Class 6 - Symbiodinium

 Outline
 1. Review of Symbiodinium and coral bleaching.
 2. Investigating diversity of Symbiodinium: past to present.

 Part 1: Review of Symbiodinium and bleaching.
 Dangers facing coral reefs:
 Global warming is raising the temperature of the ocean; this kills corals - “coral bleaching”.
 Also, as the oceans become more acidic, it is more difficult for corals to make their skeletons.
 Perhaps 90% of coral reefs will be dead by 2050.

 Diagram of iving tissue

 Numbers of zooxanthellate genera over time, increase in ZX genera of corals.
 More diverse than ever, showing benefits of symbioses.
 Believed to have started approximately 60 million years ago.
 Symbiodinium spp. in invertebrates
holobiont=host+symbiont(s)
 Corals and symbionts
 Many shallow water corals get their energy from symbiotic zooxanthellae.
 These small animals make it possible for corals to live in the warm oceans.
 But, these symbionts are sensitive to hot ocean temperatures.
 What turns the coral white?
- As a stress response, corals expel the symbiotic zooxanthellae from their tissues
- The coral tissue is clear, so you see the white limestone skeleton underneath
 What can stress a coral?
 High light or UV levels
 Cold temperatures
 Low salinity and high turbidity from coastal runoff events or heavy rain
 Exposure to air during very low tides
 Major: high water temperatures
 Thermal stress
 Corals live close to their thermal maximum limit
 If water temperature gets 1 or 2°C higher than the summer average in many parts of the world, corals may get stressed and bleach
 NOAA satellites measure global ocean temperature and thermal stress
 How warm is warm?
 How hot do you think the ocean has to get before corals start to bleach?
 GLOBAL WARMING
 Glaciers and Sea Ice are melting
 World map showing levels of
coral bleaching. Source: ReefBase
 Can corals recover?
 Yes, if the stress doesn’t last too long
 Some corals can eat more zooplankton to help survive the lack of zooxanthellae
 Some species are more resistant to bleaching, and more able to recover
 Can corals recover?
 Corals may eventually regain color by repopulating their zooxanthellae
 Algae may come from the water column
 Or they may come from reproduction of the few cells that remain in the coral
 Can corals recover?
 Corals can begin to recover after a few weeks
 Does bleaching kill corals?
 Yes, if the stress is severe
 Some of the polyps in a colony might die
 If the bleaching is really severe, whole colonies might die
 Bleaching in Puerto Rico killed an 800-year-old star coral colony in 2005
 What else can stress do to corals?
 Question: what is something that happens to people when they are highly stressed?
 What else can stress do to corals?
 Question: what is something that happens to people when they are highly stressed?
 Bleaching and coral disease
 Coral diseases are found around the world

 High temperatures and bleaching can leave corals more vulnerable to disease

 Can quickly kill part or all of the coral colony
 Bleaching and bioerosion
 We have seen that bleaching can kill part or all of a coral colony
 Areas of dead coral are more vulnerable to bioerosion (when animals wear away the coral reef’s limestone structure)
 Storms & coral bleaching
 The same warm water that causes corals to bleach can also lead to strong storms.
 Storms: a mixed blessing
 Storms: a mixed blessing
 Each passing hurricane in 2005 cooled the water in the Florida Keys.


 Part 2: Investigating diversity of Symbiodinium: past to present.
 What are zooxanthellae?
 Algae that live in the coral polyp’s surface layer
 Algae get nutrients and a safe place to grow
 Corals get oxygen and help with waste removal
 Corals also get most of their food from the algae
 Symbiosis overview
 Genus Symbiodinium
 Described in 1962 by H. Freudenthal.
 Within dinoflagellates.
 Was though there was one single species worldwide.

 Morphology & life cycle
 Host species
 Cnidaria (corals, jellyfish, anemone, zoanthids, octocorals).
 Mollusca (clams, snails).
 Platyhelminthes (flatworms).
 Porifera (sponges).
 Protista (forams).
 First genetic studies
 Rowan & Powers 1991.
 Utlized 18S ribosomal DNA.
 Sampled from corals & anemones.
 Found unexpected diversity!
 Recommended further genetic studies.

 Second wave of studies
 Used faster evolving DNA markers.
 Particularly ITS-rDNA.
 Even more diversity!
 Zooxanthellae clade
DNA analyses
Clade: A group composed of all the species descended from a single common ancestor
 Diversity
 Eight major clades known.
 Within each clade many subclades.
 Do not know what taxonomic level clades are equal to.
 Evolution and biogeography
 Many studies have catalogued diversity.
 Can now understand on many scales.
 Can predict evolution.
 Specific types
 Many subclades or types associate with similar hosts.
 Could be co-evolution.
 Symbiodinium in Zoanthus sansibaricus
 We sampled the same species from 4 locations.
 Each host colony was shown to associate with one subclade of Symbiodinium.
 Subclade C1/C3 was common in the north, and subclade A1 was dominant in the south.
 C1/C3 has been shown to be a dominant Indo-Pacific “generalist”, with C15 common in Porites spp., and A1 a shallow-water specialist.
 Modes of transmission & flexibility
 2 major types; a) vertical and b) horizontal.
 Vertical should result in more co-evolution and less flexibility.
 Also, in horizontal, ZX from environment still rare.
 Changes in ZX
over time?
 Changes have been seen over time in content of ZX within coral colonies!
 Particularly after bleaching events.
 ZX shuffling?
 Adaptive Bleaching Hypothesis (ABH).
 Very controversial, large conservation implications.
 Two ways this occurs.
 Diversity within colonies
 Same colony may have different ZX at different locations!
 Differences in types
 Since we know diversity, we can experiment with different conditions.
 Many ZX are easy to culture.
 Control light, temperature, nutrients, etc.

 Can also then experiment in situ.
 Symbiodinium spp. characters
 Believed to alternate between a free-living stage with flagella, and a non-motile stage with chlorophyll.
 Believed to sexually reproduce, although this has not been observed.
 Overall morphological condition can degrade based on non-optimal environmental conditions, in particular low (<15 º C) and high (>30ºC) sustained ocean temperatures.
 “Adaptive bleaching” hypothesis
 Bleaching may enable corals to adopt different classes of zooxanthellae, better suited for a new environment. By:
 ‘symbiont switching’ (a new clade from exogenous sources) or
 ‘symbiont shuffling’ (host contains multiple clades and a shift in dominance occurs).

 Can we protect corals from bleaching?

 Marine invertebrate - Symbiodinium spp. symbioses overview
 Symbiodinium spp. found in many clonal cnidarians (and other invertebrates) in tropical and sub-tropical oceans. Symbiodinium are the main reason coral reefs exist and have large levels of diversity.
 Symbiodinium is now divided into 8 “clades” labelled A-H (of unknown taxonomic level) with many “subclades” (designated by numbers) within each clade (see various works by Pochon et al., and LaJeunesse et al.)
 Host species’ association with various clades and subclades of Symbiodinium (often more than one) may be at least partially responsible for differences in bleaching patterns seen during bleaching events (i.e. ENSO event of 2001, etc.).
 Also, some host species have been shown to have flexible associations with Symbiodinium over biogeographical ranges (depth, latitude, etc.) or time (summer versus winter, etc.). This is part of the Adaptive Bleaching Hypothesis (ABH) (Buddemier and Fautin 2004; Baker 2001), and is very contentious.
 Need to understand Symbiodinium diversity within zoanthids before any discussion of symbiotic zoanthid ecology can be conducted.
References:
1. Rowan & Powers. 1991. Molecular genetic identification of symbiotic dinoflagellates (zooxanthellae). Marine Ecology Progress Series 71: 65-73.
2. Stat et al. 2006. The evolutionary history of Symbiodinium and scleractinian hosts - Symbiosis, diversity, and the effect of climate change. Plant Ecology, Evolution and Systematics 8: 23-43.
3. LaJeunesse 2005. ‘Species’ radiations of symbiotic dinoflagellates in the Atlantic and Indo-Pacific since the Miocene-Pliocene transition. Molecular Biology and Evolution 22: 570-581.
4. Pochon et al. 2004. Biogeographic partitioning and host specialization among foramineferan dinoflagellate symbionts (Symbiodinium; Dinophyta). Marine Biology 139: 17-27.