The shifting paradigm of epiphytic symbiosis in a changing ocean

Chair: Elvira Poloczanska

Steve S Doo (1), Alexia Graba-Landry (2), Maria Byrne (1)
1 Coastal and Marine Ecosystems Group, School of Biological Sciences, University of Sydney, NSW 2006 Australia
2 Centre of Excellence for Coral Reef Studies, Townsville, QLD 4811 Australia

Background
Epiphytic symbioses are common in tropical marine environments. Large benthic foraminifera (LBFs) commonly live as epiphytes on marine algae, and thought to benefit from this symbiosis; the current paradigm. Marine algae have large diffusive boundary layers, causing elevated pH around the algae from photosynthesis, benefiting LBFs. But how the interaction between macroalgae and epiphytes may be altered in changing climates is unknown.

Methods
In this study, we document the interaction between the calcifying green algae Halimeda tuna and a LBF, Marginopora vertebralis in current and predicted near future conditions. A fully orthogonal design with two temperature (ambient and +3°C), two pH (ambient and -0.3 pH units), and three symbiosis (H. tuna only, M. vertebralis only, and both) factors were incubated in a flow-through experiment for 15 days.

Findings
A significant increase in bleaching (visual color scoring) of H. tuna in elevated temperature and CO2 was observed when no epiphytes were present, while M. vertebralis did not exhibit bleaching. Survival rate increased significantly for H. tuna when M. vertebralis was present in elevated temperature and CO2 conditions. Elevated temperature and CO2 caused a decrease in growth (wet weight) in both H. tuna and M. vertebralis, but this effect was ameliorated in the epiphytic symbioses treatment groups. Dissolution (alkalinity anomaly measurements) occurred in both elevated temperature and CO2 treatment groups to both H. tuna and M. vertebralis. While dissolution of H. tuna and M. vertebralis may indicate an overall deleterious trend, survival (bleaching), was significantly lower in treatment groups incubated in symbiosis (both H. tuna and M. vertebralis). This suggests dissolution of M. vertebralis is aiding in the survival of H. tuna, buffering pH around the macroalgae.

Conclusions
This indicates epiphytes on calcifying macroalgae are particularly important to their survival in future climate change scenarios, shifting the current paradigm that epiphytes benefit from their hosts.