Polinski, Jennifer

Mesophotic reefs represent biodiverse ecosystems that may act as a refuge for
depth-generalist coral species threatened in shallow habitats. Despite the importance of
coral-algal symbioses, few studies focus on mesophotic zooxanthellae assemblages and
their influence on connectivity. This study compared zooxanthellae in Montastraea
cavernosa at shallow and mesophotic depths at Flower Garden Banks National Marine
Sanctuary and McGrail Bank. Mesophotic corals contained more zooxanthellae and more
chlorophyll a and c2 per unit area coral. Increased zooxanthellae within mesophotic
corals may represent an adaptive strategy to optimize light capture in low-light
environments. Genetic profiles for zooxanthellae assemblages from shallow and
mesophotic corals showed similar diversity across banks and between depths. The
dominant sequence making up assemblages was identified as Symbiodinium type C1.
Similar assemblage diversity suggests that zooxanthellae assemblages will not limit
connectivity potential between shallow and mesophotic corals at these reefs.

Algal symbionts, commonly called zooxanthellae, living in within coral tissues provide
energy to hermatypic corals through photosynthesis. Zooxanthellae density, as well as
photosynthetic pigment concentration, can be used as an indicator of coral health. This study compared zooxanthellae densities and concentrations of chlorophyll a and c between four sites and two species of coral, Montastraea cavemosa and Diploria clivosa, found at the St. Lucie Reef to establish baseline levels for comparison to future levels. No significant differences were found among sites despite increasing depth farther from the inlet, suggesting reduced light penetration closer to the inlet. Significant differences were found for zooxanthellae densities and chlorophyll concentrations per zooxanthellae cell between coral species. However, there was no significant difference for chlorophyll concentrations per unit area of coral tissue between species. This suggests that all site locations experience similar conditions despite differences in depth. It also suggests potentially different types of zooxanthellae, which is being addressed by ongoing research.