Measuring multi-year changes in the Symbiodiniaceae algae in Caribbean corals on coral-depleted reefs

Area of Science:

• Marine Biology
• Ecology
• Genetics

Background:

• Coral reef degradation is a significant ecological issue, but monitoring coral cover alone often fails to identify the root causes or inform conservation.
• Studying the genetic diversity of Symbiodiniaceae (symbiotic algae within corals) is vital for understanding coral health and resilience but is often overlooked.

Purpose of the Study:

• To investigate the temporal dynamics of Symbiodiniaceae communities in common coral species on St. John, US Virgin Islands.
• To assess how environmental disturbances, such as hurricanes and coral disease, affect coral symbiont composition.
• To evaluate the effectiveness of different sampling strategies for monitoring coral symbiont diversity.

Main Methods:

• Augmented an ecological time series by genetically identifying Symbiodiniaceae in tagged coral colonies over three years (2017-2019).
• Used Symbiodiniaceae Internal Transcribed Spacer 2 (ITS2) sequencing to identify distinct genetic types and profiles.
• Compared symbiont diversity among coral species, individuals, and over time, considering major disturbance events.

Main Results:

• Identified 525 distinct Symbiodiniaceae sequences, comprising 42 ITS2 type profiles, with diversity varying among host species and individuals.
• Observed that symbiont communities were largely preserved within coral hosts over three years, despite significant physical disturbances and the onset of stony coral tissue loss disease.
• Found variation in dominant symbionts among colonies for all host species, suggesting potential impacts on rare algal lineages if coral populations decline.

Conclusions:

• Temporal monitoring of Symbiodiniaceae is essential for understanding coral reef ecology and informing conservation, especially in the face of increasing threats.
• Coral host abundance declines may lead to the loss of rare Symbiodiniaceae lineages, particularly if these declines are symbiont-specific.
• Random sampling of numerous corals may be more effective than repeated sampling of tagged individuals for capturing the full diversity and dynamics of coral symbiont communities.