By Kyla Kelly; USC graduate student and Sea Grant Trainee
Published May 29, 2020
We feel the presence of phytoplankton every day in various ways. For instance, take two deep breaths: one to thank the trees and one to thank the phytoplankton. These microscopic, photosynthetic organisms produce roughly 50% of the oxygen we breathe and are therefore extremely important in sustaining life on this planet. However, we also occasionally feel the impacts of phytoplankton in negative ways when they appear as harmful algal blooms (HABs). Certain oceanic conditions can trigger rapid growth of phytoplankton, increasing their abundances greatly and often coloring coastal waters a red or brown hue. When the algae causing these “red” or “brown” tides also produce toxins, they turn into HABs and can become problematic.
Pseudo-nitzschia (pronounced ‘soodo-nitchia’) is one type of HAB that blooms annually on the west coast of the United States. This type of phytoplankton produces a potent neurotoxin, known as domoic acid. Domoic acid bioaccumulates in the food chain when shellfish feed on Pseudo-nitzschia cells and consequentially accumulate the toxin in their tissues. Although the shellfish generally remain unharmed, human ingestion of these contaminated shellfish can cause amnesic shellfish poisoning, a neurological disorder that leads to headaches, nausea, short term memory loss, and, in severe cases, can be fatal. In order to protect human health, when toxic Pseudo-nitzschia blooms are present, the harvest and sale of shellfish must temporarily cease. These fishery closures can lead to massive economic losses. Furthermore, this domoic acid can also be harmful to marine mammals and birds, causing erratic behavior, seizures, and mass mortality events.
Just as UV rays can be damaging to our human skin cells, it also can be harmful to phytoplankton cells by damaging their DNA.
These blooms can occur naturally with the changing of the seasons, yet human activities seem to be exacerbating their frequency and severity. Climate change is progressing at a rapid rate due to the combustion of fossil fuels and release of excess carbon dioxide into the atmosphere. The accumulation of this greenhouse gas in the atmosphere most notably causes global temperatures to rise but also can impact a variety of other components of the global ecosystem. For example, phytoplankton are increasingly becoming exposed to harmful ultraviolet (UV) radiation. Just as UV rays can be damaging to our human skin cells, it also can be harmful to phytoplankton cells by damaging their DNA. Furthermore, nitrogen coming from land-based sources, such as agricultural and sewage runoff, can change the kinds of nitrogen available to phytoplankton. This nutrient is essential to phytoplankton growth, but different forms (i.e. nitrate vs. urea) can either enhance or decrease growth and toxin production.
There is currently a limited capacity to accurately predict when these blooms will occur, however, gaining a better understanding of what triggers these toxic blooms is important as it will allow us to better monitor their presence in the ocean, forecast their occurrence, and ultimately reduce the negative consequences on human health, the ecosystem, and the economy. Previous research attempting to explain the causes of toxic Pseudo-nitzschia blooms have focused on one environmental factor at a time (e.g. just temperature, or just nutrients). However, it is clear that several factors are changing in the ocean simultaneously due to climate change. Therefore, studying Pseudo-nitzschia in the context on one environmental factor in the lab will not give us an accurate picture of what is driving blooms in the natural oceanic environment.
A recent Sea Grant funded project at the University of Southern California (USC) used a “multiple driver approach” to determine the triggers of rapid Pseudo-nitzschia growth and domoic acid production. This experimental design is unique because it uses multiple environmental conditions at once -- UV radiation, alternative nitrogen sources (nitrate vs. urea), and warm temperatures -- which more closely resemble the natural environment.
...the combination of these three environmental variables reflects a future ocean altered by climate change...
For all treatments exposed to warmer temperatures, domoic acid production increased three-fold. However, when these cells were simultaneously exposed to multiple stressors -- a combination of warming, urea (anthropogenic nitrogen), and UV radiation -- toxin production increased to an even greater degree. This indicates that these three “stressors” conditions work together to enhance toxin production more than any single factor would alone. This is significant, as the combination of these three environmental variables reflects a future ocean altered by climate change: one that is warmer, more influenced by nitrogen from land-based sources (agriculture and sewage), and experiences more intense UV radiation. Therefore, toxic blooms of Pseudo-nitzschia could worsen in the future due to climate change.
Updating forecast models with this multiple-stressor data will assist in enabling more accurate predictions of when and where these blooms will occur.
Although this data reveals more severe impacts for the future, the knowledge gained is extremely valuable in adapting to these conditions and building resilience in response to climate change. The data from this experiment will be incorporated into local Southern California HAB forecasting models that function to predict when toxic bloom events may occur. Updating the forecast models with this multiple-stressor data will assist in enabling scientists and managers to more accurately predict when and where these blooms will occur. More effective forecasting tools will allow for more accurate monitoring and successful management of HABs, reducing the negative consequences they have on human health, marine ecosystems, and the economy.