In Canada, agriculture and forestry are leading industries in pesticide use.
Currently, at the University of Manitoba, research is being done to investigate the environmental effects of these chemicals.
For the past two years, I have been examining the environmental impacts of polyethoxylated tallow amine (POEA), a surfactant paired with the glyphosate in Roundup herbicide.
The surfactant acts to break down the waxy outer layer of weeds allowing the active ingredient, glyphosate, to then rapidly enter the plant and inhibit the production of essential amino acids.
Most recently, I’ve paid particular attention to the effects POEA has on fish gills.
Water bodies close to pesticide application sites are at risk of contamination by spray drift and runoff.
Shallow ponds are considered to be at greatest risk because they have a lower ability to dilute contaminants and are also rich with life.
As it happens, small fish are key in maintaining a balance as both predator and prey in these ponds.
Monitoring the effects of pesticides on these fish can give insight to long-term impacts of pesticides to the greater aquatic ecosystem.
Their gills have two primary functions: gas exchange (oxygen uptake from the surrounding water) and ion regulation (the uptake of sodium chloride and the excretion of metabolic waste).
Since the gills are one of the first organs exposed to a hazardous substance, any changes at the cellular level can be observed to determine how bad a contaminant really is to the aquatic environment.
There are a number of tests that can determine the toxicity of a substance, but if you actually want to track the effects first hand, then histopathological techniques are your best bet.
Diseased and dying
Histopathology is defined as the study of the structure and function of cells in diseased tissues.
The word is Greek in origin and is a composite of the root words histos “tissues,” pathos “disease or suffering,” and logia “study.”
Histopathology incorporates the use of a microscope to examine sections of tissue for changes to the function of cells, which may not be outwardly visible to the naked eye.
Specialized stains are used to highlight specific tissue structures or cellular components, such as cartilage, connective tissue, carbohydrates, and lipids.
It’s often best to implement a series of histological methods in order to confirm findings.
Having the ability to apply multiple techniques to an experimental sample size that may not be considered statistically sound can add support to otherwise flimsy results.
Applying the techniques
With the help of these techniques, I am currently investigating the histopathological effects of POEA on fathead minnow gills.
Hematoxylin and eosin is one out of two stains that I use in my analysis and the most frequently associated with histopathology.
The hematoxylin stains the nuclei of the cells blue and the eosin counterstains the cytoplasm of the cells pink.
One of the most commonly observed alterations to gills is the increase of cell number and size, known as hyperplasia and hypertrophy.
It’s speculated that this swelling of cells is a mechanism to inhibit the contaminant from entering the fish and to protect internal organs.
The trade off, however, is reduction in gas exchange which seals an unfortunate fate for fish.
In order to measure this, I calculate the percentage of surface area of the gills – the smaller the percentage, the less oxygen uptake.
Another alteration associated with injury of gills is the increase of mucous cells.
It’s not difficult to guess what mucous cells do; they secrete mucus. Mucus production increases in response to the exposure of harmful substances.
This happens to us as well – just think about how much you have to blow your nose when you get sick.
Well, fish have this natural response too, and in order to increase mucus production the number of mucous cells increases.
Since mucous cells are carbohydrate-rich, I use a special stain called Periodic-Acid Schiff, which makes it easy to identify these cells.
Remember, these defences come at a cost. Cell damage in gills reduces ion exchange and impairs respiration.
Exposure to contaminants for long durations or at high concentrations can ultimately lead to fish death.
Luckily, no fish died during my exposure and the preliminary results revealed that though cellular damages are observed early on, they decrease over time.
This indicated that POEA at concentrations estimated in the environment breaks down rapidly in water and does not cause permanent damage to the gills.
This is not to say that the results would be the same in a real-life scenario where POEA may be applied up to five times over a growing season.
The design of my experiment is based only on a one-time, mid-summer herbicide application.
Polyethoxylated tallow amine’s short time in the water column may be good news for fish, but there are many other organisms to consider that live in other niches of aquatic environments – for example, sediment.
Ecosystems are delicately balanced and the slightest offset can cause a cascade of effects.
An organism as insignificant to us as a minnow may have a critical role in the healthy function of the system – remove the minnow and the system collapses.
