Rachael M. Heuer

Research Interests: The interface of physiology, ecotoxicology, and human health

Research description: My research uses physiology to examine organismal and behavioral responses to environmental stressors. I believe that harnessing the utility of aquatic animal model organisms such the California sea hare (Aplysia californica, “Aplysia”) and the zebrafish (Danio rerio) can advance our understanding of both human health challenges, and the responses of marine and freshwater species to climate change and environmental pollutants.

Research aims: 

Aim 1. Impacts of ocean acidification and ocean warming on the physiology and behavior of freshwater and marine organisms

Marine organisms vary in their ability to cope with CO₂. Generally, fish and active invertebrates are considered acid-base “regulators” and dynamically maintain pH homeostasis by accumulating bicarbonate (HCO₃^–). While protecting pH, the sustained elevation of internal pCO₂ and HCO₃^– following CO₂ compensation is hypothesized to cause a variety of secondary impairments.

Impacts on ion balance: Although it may sound odd, marine fish must drink seawater to stay hydrated. In this process they move ions across the intestine to help with water absorption. I examine how ocean acidification effects this ion movement in the intestine using the Gulf toadfish (Opsanus beta). My research has shown that efforts to accumulate HCO₃^– in the blood to maintain internal pH homeostasis, leads to a counterproductive HCO₃^– loss through the intestine. This increased ion movement ends up increasing the oxygen consumption rate of isolated intestinal tissue in CO₂-exposed animals. Ultimately, this increase is an added metabolic cost for the animal. Read more here

As a part of routine osmoregulatory processes marine fish produce calcium carbonate in their intestine. I am also interested in how CO₂ exposure and ocean warming impacts the production of fish carbonates and what this could mean for carbon cycling in future oceans. In toadfish, increased temperature leads to increased carbonate production rates. Read more here

Intestinal calcium carbonates produced by Gulf toadfish in the tank bottom (left) and cross-sectional image via scanning electron microscopy (right)

Impacts on behavior: Impaired behavior following exposure to CO₂ has been reported in more than 130 studies to date in marine organisms at ocean acidification-relevant CO₂ levels (<1900 uatm CO₂). The majority of these studies have focused on marine fish, demonstrating that endpoints such as vision, olfaction, lateralization, and learning are affected. Invertebrates are also impacted. The underlying cause is thought to result from changes to internal ion gradients that occur when fish maintain pH homeostasis following CO₂ exposure. I examine the link between these ion gradients and behavior and try to understand more about why these behavioral anomalies are occurring. Read more here

My research currently uses well-established aquatic model organisms such the California sea hare (Aplysia californica, “Aplysia”) and the zebrafish (Danio rerio) to advance our understanding of why climate change stressors impact physiology and behavior.

Aplysia have well-mapped, simple nervous systems and have been used for decades to study the cellular basis of learning and memory. Our research has shown that Aplysia exposed to elevated CO₂ show an altered tail withdrawal reflex and relaxed their tails more quickly compared to controls. Read more here

Rachael Heuer and former undergraduate student Rebecca Zlatkin holding California sea hares in the National Resource for Aplysia, and measuring hemolymph chemistry in the lab. Photo credits: Diana Udel, University of Miami

Zebrafish also offer advantages for exploring how climate change may impact behavior. They are easily imaged during their transparent early life stages and there are established tools for genetic manipulations. I am interested in using this species to better understand both climate change stressors and responses to toxicants like crude oil (see aim 2).

Heuer working with Zebrabox to conduct behavioral testing on larval zebrafish following CO₂ exposure

Aim 2. Impacts of crude oil on fish performance and behavior

The Deepwater Horizon oil spill is considered one of the worst ecological disasters in U.S. history. I examine how this oil spill affects performance and the cardiovascular system in fish, using the pelagic Mahi-mahi (Coryphaena hippurus). Early research indicated that mahi exposed to crude oil show compromised athletic performance. I found that individual heart cells (cardiomyocytes) exposed to crude oil show reduced contractility and  electrophysiological performance. Read more here

 

Heuer in the lab isolating individual cardiomyocytes (heart cells). Photo credit: Dan Dinicola (left) and graphical abstract from Heuer et al 2019

 

Cardiovascular impacts of oil are now well-documented in fish, but effects on neurosensory and behavioral endpoints are still emerging. I am interested in using zebrafish and Aplysia to better understand neurosensory and behavioral impacts of crude oil on fish and invertebrates. These findings have implications for human health (see Aim 3)

Aim 3: Using aquatic animal models that inform adverse human health outcomes

Exposure to the harmful compounds in oil are not limited to aquatic species. Humans exposed during crude oil clean up efforts report neurological symptoms. Non-occupational exposure occurs through tobacco smoke, forest fires, and motor vehicle emissions. In-utero exposure to these compounds is correlated with neurological impairments in children. In addition to neurological effects, growing epidemiological and clinical studies suggest that particulate matter in air pollution containing toxic PAHs are triggers of cardiovascular disease. Thus, a better understanding of the mechanisms leading to neurological and cardiovascular impairments could be informed by using animal models such as Aplysia and zebrafish.