling, numbing or aching of extremities, incoherence, and respiratory paralysis.[13] Reports of skin irritation after swimming in the ocean during a red tide are common, so people should try to avoid the red tide when it is in the area.[14]

When the red tide cells rupture, they release extracellular brevetoxins into the environment. Some of those stay in the ocean, while other particles get aerosolized. During onshore winds, brevetoxins can become aerosolized by bubble-mediated transport, causing respiratory irritation, bronchoconstriction, coughing, and wheezing, among other symptoms.[14] On a windy day, avoiding contact with the aerosolized toxin is recommended. These individuals report a decrease in respiratory function after only 1 hour of exposure to a K. brevis red-tide beach and these symptoms may last for days.[15] People with severe or persistent respiratory conditions (such as chronic lung disease or asthma) may experience stronger adverse reactions. The National Oceanic and Atmospheric Administration's National Ocean Service provides a public conditions report identifying possible respiratory irritation impacts in areas affected by red tides.[16]

The ICD-10 Diagnosis Code as provided by the Center for Disease Control (CDC) is Z77.121.[17] It is applicable to the following:[18]

• Contact with and (suspected) exposure to (harmful) algae bloom NOS
• Contact with and (suspected) exposure to blue-green algae bloom
• Contact with and (suspected) exposure to brown tide
• Contact with and (suspected) exposure to cyanobacteria bloom
• Contact with and (suspected) exposure to Florida red tide
• Contact with and (suspected) exposure to pfiesteria piscicida
• Contact with and (suspected) exposure to red tide

The dinoflagellate labeled above is the microscopic alga Karenia brevis. It is the cause of red tide in the Gulf of Mexico. The algae propel themselves using a longitudinal flagellum (A) and a transverse flagellum (B). The longitudinal flagellum lies in a groove-like structure called the cingulum (F). The dinoflagellate is separated into an upper portion called the epitheca (C) where the apical horn resides (E) and a lower portion called the hypotheca (D).

Red tide is a colloquial term used to refer to one of a variety of natural phenomena known as harmful algal blooms. The term specifically refers to blooms of a species of dinoflagellate.[19] It is being phased out by some researchers because:

1. Red tides are not necessarily red and many have no discoloration at all.
2. They are unrelated to movements of the tides.
3. The term is imprecisely used to refer to a wide variety of algal species that are known as bloom-formers.

As a technical term, it is being replaced in favor of more precise terminology, including the generic term "harmful algal bloom" for harmful species, and "algal bloom" for benign species.

The term red tide is most often used in the US to refer to Karenia brevis blooms in the eastern Gulf of Mexico, also called the Florida red tide. K. brevis is one of many different species of the genus Karenia found in the world's oceans. [20] Major advances have occurred in the study of dinoflagellates and their genomics. Some include identification of the toxin-producing genes (PKS genes), exploration of environmental changes (temperature, light/dark, etc.) have on gene expression, as well as an appreciation of the complexity of the Karenia genome.[20] These blooms have been documented since the 1800s, and occur almost annually along Florida's coasts.[20] There was increased research activity of harmful algae blooms (HABs) in the 1980s and 1990s. This was primarily driven by media attention from the discovery of new HAB organisms and the potential adverse health effects of their exposure to animals and humans.[21] The Florida red tides have been observed to have spread as far as the eastern coast of Mexico.[20] The density of these organisms during a bloom can exceed tens of millions of cells per litre of seawater, and often discolor the water a deep reddish-brown hue.

Red tide is also sometimes used to describe harmful algal blooms on the northeast coast of the United States, particularly in the Gulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. These blooms of organisms cause severe disruptions in fisheries of these waters, as the toxins in these organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin.[22] The related Alexandrium monilatum is found in subtropical or tropical shallow seas and estuaries in the western Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, and the eastern Pacific Ocean.

Red tide (NOAA)

Red tides contain dense concentrations of organisms and appear as discolored water, often reddish-brown in color. It is a natural phenomenon, but the exact cause or combination of factors that result in a red tide outbreak are not necessarily known.[2] However, three key factors are thought to play an important role in a bloom - salinity, temperature, and wind. Red tides cause economic harm, so outbreaks are carefully monitored. For example, the Florida Fish and Wildlife Conservation Commission provides an up-to-date status report on red tides in Florida.[23] The Texas Parks and Wildlife Department also provides a status report.[24] While no particular cause of red tides has been found, many different factors can contribute to their presence. These factors can include water pollution, which originates from sources such as human sewage and agricultural runoff.[25]

The occurrence of red tides in some locations appears to be entirely natural (algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents)[26][27] while in others they appear to be a result of increased nutrient pollution from human activities.[28] The growth of marine phytoplankton is generally limited by the availability of nitrates and phosphates, which can be abundant in agricultural run-off as well as coastal upwelling zones. Coastal water pollution produced by humans and systematic increase in seawater temperature have also been implicated as contributing factors in red tides. Other factors such as iron-rich dust influx from large desert areas such as the Sahara Desert are thought to play a major role in causing red tides.[29] Some algal blooms on the Pacific Coast have also been linked to occurrences of large-scale climatic oscillations such as El Niño events. While red tides in the Gulf of Mexico have been occurring since the time of early explorers such as Cabeza de Vaca,[30] what initiates these blooms and how large a role anthropogenic and natural factors play in their development is unclear. Whether the apparent increase in frequency and severity of algal blooms in various parts of the world is in fact a real increase or is due to increased observation effort and advances in species identification methods is also debated.[31][13]

Increasing temperature, enhanced surface stratification, alteration of ocean currents, intensification or weakening of local nutrient upwelling, stimulation of photosynthesis by elevated CO₂, reduced calcification through ocean acidification, and heavy precipitation and storm events causing changes in land runoff and micronutrient availability may all produce contradictory species- or even strain-specific responses.[32] In terms of harmful algal blooms (HABs), we can expect: (i) range expansion of warm-water species at the expense of cold-water species, which are driven poleward; (ii) species-specific changes in the abundance and seasonal window of growth of HAB taxa; (iii) earlier timing of peak production of some phytoplankton; and (iv) secondary effects for marine food webs, notably when individual zooplankton and fish grazers are differentially impacted by climate change.[32] However, the potential consequences of these changes for HABs have received relatively little attention and are not well understood. Substantial research is needed to evaluate the direct and indirect associations between HABs, climate change, ocean acidification, and human health.[33]

A multi-partner project funded by the federal EcoHab program (NOAA) and published by the Mote Marine Laboratory shows a list of what feeds red tides.[34] A study from the Florida FWC shows the Karenia brevis algae red tide found in Florida is fed and worsened by nitrogen (N) and phosphorus (P).[35]