Partnertausch: Hitzetolerante Algen-Symbionten schützen vor Korallenbleiche

Eines unserer Ziele hier im Kalte-Sonne-Blog ist die Bekanntmachung von klimawissenschaftlichen Resultaten, die es nicht in die Mainstream-Presse schaffen. Hierdurch versuchen wir eine inhaltliche Schieflage auszugleichen. Im heutigen Beitrag soll es um die Korallenbleiche gehen. Ein Jahr lang haben wir gesammelt und waren erstaunt, wie sehr sich die Sichtweise hier geändert hat. Der Grundmechanismus ist natürlich geblieben: Immer wenn es Korallen zu heiß wird, schmeißen sie ihre Algen-Symbionten raus und erbleichen dabei. Da die Korallen von den Symbionten als Energie- und Nährstofflieferant abhängig sind, müssen die Vakanzen schnell wieder gefüllt werden. Entweder mit den ursprünglichen Algen (wenn es wieder abkühlt) oder mit neuen Algen, die an erhöhte Temperaturen angepasst sind. Nach diesem Prinzip operierend haben die Korallen schon etliche hundert Millionen Jahre überlebt.

Die University of Miami Rosenstiel School of Marine & Atmospheric Science beschrieb den Prozess im Rahmen einer Pressemitteilung am 4. Juni 2015:

New study uncovers why some threatened corals swap ‚algae‘ partners

Research findings provide new information on coral survival in a warming ocean

A new research study showed why threatened Caribbean star corals sometimes swap partners to help them recover from bleaching events. The findings are important to understand the fate of coral reefs as ocean waters warm due to climate change.

The University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science research team placed colonies of Caribbean star coral (Orbicella faveolata) in a heated tank for one to two weeks to replicate ocean conditions that would lead to both mild and severe coral „bleaching“ – when corals turn white as a result of the loss of symbiotic algae living in their tissues. The corals, collected from waters off Miami, were then allowed to recover at two different water temperatures, below and above the local average, to see if they recovered with the same or different algal partners. „Since ’symbiont shuffling‘ occurs in only some cases, we wanted to understand what drives this process and whether it could help corals adjust to climate change,“ said Ross Cunning, a UM Rosenstiel School alumnus and lead author of the study. „We discovered that partner switching in Caribbean star corals is dependent upon the severity of the bleaching event and the temperature during recovery.“

The researchers discovered that severe bleaching and warmer water recovery temperatures caused corals to shuffle their symbionts in favor of more heat-tolerant algae, which belong to a group of symbionts called clade D, while mild bleaching and cooler recovery drove shifts toward the less heat-tolerant algae, in clade B. The study, published in the June 3 issue of the journal Proceedings of the Royal Society B, suggests that increases in heat-tolerant symbionts in the Caribbean star coral are greatest when bleaching is more severe and the recovery environment is warmer.

Corals depend on symbiotic algae to survive and build coral reefs. Increased ocean temperatures due to climate change can cause these symbiotic algae to be expelled from the coral, an event known as bleaching, which often leads to death. Climate change is one of the main threats to the Caribbean star coral (O. faveolata), which was were recently listed as a ‚threatened‘ species under the U.S. Endangered Species Act. „These findings help resolve a long-standing debate over why some corals switch partners after bleaching, while others do not,“ said Andrew Baker, UM Rosenstiel School associate professor of marine biology and ecology and a Pew fellow in marine conservation. „They show that, as the oceans continue to warm and bleaching events become more severe, we might expect heat-tolerant symbionts to become a common feature of recovering reefs. Corals that can ‚buddy up‘ with different algae might be more resistant to bleaching in the future.“

Two more recent studies, also conducted in Baker’s Coral Reef Futures lab at UM, showed how corals modify their symbionts in response to environmental changes. The first, published earlier this year in the journal Global Change Biology, showed how changes in symbiont partners following bleaching directly increased corals‘ thermal tolerance. The second, published in the May issue of the journal Ecology, showed that, in addition to changing the types of algae they partner with, corals also fine-tune the number of algae they contain to deal with an ever-changing environment. „Together, these studies suggest that that the rate of warming, timing between bleaching events, and severity of each bleaching event, will play an important role in determining coral survivorship,“ said Baker. „We need to better understand these changes in order to accurately predict coral reef futures.“

Bereits einige Monate zuvor hatte die University of Southampton der Öffentlichkeit besonders wärmeresistente Algen-Symbionten vorgestellt:

New algae species helps corals survive in the hottest reefs on the planet

A new species of algae has been discovered in reef corals of the Persian (Arabian) Gulf where it helps corals to survive seawater temperatures of up to 36 degrees Celsius — temperatures that would kill corals elsewhere.

Researchers from the University of Southampton and the New York University Abu Dhabi identified the symbiotic algae in corals from Abu Dhabi, United Arab Emirates, the world’s warmest coral reef habitat. The paper, which reports the breakthrough discovery, was published this week in the journal Scientific Reports. „We found that commonly applied molecular methods did not give enough resolution to distinguish the dominant symbionts of Gulf corals from those in other parts of the world’s oceans,“ explains Professor Jörg Wiedenmann, Professor of Biological Oceanography and Head of the Coral Reef Laboratory at the University of Southampton. „However, when analysed by alternative molecular biological approaches, we found pronounced differences that set this heat tolerant species clearly aside. We named it Symbiodinium thermophilum in reference to its ability to survive unusually high temperatures.“

Reefs are made up of many coral species, most of which live in a mutually beneficial relationship with microscopically small algae hosted in their tissue. These symbiont algae produce sugars that contribute to the diet of the coral in return for shelter and nutrients that are vital for algal growth. However, the symbiotic association is vulnerable to changes in environmental conditions, in particular to increases in seawater temperature. Heat-stress induced loss of the algal partners from the coral host can result in the often fatal process known as ‚coral bleaching‘.

„Understanding how corals survive under the extreme temperatures in the Gulf will give us important insights into the ability of reef corals to handle the heat stress, which is threatening their survival in the oceans that are warming up in response to climate change,“ explains Professor Wiedenmann. „We monitored the symbiotic partnership over several seasons to ensure that this association was stable through a range of thermal conditions,“ comments Professor John Burt from the New York University Abu Dhabi. „We can confirm that this new type of alga is indeed the year-round prevalent symbiont across several dominant coral species from the Abu Dhabi coast of the United Arab Emirates,“ he adds.

„It gives hope to find that corals have more ways to adjust to stressful environmental conditions than we had previously thought,“ adds Professor Wiedenmann. „However, it is not only heat that troubles coral reefs. Pollution and nutrient enrichment, overfishing and coastal development also represent severe threats to their survival. Only if we manage to reduce these different forms of stress will corals be able to benefit from their capacity to adjust to climate change.“

Eine ähnliche Studie erschien 2015 auch von Silverstein et al. im Fachblatt Global Change Biology. In der Karibik hat sich zwischenzeitlich eine Mikroalge ausgebreitet, die die dortigen Korallen gegen Hitzestress schützt. Penn State berichtet darüber am 1. Juni 2015 in einer Pressemitteilung:

Invasive microbe protects corals from global warming, but at a cost

An invasive species of symbiotic micro-alga has spread across the Caribbean Sea, according to an international team of researchers. These single-cell algae, which live within the cells of coral animals, are improving the resilience of coral communities to heat stress caused by global warming, but also are diminishing the abilities of corals to build reefs.

„The results raise a potentially contentious issue about whether this invasion is relatively good or bad for the long-term productivity of reef corals in the Atlantic Ocean and the ecosystems they support,“ said Todd LaJeunesse, associate professor of biology, Penn State. The team’s findings appear in today’s (JUNE 1) issue of the Proceedings of the National Academy of Sciences. According to LaJeunesse, relationships between corals and photosynthetic algae evolved over millions of years and are generally mutually beneficial. Corals derive energy and nutrients from algae, and in turn algae obtain nutrients and protection by living in the tissues of corals.

„Coral reefs are highly important to the biosphere, and they also have enormous economic and societal value in the form of tourism, recreation and coastal protection, and as a source of food and pharmaceuticals,“ said LaJeunesse. „Currently, these ecosystems are threatened by synergistic effects of diminished water quality, increased temperature and reduced ocean alkalinity.“ LaJeunesse and his team, which includes researchers from the University of Delaware and the Universidad Nacional Autonoma de Mexico (UNAM), used DNA sequencing techniques to document this possible new threat to coral reefs — the spread of non-native, Symbiodinium trenchii, which comes from the Indo-Pacific.

To determine that S. trenchii’s presence in the Caribbean likely came from a limited introduction and then began to spread, the team used population genetic markers to analyze the genetic diversity among populations of S. trenchii in the Indo-Pacific and compared this diversity with that in the Caribbean Sea. „We found that the Caribbean population of S. trenchii contains very little genetic diversity and is highly inbred,“ said Tye Pettay, recent Ph.D. in biology, Penn State and current postdoctoral fellow, University of Delaware. „In contrast, S. trenchii in the Indian and Pacific oceans is extremely diverse and contains far more genetic diversity on a single reef the size of a football field than it does in the entire Caribbean Sea. Our evidence indicates that the introduction of S. trenchii to the Caribbean was relatively recent. There has been no time for it to evolve any novel genetic diversity.“

In its new home in the Caribbean, the team found, S. trenchii behaves opportunistically and proliferates within coral colonies during periods of increased sea-surface temperatures, enabling the corals to survive these episodes. During these warming events, S. trenchii replaces the more sensitive native species of algae, which are expelled by their hosts when the environment becomes too warm — a process known as bleaching. S. trenchii eventually is replaced by native species of algae after environmental conditions return to normal. „For some time now researchers have focused on identifying stress-tolerant Symbiodinium that may allow reef corals to better cope with future increases in temperatures,“ said Pettay. „Symbiodinium trenchii in the Caribbean possesses those attributes. It turns out, however, that because this species was introduced to the region, its symbioses may be suboptimal, which may significantly diminish coral growth under normal conditions.“

Specifically, the researchers found that S. trenchii benefits some colonies of coral by providing them with thermal tolerance under conditions up to 3.6 degrees Fahrenheit higher than normal, yet the team also found that for one group of dominant reef-building corals, Mountainous Star Coral, S. trenchii reduces rates of calcification — the process by which reefs are built — by half. „Our results indicate that S. trenchii may not translocate as many nutrients to the coral host as do native species, which is why we see reduced calcification rates among the corals,“ said LaJeunesse.

The retention of more nutrients to invest in its own growth and physiology may explain, in part, why S. trenchii can continue to function under heat stress and is not expelled by the host during warming events, he added. As a result, corals with S. trenchii tend not to bleach. „Invasive species pose major threats to biodiversity, ecosystem function and economic well-being,“ said LaJeunesse. „Growing evidence indicates that microbes, which include micro-algae, are being successfully introduced to new places around the world, but we still have little understanding of the negative or positive outcomes from such introductions. This work highlights how microbial introductions, many of which may be unknown to science, can affec ecosystem stability and function — in this case, reduced calcification of corals in the Caribbean.“

Passend zum Thema eine Studie mit dem Titel von Cruz et al. 2015 in Marine Biology:

„White but not bleached: photo-physiological evidence from white Montastraea cavernosa reveals potential overestimation of coral bleaching“ 

Den Abstoßungs-Akt zwischen Koralle und Alge hat die Queensland University of Technology schön dokumentiert. In einer Pressemitteilung vom 16. August 2016 gab die Universität bekannt:

Caught in the act: first videos of coral bleaching behaviour

Coral researchers have for the first time captured the specific behaviour of a coral as it’s bleaching.

The team from QUT in Australia used a clever combination of microscope, digital camera and smart tablet to record close-up, detailed time-lapse videos of a coral species’ physical reaction to heat stress, showing evidence for the first time that it employs pulsed inflation. To simulate rising sea surface temperatures, researchers Brett Lewis and Dr Luke Nothdurft from QUT’s marine facility in the School of Earth, Environmental and Biological Sciences placed solitary corals, Heliofungia actiniformis, into controlled aquaria, before heating the water up. Their resulting videos, described in the peer-reviewed Coral Reefs, show the unhappy corals belching Symbiodinium, tiny algae cells that live within coral tissue and give corals their vibrant colours.

“What’s really interesting is just how quickly and violently the coral forcefully evicted its resident symbionts,“ said Mr Lewis, from QUT’s Science and Engineering Faculty. “The H. actiniformis began ejecting the symbionts within the first two hours of us raising the water temperature of the system.” Mr Lewis said previous studied had shown H. actiniformis was one of the very few corals on the Great Barrier Reef considered to be relatively resilient to bleaching, even as neighbouring species suffered the full effects. “Our observations suggest this resilience could be due to the rapid expulsion of the coral’s algal symbionts during thermal stress, and could very well increase H. actiniformis’s chance of survival during abnormally high sea temperatures.”During the experiments, the team raised the water temperature in a 10-litre aquarium system from 26oC to 32oC over 12 hours, where it remained for up to eight days.

While scientists have known for some time that coral bleaching occurs when the relationship between the coral and their Symbiodinium breaks down as ocean temperatures rise, the QUT team’s time-lapse videos show for the first time how this coral removes the algae. “Our H. actiniformis used a pulsed inflation to expel Symbiodinium over time (seen as greenish plumes in the video) – inflating their bodies to as much as 340 per cent of their normal size before suddenly and violently contracting and ejecting Symbiodinium through their oral openings over the four to to eight day duration of the experiments” Dr Nothdurft said.  

Dr Nothdurft said reef-building corals and their algal Symbiodinium had evolved to form a mutually-beneficial relationship. “Coral provide Symbiodinium with protection and surface area for photosynthesis, while the excess sugars created by the algae supply the majority of the coral’s daily food requirements,” he said. He said expulsion of the algae removed the pigment from the corals tissue, rendering them white or transparent, referred to as coral bleaching. If environmental conditions return to normal quickly enough, some corals may regain their Symbiodinium and associated colour. “If the Symbiodinium is removed from the host and does not recolonise quickly, the corals can die. “Mass coral bleaching events are a concern for scientists globally with recent events on the Great Barrier Reef highlighting the threat of elevated water temperatures to the heath of reef ecosystems.” Mr Lewis and Dr Nothdurft have captured a wide variety of coral behaviours with time-lapse photography, including how they eat and how they fight over limited space. Watch the videos:

Eine gute Zusammenfassung zum Thema Korallenbleiche und Klimawandel stammt von Jim Steele. Die überraschend gute Hitzetoleranz von Phytoplankton war auch Thema einer Pressemitteilung der University of Exeter vom 27. November 2015:

Don’t forget plankton in climate change models, says study

A new study found that phytoplankton – microscopic water-borne plants – can rapidly evolve tolerance to elevated water temperatures.

Globally, phytoplankton absorb as much carbon dioxide as tropical rainforests and so understanding the way they respond to a warming climate is crucial. Phytoplankton subjected to warmed water initially failed to thrive but it took only 45 days, or 100 generations, for them to evolve tolerance to temperatures expected by the end of the century. With their newfound tolerance came an increase in the efficiency in which they were able to convert carbon dioxide into new biomass. The results show that evolutionary responses in phytoplankton to warming can be rapid and might offset some of the predicted declines in the ability of aquatic ecosystems to absorb carbon dioxide as the planet warms.

Dan Padfield, a PhD student at the Environment and Sustainability Institute at the University of Exeter’s Penryn Campus in Cornwall, said: “Our findings suggest that evolution could play a key role in shaping how aquatic ecosystems respond to climate change. The phytoplankton in our study adapted to warmer water in the lab and evolved the ability to capture more atmospheric carbon dioxide. “Our results demonstrate that evolutionary responses of phytoplankton to warming should be taken into account when developing models of how climate change will affect aquatic ecosystems. This experimental work provides the empirical basis for incorporating evolution into the models used to forecast future ocean productivity.”

The researchers exposed Chlorella vulgaris, a model species of phytoplankton, to temperatures of 20 – 33 degrees. Initially rates of growth peaked at 30 degrees, while 33 degrees was stressful and limited growth. After 100 generations (45 days) growth increased to levels expected from the exponential effects of temperature on physiological rates, showing that the algae had evolved the ability to thrive at the increased temperatures. The underlying mechanism for the ability to tolerate warmer temperatures was an increase in the efficiency in which the alga was able to convert carbon dioxide into new biomass by reducing rates of respiration (production of carbon dioxide). It is this shift in the relative rates of respiration and photosynthesis that enabled the phytoplankton to cope with warmer temperatures. While these experiments focused on a single species and strain of phytoplankton, the researchers believe that the rapid evolution of carbon-use efficiency will apply to other species of phytoplankton and substantially improve models describing ecological and biogeochemical effects of climate change.

Nur drei Wochen später legte die Uni Exeter noch einmal nach. Wieder ein unerwartet positives Resultat: Höhere Wasssertemperaturen steigern die Biodiversität und Photosyntheseleistung von Phytoplankton:

Phytoplankton like it hot: Warming boosts biodiversity and photosynthesis in phytoplankton

Warmer temperatures increase biodiversity and photosynthesis in phytoplankton, researchers at the University of Exeter and Queen Mary University of London (QMUL) have found. Globally, phytoplankton – microscopic water-borne plants – absorb as much carbon dioxide as tropical rainforests and so understanding the way they respond to a warming climate is crucial.

The groundbreaking study, published in the journal PLOS Biology, was carried out over five years using artificially warmed ponds that simulated the increases in temperature expected by the end of the century. The researchers found that phytoplankton in ponds that had been warmed by four degrees, had 70% more species and higher rates of photosynthesis, and as a result, have the potential to remove more carbon dioxide from the atmosphere. Phytoplankton were counted, measured and identified under a microscope, and the production or consumption of oxygen was measured to determine rates of photosynthesis and respiration.

The study found that phytoplankton communities in the warmed ponds were more species rich, had greater evenness in species abundance, greater biomass and were dominated by larger species. In contrast to previous work conducted in small scale, short-term laboratory experiments, these findings demonstrate that future global warming could actually lead to increases in biodiversity and photosynthesis in some locations. These results cannot be extrapolated to the global scale as declines might occur in other places where different ecological mechanisms prevail.

The authors attribute their findings to the fact that the experiments were conducted in open outdoor ecosystems where local extinctions of species can be replaced by new immigrants from surrounding locations. Dr Gabriel Yvon-Durocher from the Environment and Sustainability Institute at the University of Exeter said: “The increases we’ve seen in phytoplankton biodiversity appear be driven primarily by the effects of warming on zooplankton – the microscopic animals that eat phytoplankton. “Higher grazing rates by the zooplankton, which prefer small abundant phytoplankton species, prevent the ecosystem being dominated by just a few of these highly competitive species, allowing species which are inferior competitors for resources to coexist. “What our study clearly shows is that future global warming is likely to have a major impact on the composition, biodiversity and functioning of plankton, which play a pivotal role in aquatic ecosystems.” Professor Mark Trimmer from QMUL’s School of Biological and Chemical Sciences said: “Our warming facility at QMUL has been running for 10 years now and it is quite remarkable what such a simple experiment has enabled us to uncover about how climate warming alters the cycling and balance of the key bio-elements that sustain life on Earth.”