Our world depends on algae. Without it, our climate, ecosystems, and life as we know it would all look very different.

Often mistaken for plants, algae are a group of photosynthetic organisms of the kingdom Protista that range in size from microscopic plankton to giant kelp forests that exceed 60 metres in length. They serve as the base of food webs worldwide, contribute to the carbon pump by removing CO2 from the atmosphere, and are a valuable source of crude oil for industrial action across the globe.

In the right conditions, namely, high nutrients and light availability, microscopic algae can rapidly accumulate, or “bloom”. These algal blooms can reach such proportions that it is possible to view them from space via satellite imagery.

A toxic algal bloom in Lake Erie viewed from space. USGS

The presence of algae in the form of microscopic phytoplankton, provides an abundance of food for higher trophic organisms and can support complex marine food webs.

But it’s not all good news. Some species of algae have the potential to cause harm to marine ecosystems and mankind alike. If the right conditions are met, these species can bloom and even stain the water red.

“And all the waters that were in the river were turned to blood. / And the fish that was in the river died; and the river stank” – Exodus 7:21

What is a Red Tide?

A red tide is a type of harmful algal bloom (HAB), appropriately named for its tendency to discolour the sea surface. Typically, red tides occur along coastal regions, in areas of terrestrial runoff. This runoff comprises of fertilizer, sewage, and livestock wastes all of which contain high concentrations of fixed nitrogen and phosphate, the two essential nutrients for algal growth.

When introduced into the aquatic environment, these nutrients trigger an algal bloom; a process known as eutrophication. In the majority of cases, this anthropogenic-induced bloom has the potential to wreak havoc on local ecosystems as well as cause a myriad of problems for mankind.

The so called, harmful algal blooms can directly or indirectly harm other aquatic organisms. When a thick algal mat is formed on the surface of a body of water, the amount of sunlight that reaches the deeper waters is significantly decreased. This in turn causes the death of photosynthesising organisms within the water and may also indirectly lead to animal mortality. Aerobic bacteria then decompose the deceased organisms, thereby depleting dissolved oxygen concentrations and rendering the habitat anoxic, commonly called a “dead-zone”. As a result, the habitat has lost a great amount of diversity; the implications for the fishing industry are obvious.

In more extreme cases, HABs are associated with algal-toxins, or phycotoxins, which are produced by a number of phytoplankton, most commonly dinoflagellates. When exposed to high nutrient concentrations, these organisms bloom and can “dye” their inhabited waters red. The majority of red tides are, however, not red; the colour of the bloom is dependent on the chlorophyll pigment characteristic of the species. Putting aside the misleading name, one thing is for sure: red tides pose a serious problem in desperate need of a solution.

The Culprits

Karenia brevis

Karenia brevis is a microscopic dinoflagellate, found in the waters of the Gulf of Mexico. This species is guilty of causing the “Florida red tides” that run rampant along the Gulf coasts of Florida and Mexico. Each individual possesses two flagella, which facilitate movement through the water column; K. brevis can travel great lengths around the Florida peninsula, being found as far north as the Carolinas.

K. brevis is capable of producing an array of lethal neurotoxins which are known as brevetoxins. These toxins have the potential to cause harmful gastrointestinal, respiratory, and neurological effects. As such, the presence of K. brevis red tides has been linked to mass mortalities of seabirds and larger marine organisms such as manatees. In addition to this, K. brevis can accumulate in filter-feeding molluscs such as mussels, where it is then passed on through human consumption where it causes neurotoxic shellfish poisoning.

A Karenia brevis red tide off the coast of Texas. NOAA

Pfiesteria piscicida

Pfiesteria piscicida is yet another dinoflagellate, responsible for harmful algal blooms in the esturaies and coastal waters of North Carolina and Maryland. Unlike Karenia brevis, this dinoflagellate lacks photosynthetic pigments, making it more akin to a flagellated animal.

P. piscicida is largely held responsible for major fish kills and a myriad of human illnesses. As such, this little organism earned itself a nasty reputation in North Carolinian media in the 1990s and was known as “the cell from hell”.

The life-history of P. piscicida has been a subject of debate for many years. When it was first described, P. piscicida was believed to have as many as 24 life cycle stages including cyst, amoeboid, and the regular dinoflagellate form. However, further studies did not find any evidence of these unusual life cycle stages, instead finding that of a typical marine dinoflagellate.

The exact extent and mechanism employed when killing marine organisms is also up for debate. DNA analysis suggests that this organism lacks the ability to produce the toxins associated with most dinoflagellate species. In fact, a summary on the research of P. piscicida concluded that Pfiesteria is not a threat to human or animal health as was originally believed. Fish kills and lesions observed in laboratory conditions were only recorded when Pfiesteria were in considerably higher numbers than found in natural conditions. The true nature of Pfiesteria piscicida is still the subject of much marine research.

A single Pfiesteria piscicida under an electron microscope. UNC Sea Grant College Program

What can be done?

The National Oceanic and Atmospheric Administration estimates that harmful algal blooms account for the loss of 82 million dollars in the seafood, restaurant, and tourism industries every year. NOAA scientists continue to monitor HABs so as to avoid drastic consequences. The FWC Fish and Wildlife Research Institute provide daily sample maps and a forecast of red tides. In conjunction with increased awareness and research efforts, monitoring and reporting HABs help protect coastal ecosystems and may yet mitigate this infamous effect of human influence on our oceans.

Written by Lucas King