Algal Blooms: How One-Celled Microorganisms Can Crash an Ecosystem
News reports have been covering the marine life dying on the shores of southwest Florida due to a toxic red tide. What exactly is a red tide? Why is it toxic? Here, we answer some common questions about these events:
What Is a Red Tide?
A red tide is one form of a harmful algal bloom, which occurs when algae microorganisms quickly grow to large numbers in a given ecosystem. The microorganisms can be cyanobacteria, dinoflagellates or diatoms; these are all referred to as algae, a large group encompassing many phyla of photosynthetic microorganisms. Red tides are typically produced by dinoflagellates but can also be caused by diatoms.
Does the Water Actually Turn Red?
The accumulation of dinoflagellates can create a reddish-brown color in the water; very large blooms can even be viewed from space. However, red tides don’t always turn red, and harmful effects, such as the toxins associated with some red tides (see below), can be present even if there is no discoloration in the water.
Why Do Red Tides Occur?
Red tides occur when the conditions are optimal for algal growth: the right combination of lots of sun, warmth and nutrients encourage blooms.
Although they occur naturally, human activity can aid the development of red tides and harmful algal blooms. Increased levels of nitrogen or phosphorus nutrients from agricultural runoff or sewage can encourage algal growth and have been directly associated with harmful algal blooms. Climate change has also increased water temperatures in many regions, increasing the range of some microorganisms.
Where Do Red Tides Occur?
Red tides occur worldwide. Some locations, like the Florida peninsula, see red tides annually, while other areas have experience red tides due to waste nutrient additions to ecosystems. The duration of red tides can vary from less than a week to a few months.
Why Are Some Red Tides Deadlier than Others?
Harmful algal blooms are dangerous because they quickly consume the available oxygen in a given ecosystem. The large number of microorganisms can block sunlight to the plant life on the seafloor, leading to less oxygen production. After the bloom, when large numbers of algae are dying, heterotrophic decomposing microbes use oxygen as they consume the algal carcasses, leading to a rapid and drastic drop in available oxygen for the marine life that need it to survive. This oxygen depletion is one reason that red tides and harmful algal blooms can lead to large die-offs of fish, shellfish and other sea dwellers.
Some red tides are caused by microorganisms that release toxins as part of their growth cycle, and these toxin-producing blooms can therefore be even more dangerous. These toxins are thought to be part of a competitive strategy to defend against the marine life that grazes on algae, and toxin production often increases as phosphorous or nitrogen become limiting. This means that algal bloom growth may be initiated by influx of these nutrients, but as nutrients concentrations decrease, the production of algal toxins can increase.
The effects of toxins in people can vary, but often cause gastrointestinal and neurological symptoms such as vomiting, diarrhea, and ataxia when ingested. Toxin inhalation can lead to respiratory symptoms such as cough and bronchospasm.
What Is Causing the Red Tide off the Coast of Florida Right Now (August 2018)?
Karenia brevis electron micrograph. Source.
The current red tide is caused by Karenia brevis, a dinoflagellate that releases a toxin called a brevetoxin. Brevetoxin is a neurotoxin that binds voltage-sensitive sodium channels and disrupts nervous system signaling.
Brevetoxin is produced in vesicles and released through exocytosis, and is thought to enter animals through ingestion and even inhalation. The toxin can accumulate in shellfish and in lower levels in fish that eat plankton.
The signal to release brevetoxin vesicles seems to be the sun itself. Pedro Verdugo of the University of Washington told Microbe magazine that blue light flashed onto K. brevis cells for a few seconds will trigger the release of brevetoxin. “Blue light stimulation implies that these cells must have a photoreceptor, most likely associated with the cell structures known as chloroplasts,” he said.
What Effects Do Red Tides Have on Peoples’ Health?
The main health concern for people in an area affected by a red tide is aerosolized toxins, which can irritate the respiratory system. This is usually temporary, but can be serious for people with respiratory conditions like asthma. Eating shellfish affected by red tide toxins can cause neurotoxic shellfish poisoning. Brevetoxins cannot be detected by taste or smell and are not removed or made less toxic by cooking. Red tide water can also irritate the skin and eyes. While exposure to the water itself isn’t considered dangerous for most people, being in or on water affected by a red tide would expose a person to the highest levels of aerosolized toxins.