Harmful Algal Blooms (HABs) are impacting the economy and marine health of Washington’s Pacific Coast. You may have heard this term before, but have you ever delved into the science behind these blooms? What precisely triggers them, and what lies within these flourishing clusters? One particular genus, Pseudo-nitzschia, H. Peragallo, 1900, has emerged as a fascinating enigma within this ecological puzzle.
HABs occur when a type of algae grows rapidly and overwhelmingly in water bodies. These blooms are notorious for their capacity to produce toxins detrimental to freshwater and marine life and human well-being. In coastal water, HABs caused by Pseudo-nitzschia create toxins that may be deadly.
Pseudo-nitzschia: A Microscopic Marine Marvel
Pseudo-nitzschia is a marine diatom with a widespread distribution. The valves are lanceolate, fusiform, or linear in shape and typically are lightly silicified. These diatoms feature a distinctive arrangement of two plate-like chloroplasts within their living cells that are arranged symmetrically. A noteworthy trait of this genus is its tendency to form filamentous colonies where adjoining cells overlap, creating a stepped pattern – a trait that is typical to the species, except for P. americana.
The Neurotoxic Connection: Domoic Acid
Pseudo-nitzschia’s claim to notoriety is its production of domoic acid (DA, CAS 14277-97-5, C15H21NO6), a potent neurotoxin. Neurotoxins impact animals’ nervous systems, including their brains. Interestingly, Pseudo-nitzschia has species that are both toxic and non-toxic. This genus has at least 52 confirmed species of which 16 have been reported to produce DA. Therefore, it’s crucial to identify these species for effective monitoring and mitigation of toxic events. This toxin can accumulate in the hepatopancreas of shellfish, zooplankton, and finfish that feed on Pseudo-nitzschia. DA can impact the nervous system of the consumers eating Pseudo-nitzschia by causing excitotoxicity (fyi excitotoxicity= neuronal cell damage resulting from interaction with glutamate receptors on nerve cell terminals). Birds, mammals, and humans that consume these contaminated organisms experience a syndrome termed amnesic shellfish poisoning. This syndrome is characterized by short-term memory loss along with other neurological effects. Sadly, DA poisoning is widespread along the west coast of US causing illness and killing of marine birds and mammals on an annual basis. Severe cases can lead to death in humans, as well, a grim reminder of the havoc these toxins can wreak.
Distribution & Global Warming
Pseudo-nitzschia’s presence has been reported across a wide range of temperature and salinity conditions globally from the open ocean to coastal waters (Bates et al., 2018). Blooms often coincide with elevated nutrients, high light levels, and warm seawater temperatures. HABs caused by this genus has been reported from North America, South America, Australia, Asia, and Europe, indicating a worldwide phenomenon. The very first Pseudo-nitzschia bloom was observed on the North American west coast in 1991 extending from Washington State all the way to Baja California.
The ongoing trend of global warming has breathed new life into these HABs. The increases in bloom frequency and severity are a serious concern for ocean health. The correlation between rising ocean temperatures and Pseudo-nitzschia‘s flourishing is becoming increasingly evident. A noteworthy example is the historic, widespread HAB of P. australis in early May 2015 along nearly the entire west coast of the United States stretching from central California to the Alaska Peninsula. This event was the largest and longest ever recorded in the region; it coincided with North Pacific Ocean Warm Anomaly that was nicknamed ‘the Blob’ and nutrient-rich upwelling (McCabe et al., 2016). Domoic acid concentrations during this period reached staggering levels, far surpassing those expected in a typical Pseudo-nitzschia bloom.
Relationship with Temperature
A closer examination reveals that changes in environmental conditions, specifically temperature, play a pivotal role in its growth rate (Thorel et al., 2014). Pseudo-nitzschia australis collected from the English Channel and studied in semi-continuous culture showed highest growth rates between 13.5 and 18.6 °C. Further research into P. australis uncovered a temperature-dependent relationship between growth rates and DA production, where DA production increased exponentially with the growth rate (Thorel et al., 2014). Notably, P. australis isolates from Southern California studied in an incubation experiment showed maximum growth at 23°C, while DA concentrations soared at 30°C – conditions strikingly reminiscent of the 2014–2015 warming anomaly (Zhu et al., 2017).
Expanding Horizons: Pseudo-nitzschia HABs, Not Limited to Washington’s Pacific Coast
The phenomenon of HABs produced by Pseudo-nitzschia is not just prevalent along Washington’s Pacific Coast, the US West coast, and the US Gulf coast. It is now reported in new regions where they haven’t existed before. Take for example the recent HAB caused by P. australis in Gulf of Maine in 2013 and 2016. This HAB produced an unprecedented Domoic acid event that resulted in closure of shellfishery across Gulf of Maine in 2016. Apparently 14 species of Pseudo-nitzschia species had been identified in the region over the years of monitoring but this was the first time a bloom was ever formed in that region. Since then, P. australis blooms have occurred every year resulting in additional closures in 2017-19 (Bates et al., 2018; Clark et al., 2019, 2021).
Narragansett Bay in Rhode Island also had a Pseudo-nitzschia HAB in 2016, marking its first encounter with such an event and resulting in closure of shellfish fishery. This pattern echoes the temperature-associated blooms observed on the Pacific coast. Rising temperatures and shifts in nutrient concentrations, attributed to climate change and sewage treatment adjustments, have woven a similar narrative for Narragansett Bay (Roche et al., 2022).
Pseudo-nitzschia australis: The Common Thread
Amid these complex scenarios, P. australis emerges as a common thread, implicated in HABs and DA production events along both eastern and western coasts. These strains thrive in higher temperature ranges, spanning from the Gulf of Maine at 15°C (Clark et al., 2021) to Monterey Bay, California at optimal growth temperatures at 17 – 18°C (McCabe et al., 2016), and in Southern California at 23 – 26°C (Zhu et al., 2017). In contrast to all of this, Narragansett Bay bucks this trend, with P. australis blooms occurring at lower thermal ranges 1.6 – 15.4°C than other species.
A Delicate Balance
In conclusion, the story of Pseudo-nitzschia unfolds as a tale of intrigue within the realm of Harmful Algal Blooms. This enigmatic genus, with its toxic potential and sensitivity to environmental shifts, serves as a poignant reminder of the delicate balance that sustains aquatic ecosystems. As we navigate the complexities of climate change and ecological resilience the Pseudo-nitzschia‘s enigma continues to captivate, urging us to unravel its mysteries for the sake of our oceans and ourselves.
References:
Bates S. S., Bird C. J., de Freitas A. S. W., Foxall R., Gilgan M., Hanic L. A., et al. (1989). Pennate Diatom Nitzschia Pungens as the Primary Source of Domoic Acid, a Toxin in Shellfish From Eastern Prince Edward Island, Canada. Can. J. Fisheries. Aquat. Sci. 46 (7), 1203–1215. doi: 10.1139/f89-156
Clark S., Hubbard K. A., McGillicuddy D. J., Ralston D. K., Shankar S. (2021). Investigating Pseudo-Nitzschia Australis Introduction to the Gulf of Maine with Observations and Models. Continental. Shelf. Res. 228, 104493. doi: 10.1016/j.csr.2021.104493
McCabe R. M., Hickey B. M., Kudela R. M., Lefebvre K. A., Adams N. G., Bill B. D., et al. (2016). An Unprecedented Coastwide Toxic Algal Bloom Linked to Anomalous Ocean Conditions. Geophysical. Res. Lett. 43 (19), 10366–10376. doi: 10.1002/2016GL070023
Roche M. K, Sterling A. R., Rynearson T. A, Bertin M.J., Jenkins B.D. (2022). A Decade of Time Series Sampling Reveals Thermal Variation and Shifts in Pseudo-nitzschia Species Composition That Contribute to Harmful Algal Blooms in an Eastern US Estuary. Front. Mar. Sci. Sec. Aquatic Microbiology. Volume 9 – 2022 | https://doi.org/10.3389/fmars.2022.889840
Thorel M, Fauchot J., Morelle J., Raimbault V., Roy B. Le., Miossec C., Kientz-Bouchart V., Claquin P. (2014). Interactive effects of irradiance and temperature on growth and domoic acid production of the toxic diatom Pseudo-nitzschia australis (Bacillariophyceae) Harmful Algae, 39, pp. 232-241, 10.1016/j.hal.2014.07.010
Zhu Z., Qu P., Fu F., Tennenbaum N., Tatters A. O., Hutchins D. A. (2017). Understanding the Blob Bloom: Warming Increases Toxicity and Abundance of the Harmful Bloom Diatom Pseudo-Nitzschia in California Coastal Waters. Harmful. Algae. 67, 36–43. doi: 10.1016/j.hal.2017.06.004
© Heera Malik, September 2023