Written by Danielle Salley, Four Green Steps

Tropical coral reefs are among the most biologically diverse ecosystems on the planet and of immense economic value to an estimated 500 million people worldwide.  At present, coral reefs are deteriorating due to a multitude of anthropogenic stressors, mainly the rise of atmospheric CO_2, resulting in global warming and ocean acidification.  As the sea surface temperatures (SSTs) rise even 1-2°C above ambient temperatures, coral reefs species are being pushed to their thermal limits, which results in severe coral bleaching.  Coral bleaching is not a new phenomenon; it is part of the natural cycle, like plant death, decomposition, and regrowth.  However, the concerns stem from the increase in abundance and severity of coral mortality due to coral bleaching, and thus the dwindling capacity for regeneration of the healthy coral reefs.

Here’s some background information about coral bleaching:

There are numerous species of tropical coral reefs present across the globe, most prominently found in the Indo-Pacific and the Caribbean.  Species differ in their structure and solidarity of survival in terms of temperature fluctuations (perhaps initiated by El Nino Oscillation (ENSO) events, or global warming trends), and natural disasters.

Coral species live symbiotically with zooxanthellae, which are photosynthetic algae.  They have a mutually beneficial relationship wherein the coral reef provides shelter, food (from the coral’s nitrogenous waste), and protection for the algae; whereas, the algae provides carbohydrates via photosynthesis, which provides energy for the reef’s metabolic processes to enable calcification and growth.  Plain and simple, they need each other to be happy and healthy.  The algae contain chlorophyll-b, which is what gives the coral reef its pigment.  After an ENSO event, the SSTs rise and the coral host expels the algae.  In doing so, the coral reef loses its pigment thus coral bleaching occurs and the reef appears pale and white.  Since the coral relies on the algae for survival, expelling mass algae threatens the reefs survival thus permanent coral mortality is possible.

The most trying times occur after the event, in assessing the degree of coral mortality.  During extreme ENSO events, there has been up to 60% of the reef algae expelled which disrupts the functioning of the reef, leading to severe coral mortality.  This can be reversed if the algal symbionts recolonize the reef; however, this takes from months to years for a reef to regenerate into a healthy system.

The multitude of coral species enables a genetic variability that provides evidence for why certain corals are less affected by thermal stress than others.  But is it the coral host or the algal symbionts that permit such resilience?  Scientists have done numerous studies to identity the answer.  It turns out that the algal symbionts are more prone to adaptation via natural selection than their coral hosts because they have a large population with short clonal generation times.  Naturally, this is an ideal situation for heritable traits to be passed on when new environmental stressors arise (such as an ENSO event), or are amplified over time, evident from rising SSTs due to global warming.

It is conclusive that the long generation time of coral hosts decreases its overall heritability of traits which raises concerns for timely adaptation of the coral-algal symbiosis.  The potential to be outpaced by the rate of environmental change is of great concern because this could easily be translated into species extinction and loss of biodiversity over time.  Nonetheless, there is hope! It seems clear that genetic variability among algal symbionts will become of outmost importance for coral reef resilience in the near future.

Image courtesy of Creative Commons.

Sources:

Article:  “Estimating the Potential for Adaptation of Corals to Climate Warming”

By: Nikolaus B. M. Császár, Peter J. Ralph, Richard Frankham, Ray Berkelmans, and Madeleine J. H. van Oppen

Article:  “Discovery of how coral reefs adapt to global warming could aid reef restoration”

http://www.eurekalert.org/pub_releases/2010-06/ps-doh061610.php

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