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Chemistry

Not better together: complex pollutant soup spells trouble for marine phytoplankton

Echeveste, P., Galbon-Malagon, C., Dachs, J., Berrojalbiz, N., & Agusti, S. 2016. Toxicity of natural mixtures of organic pollutants in temperate and polar marine phytoplankton. Science of the Total Environment, 571, 34-41. http://dx.doi.org/10.1016/j.scitotenv.2016.07.111

Killer combos
Thelma and Louise. Gru and his Minions. Peanut butter and jelly. Your iPhone and its case. Everyday we encounter examples of combinations that are just plain better together than apart. Yet researchers have recently discovered that the complex mixtures of persistent organic pollutants (POPs) in oceanic waters are decidedly worse for some of the ocean’s most important primary producers, marine phytoplankton.

Previous research has typically focused on how one individual contaminant type or family could impact these important microscopic plants; these studies found that individual POPs can alter how phytoplankton take up carbon or silicate, grow, or perform photosynthesis. However, these effects were possible only at concentrations well above current oceanic contaminant concentrations.

The recent work of Echeveste et al. seeks to provide more realistic information about the impact of POP mixtures on phytoplankton, such as these pennate and centric diatoms from Antarctica. Photo credit: Wikipedia

The recent work of Echeveste et al. seeks to provide more realistic information about the impact of POP mixtures on phytoplankton, such as these pennate and centric diatoms from Antarctica. Photo credit: Wikipedia.org

Enter the work of Spanish researcher Pedro Echeveste from Pontificia Universidad Católica de Chile and his international colleagues. They realized that phytoplankton are not subjected to just one or two types of POPs in natural settings; they are inundated with a veritable POP soup composed of hundreds or thousands of different contaminants at variable concentrations. Additionally, some of these compounds have a ‘1+1=3’ behavior, where the combination is more potent than sum of the individual parts. In this study, they set out to figure out how these realistic POP combinations could be impacting phytoplankton from different areas of the world ocean.

Shipboard science around the globe
Echeveste and his colleagues employed a multi-year, global strategy to address their research questions. They collected water from the Northeast Atlantic, the coastal Mediterranean Sea, and two locations in the Southern Ocean: one in the Weddell Sea and one in the Bellingshausen Sea. They used a suite of chemical techniques to extract the bulk contaminant load from seawater to replicate the complex mixture of POPs in their future experiments. They didn’t identify each compound extracted; they just divided the extract into polar and non-polar groups based on chemical properties. They also measured the concentrations of some specific polar and non-polar organic contaminants dissolved in water from these different locations. Not surprisingly, water from the Atlantic and Mediterranean locations near large-scale human development contained much higher POP concentrations.

They then took phytoplankton samples from each location and exposed them to variable levels of the polar and nonpolar contaminant soup that they isolated from each region. Phytoplankton were subjected to anywhere from 5 to 890 times the natural oceanic contaminant concentration. They measured cell abundance, chlorophyll a concentrations, cell death, and growth and decay rates across the treatments as indicators of pollutant impacts. They performed these experiments aboard a ship within the region where phytoplankton were collected so that samples could be kept at native light levels and water temperatures.

Little cells taking the biggest hit
With these experiments, Echeveste and his group found that most size classes of the phytoplankton community and the community at large from all four locations experienced either a population decline or decreased growth rates when subjected to polar and/or non-polar contaminant mixtures at five to ten times natural concentrations, well below concentration thresholds from previous studies focusing on the effects of one or a few POPs. They also noticed that smaller phytoplankton, or, picophytoplankton, were particularly sensitive to the pollutant mixtures

Picophytoplankton are very tiny members of the phytoplankton community, ranging from 0.2 to 2 µm in size. Photo credit: Wikipedia.org

Picophytoplankton are very tiny members of the phytoplankton community, ranging from 0.2 to 2 µm in size. Photo credit: Wikipedia.org

This result makes sense: smaller phytoplankton cells have a decreased surface area to volume ratio, meaning they have less volume to “dilute” any incoming contaminants. Think of it like a Great Dane and a Chihuahua both eating a big box of chocolates; generally, the larger Great Dane is less likely to get sick from the same amount.

Smaller sized plankton are more susceptible to contaminant mixtures, a lot like a smaller dog may be more susceptible to eating chocolate compared to a larger dog. Photo credit: Wikipedia.org

Although this result isn’t a surprise, it doesn’t bode well for phytoplankton or ecosystem health. Smaller sized phytoplankton groups are generally better at handling nutrient-poor or stressful conditions than their larger counterparts and are therefore expanding their dominance across the world ocean as a result of pervasive global change. This means larger areas of the oceanic food web are becoming increasingly reliant on these smaller primary producers that are more likely to be impacted by complex POP mixtures.

Bigger isn’t necessarily better

However, size isn’t the whole story here. Echeveste also found that exposure history played a role in how phytoplankton communities responded to the contaminant mixtures. The Mediterranean Sea coastal community, composed primarily of picophytoplankton, was more tolerant of the polar + non-polar contaminant soup compared to the other three communities from the NE Atlantic and the Southern Ocean. This is eyebrow-raise worthy because the phytoplankton communities from the Southern Ocean are dominated by much larger diatoms, which should have been more resistant if cell size alone predicted how phytoplankton were affected. This suggests that phytoplankton communities in the remote, relatively pristine Southern Ocean may be less resistant to contaminant mixtures, while phytoplankton communities closer to human contaminant sources have had the chance to develop coping strategies.

The ol’ one-two punch

Despite the suggested tolerance of some phytoplankton communities historically exposed to POPs, the authors still suggest that the health of phytoplankton is being impacted by current oceanic concentrations of organic pollutants. The authors are quick to point out that POP mixtures aren’t the only stressor impacting phytoplankton communities. UV radiation and temperature have both been observed to exacerbate the effects of POPs on phytoplankton communities, and both of these stressors could intensify in some regions as climate change manifests itself dynamically across the globe. The experiments in this study revealed appreciable effects at five to ten times measured POP field concentrations, yet these experiments didn’t take into account these other documented stressors. Acting in concert, variables such as light, temperature, and complex POP mixtures are likely appreciably degrading current global phytoplankton communities in some capacity. Given the importance of phytoplankton as the basis of the marine food web and their role in the global carbon cycle, these results are sobering to say the least and suggest continued research is necessary to understand phytoplankton responses to realistic, complex POP mixtures in combination with other natural stressors.

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