Interactions between phenanthrene exposure and historical chemical stress: Implications for fitness and ecological resilience of the sentinel species Daphnia magna

Polycyclic aromatic hydrocarbons (PAHs) arise from incomplete combustion
of oil, coal, and gasoline, with lipophilic properties facilitating their
widespread distribution and persistence. Due to their biochemical
attributes, PAHs can accumulate in animal tissues, potentially causing
mutagenic and carcinogenic effects. Since the industrial revolution, PAH
concentrations in the environment have risen, with some areas showing
levels from 0.159 to 33,090 µg/kg sediment. Despite acute toxicity studies
showing adverse effects on freshwater organisms, the long-term impacts and
synergistic interactions with other pollutants remain largely
unexplored.  This study investigates the impact of phenanthrene
(PHE), a prominent PAH found in aquatic environments, on Daphnia magna, a
species of significant ecological importance in freshwater ecosystems
globally, being both a sentinel species for chemical pollution and a
keystone organism in freshwater aquatic ecosystems. Leveraging the
dormancy of D. magna, which spans decades or even centuries, we exposed
strains with diverse histories of chemical contaminant exposure to
environmentally relevant concentrations of PHE. Initially, acute exposure
experiments were conducted in accordance with OECD guidelines across 16
Daphnia strains, revealing substantial variation in acute toxic responses,
with strains naïve to chemical pollutants showing the lowest toxicity.
Utilizing the median effect concentration EC10 derived from acute
exposures, we assessed the impacts of chronic PHE exposure on the life
history traits and ecological endpoints of the 16 strains. To elucidate
how historical exposure to other environmental stressors may modulate the
toxicity of PHE, temporal populations of D. magna resurrected from a lake
with a well-documented century-spanning history of environmental impact
were utilized. Our findings demonstrate that PHE exposure induces
developmental failure, delays sexual maturation, and reduces adult size in
Daphnia. Populations of Daphnia historically exposed to chemical stress
exhibited significantly greater fitness impacts compared to naïve
populations. This study provides crucial insights into the augmented
effects of PAHs interacting with other environmental stressors.