Toxic bloom off the southern coast of Australia

The Australian summer ends with a dinoflagellate bloom on the southern coast. It is a “red tide” type of bloom, although the more noticeable symptom was the accumulation of bacterial foam. Primarily affected were oceanic fish and invertebrates. There have been no human fatalities, but over a hundred people reported flu-like symptoms.

Alarming signs

In mid-March, the media circulated photos of dead Australian seahorses. The bodies of these iconic seadragons of the Great Australian Bight, known to residents of South Australia as leafy sea dragons, were washed ashore in significant numbers on beaches such as Parsons and Waitpinga near the state capital, Adelaide.

At the same time, foam began appearing on the beaches. This was not the typical sea foam formed by crashing waves, but a thick layer, several dozen centimeters deep, resembling beaten egg whites. Australian experts investigating the phenomenon jokingly refer to it as bacterial smoothie. While the name may not sound appealing, Australian beachgoers often treat it as a curiosity.

Bacterial foam can sometimes contain toxic substances. This time, the toxicity did not affect only the seadragons. Dead fish of other species and dead octopuses were also found on the beach. Moreover, dozens of surfers visited doctors with flu-like symptoms such as sore throats, dry coughs, or irritated eyes. Several dozen others reported similar symptoms despite not entering the water, having only walked along the beach and inhaled the sea air.

The situation seemed quite clear – it looked like a toxic bloom occurring at the end of the Australian summer. The Adelaide area is the place where this phenomenon was first correctly identified. In 1878, local chemist George Francis described cases of pet poisoning from drinking water from Lake Alexandrina in the journal Nature. The only task remaining was to determine which species caused it. It was also possible that the foam hosted pathogenic bacteria.

Karenia mikimotoi – a troublesome newcomer

Water samples collected by South Australian sanitation services were sent to the University of Technology Sydney, where researchers examined them microscopically and genetically. They discovered high concentrations of Karenia mikimotoi cells. This species of dinoflagellate is known for creating toxic ocean blooms referred to as red tides. In reality, red tides are often more brown than red in color, depending on the dominant dinoflagellate species, whose color is determined by a combination of carotenoids and chlorophyll.

This species was discovered nearly a hundred years ago off the coast of Japan. It is a small, flagellated organism, up to 40 micrometers in diameter, belonging to dinoflagellates – single-celled algae covered with cellulose plates. In addition to photosynthesis, its representatives are capable of heterotrophic feeding, and some produce toxins. During warmer months, it can dominate phytoplankton and create blooms.

Initially, its toxic blooms were known mainly in Japan and Korea. In the late 1950s, it was first identified in the Atlantic – off the coast of the United States, and a few years later in Norway. Since the 1970s, it has occasionally caused mass fish deaths around the British Isles. It is suspected that its widespread distribution is not natural but has been spread through maritime transport in ballast water. Its broad tolerance to temperature and salinity ranges supports this. It likely overwinters without a resting stage.

It has not disappeared from the coasts of the Far East, where it continues to cause significant damage to aquaculture and fisheries. Among other things, its blooms became a testing ground in the late 20th century for control methods that were also trialed in Poland against Prymnesium parvum. Today, it is found almost worldwide, including around Australia. In the Great Australian Bight, it is common, though rarely becomes the dominant species.

Despite a century of research, the mechanism of its toxicity remains poorly understood. It is likely related to reactive oxygen species. While in humans a strong oxidant can irritate mucous membranes, causing tearing or coughing, in gill-breathing organisms it causes cell death. Reactive oxygen species also damage digestive tracts, but gill injuries are usually the fatal factor.

A few hours of exposure to Karenia mikimotoi toxins is enough to kill a fish. This type of toxicity is acute – it acts quickly but does not persist. The toxic substances do not remain in the environment for long. However, it is possible that this species also produces other, more specific toxins belonging to lipids. Additionally, among its secretions are complex sugars responsible for creating bacterial foam. Karenia mikimotoi itself is not a bacterium, but the foam it generates becomes densely inhabited by these microorganisms. If they are pathogenic, they can amplify the harmful effects of the bloom.

In the Baltic Sea, this species is relatively rare. Here, toxic blooms are still mostly caused by cyanobacteria.