Ocean acidification due to climate change is an increasing problem in aquatic ecosystems. Low oceanic pH levels, which are projected to drop by 0.4 pH units by 2100, have detrimental effects on organisms that depend on calcium carbonate minerals for shells or exoskeletons. Additionally, ocean acidification has been shown to cause the loss of dinoflagellate symbionts from coral hosts, contributing to coral bleaching. However, the effect ocean acidification has on additional symbiotic systems remains largely uninvestigated. The present study aims to extend these investigations to the symbiotic Euprymna scolopes-Vibrio fischeri model system. The Hawaiian bobtail squid and its bioluminescent symbiotic bacteria exhibit a highly specific relationship. Upon exposure and horizontal transmission of the symbiont shortly after hatching, the squid undergoes a series of physiological changes in order to facilitate the establishment of the mutualistic association. We hypothesize that if the squid are under acidic stress due to ocean acidification conditions, the establishment and maintenance of the squid-Vibrio symbiosis will be detrimentally affected. To test this hypothesis, squid were exposed to both ambient and low pH conditions for 5 days. After exposure, tissues were collected to evaluate expression levels of key factors involved in symbiosis establishment as well as stress-related genes.
Thus far, we have identified several transcripts in Euprymna scolopes that are homologues to known stress response proteins, such as a sodium-hydrogen exchanger, an ABC transporter, V-type ATPase, and the rhesus-like protein. Preliminary results show that the gills of the squid exhibit normally high expression levels of stress-related factors such as the ammonia transporter, rhesus-like protein, and an ABC transporter, while the skin has notably low expression levels of these same transcripts.
All squid tested were able to acclimate and survive for the duration of the exposure to extremely low pH levels of 7.0. Further preliminary results of quantitative expression levels indicate that peptidoglycan recognition protein 2 and a lipopolysaccharide binding protein, factors of the squid-Vibrio symbiosis, are upregulated in squid that were exposed to acidic environmental conditions. Additionally,results suggest that sodium-hydrogen exchanger 3 is expressed at higher levels in squid that were exposed to seawater at a pH of 7.0 than those at a pH of 8.0. These results suggest that ocean acidification may elicit an upregulation of select stress genes in squid to mitigate effects of harmful conditions. Additionally, a change in expression of important symbiosis-associated factors indicates potential for an effect on the squid-vibrio association.