Abstract
Climate change affects marine microorganisms in a number of ways. Evidences are accumulating on changes in composition, diversity, productivity, rate of metabolism and food web dynamics; and frequent occurrence of eutrophication, harmful blooms, diseases and coral bleaching in marine ecosystems. Crucially, microorganisms have the capacity to alter the climate. It has been recognised that microbes can accelerate the pace of climate change by adding more greenhouse gases into seawater. In spite of their important role in the contemporary climate change, marine microorganisms have been neglected in discussions of climate science. This paper calls for an intensive research to bridge the gap between microbiology and climate science.
Keywords: Greenhouse gases, global warming, microbes, food web, biogeochemical cycle
Introduction
There is now a widely-held consensus among scientists and policy-makers that human activities are increasing the levels of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. In its Fourth Assessment Report (FAR), the Intergovernmental Panel on Climate Change (IPCC., 2007) stated that warming of the climate system is unequivocal and that emission levels of greenhouse gases such as carbon dioxide, methane and nitrous oxide have increased markedly in the atmosphere as a result of human activities. These changes, in turn, links to increase in seawater temperature, acidification and sea level rise in addition to changes in current and water mass movement in the oceans. If we change the environment, the habitat changes, that is everything from the chemistry to the physics of the habitat changes and affects all the organisms that live. In recent years, there has been an increasing recognition of the impact of climate change on marine microorganisms as well as the key role they play in regulating the climate of the earth.
Impact of climate change on marine microbes and other microorganisms
From experimental evidences, we know now that changing environmental conditions deselect certain groups of microorganisms. For example, some microorganisms prefer to live in acidic conditions and some others in warm water conditions. Thus it is likely that climate change is already altering the microbial diversity. As a result of increasing sea water temperatures, acidification of the ocean and salinity changes, the dominant species of bacteria, and viruses may become dormant and completely unknown species may become dominant.
The increase in atmospheric CO2 concentrations may affect marine microorganisms in a number of ways. In the first place, it might stimulate primary producers, but an increase in primary production happens only when CO2 is the limiting factor. This is unlikely to be the case for many primary producers and for many prevailing environmental conditions. Even if primary production were enhanced, the resulting phytoplankton blooms, even if not toxic, often result in poor water quality. Decomposition of the increased organic matter would lead to oxygen depletion and to anaerobic processes that in the marine environment would lead to formation of toxic sulphide and eutrophication (Glockner et al., 2012).
Basically, elevated seawater temperature will increase the rates of metabolism of ecosystems, thereby changing the balance between production and consumption of oxygen. Considering the current knowledge of impacts climate change on microbial communities, the two most important factors are likely to be temperature and nutrient enrichment.
Global warming is predicted to drive frequent eutrophication and harmful algal blooms, as higher temperatures lead to more precipitation and greater run-off of nutrient salts into the marine environment. It is suspected that changes in climate may be creating a marine environment particularly suited to Harmful algal blooms (HAB)-forming species of algae (Hallegraeff, 1993). In the past three decades, occurrence of eutrophication and HAB seem to have become more frequent, intense and widespread. In the Indian sea waters, harmful algal blooms are increasing and causing considerable mortality of fish (Padmakumar et al., 2009). The southwest coast of India is particularly exposed in this respect.
Coral reefs are highly sensitive to climatic influences and are among the most sensitive of all ecosystems to temperature changes, exhibiting the phenomenon known as bleaching when stressed by higher than normal sea temperatures. Reef-building corals are highly dependent on a symbiotic relationship with microscopic algae (type of dinoflagellate known as zooxanthellae), which live within the coral tissues. The corals are dependent on the algae for nutrition and colouration. Bleaching results from the ejection of zooxanthellae by the coral polyps and/or by the loss of chlorophyll by the zooxanthellae themselves. Corals usually recover from bleaching, but die in extreme cases. Indian coral reefs have experienced 29 widespread bleaching events since 1989 and intense bleaching occurred in 1998 and 2002 when the sea surface temperature (SST) was higher than the usual summer maxima. By using the relationship between past temperatures and bleaching events, and the predicted SST for another 100 years, Vivekanandan et al., (2009) predicted that reefs should soon start to decline in terms of coral cover and appearance. They also predicted that the reefs in the Indian seas are likely to become remnant during the years 2050-2060.
The sensitivity of most marine microbes to temperature is extremely difficult to assess as precise thresholds are not known for most of the species. A 1-2°C increase in SST may have profound effects on biogeochemical cycles and microbial loop (Webster and Hill, 2007). It has recently been highlighted that climate change parameters such as elevated SST may also have direct and indirect consequences on marine viruses, with potentially cascading impacts on food webs, biogeochemical cycling, carbon sequestration and the metabolic equilibrium of the ocean (Danovaro et al., 2009).
Surface water warming and the consequent increase of water column stability can favour the coalescence of marine snow (small amorphous aggregates with colloidal properties) into marine mucilage (large marine aggregates representing an ephemeral and extreme habitat). Marine mucilage characterizes aquatic systems with altered environmental conditions. Danovaro et al., (2009) investigated, by means of molecular techniques, viruses and prokaryotes within the mucilage and in surrounding seawater to examine the potential of mucilage to host new microbial diversity and/or spread of marine diseases.
They found that marine mucilage contained a large and unexpectedly exclusive microbial biodiversity and hosted pathogenic species that were absent in the surrounding seawater. They also investigated the relationship between climate change and frequency of mucilage in the Mediterranean Sea over the last 200 years and found that the number of mucilage outbreaks increased almost exponentially in the last 20 years. The increasing frequency of mucilage outbreaks is closely associated with the temperature anomalies. The mucilage can act as a controlling factor of microbial diversity and has the potential to act as a carrier of specific microorganisms, thereby increasing the spread of pathogenic bacteria.
The frequency of disease outbreaks affecting marine organisms is increasing. It is not clear whether this is being driven by changing climate. However, for some populations such as corals, disease outbreaks have now been empirically linked with increased seawater temperatures. With predicted higher temperatures in the future, microbial mediated disease on marine organisms is likely to increase (Webster and Bourne, 2012).
Coral bleaching off Androth Island, Lakshadweep in April 2010 (Photo: K.P. Said Koya, CMFRI)
Impact on food web
Marine microorganisms dominate the marine food web and are the basis of production of food for all life in the ocean. Not surprisingly, marine microorganisms are of great importance to the fisheries and aquaculture sectors and a better knowledge of their role will be critical to secure the necessary food supply from marine living resources.
Laboratory experiments on seven species of phytoplankton at lower (24°C) and higher (29°C) seawater temperatures showed that at higher temperature, the rate of multiplication was faster and cell density was higher (CMFRI, 2009). However, the decay set-in earlier and the cycle was completed faster at higher temperature. For instance, the decay of microalgae was on Day 12 at 24°C, but on Day 10 at 29°C. Moreover, the species dominance within the culture period was different between the two temperatures. This study indicates the potential response in the growth rate, species composition and longevity of phytoplankton to higher temperature. Other factors such as light, current and nutrient availability will also affect the amount and composition of phytoplankton. The availability of phytoplankton influences the food availability up through various trophic levels.
The transport and abundance of zooplankton, the main consumers of phytoplankton, must synchronise with the phytoplankton bloom, otherwise zooplankton cannot survive, thus depriving food for organisms at higher trophic levels. In nature, the phytoplankton blooms, and the occurrence and abundance of zooplankton are always well-timed. Any potential mismatch would offset the food web. Synchrony between timing and abundance of peak zooplankton determines the larval recruitment as well as the abundance of several adult fishes.
Fisheries depend on adequate functioning of microorganisms. Yet, despite their central role in marine food webs, we still know very little about functioning of marine microorganism communities and how alterations, as a result of climate change, might affect commercial fish stocks.
Impact of microbes on climate
Crucially, microbes have the capacity to alter the environment in profound ways. Marine microbes comprise 98 per cent of the biomass of the world's oceans, supply more than half the world's oxygen, are the major processors of the world's greenhouse gases and have the potential to mitigate the effects of climate change. Microbes are involved in many processes, such as the carbon and nitrogen cycles, and are responsible for production as well as consumption of greenhouse gases such as carbon dioxide and methane. Microbes have positive and negative feedback responses to temperature, but the extent of these is not completely understood. The lack of clarity is because the microbes live in very diverse communities that interact with other organisms and environment in complex ways, which makes it difficult to make predictions about the effects of microbes on climate change. However, scientists are trying to include microbial activity in climate change models. What is certain is that human activities have helped to increase the production of greenhouse gases by microbes. As the CO2 and seawater temperature increase, the microbes become more active, grow faster and release more CO2 into seawater (Zimmer, 2010). The oceans will thus be able to absorb less carbon dioxide from the atmosphere. As a result, the carbon dioxide levels in the atmosphere will climb, warming the planet. It is thus possible that microbes can accelerate the pace of climate change.
Microbial activity affects atmospheric concentrations of the greenhouse gas nitrous oxide (N2O). N2O has been identified as the dominant ozone-depleting compound and it is projected that it will continue to be throughout the 21st century. Thus, understanding the processes controlling emissions of N2O from coastal and marine systems is important for evaluating climate change scenarios.
Methane (CH4) a powerful greenhouse gas (GHG), is both produced and consumed in anoxic coastal sediments via microbial processes. Sources and fates of methane in the sea and the role of marine microorganisms in methane regulation have not been fully understood. As the evidence for warming climate was established in the later part of the 20th century, it is cautioned that large quantities of methane might be liberated by widespread destabilization of climate-sensitive gas hydrate deposits trapped in marine sediments (Mascarelli, 2009). Even if only a fraction of the liberated CH4 were to reach the atmosphere, the potency of CH4 as a greenhouse gas has heightened the concerns related to climate change.
Future research
There is clear evidence that microbes can have an enormous impact on climate. However, microbes have been neglected in discussions of climate science. As there is need to understand and find ways to mitigate climate change, research on the role of microbes in global biogeochemical cycles, can no longer be ignored.
Lack of research effort into marine microbial communities makes the prediction of global climate change impacts extremely difficult. There is an urgent need for research on how climate change will affect marine microbial populations and to bridge the gap between microbiology and climate science using models. There is also the need to develop a monitoring/data collection strategy, implement validation processes to integrate data collection, modeling and experimentation and further develop technologies such as remote sensing (Reid, 2011). Climate shift experiments in the laboratory on microbial systems with control over aquatic and atmospheric environments enabling short-term and sustained experiments by altering factors such as temperature, ocean acidity, salinity, sedimentation and contaminants will be rewarding (Webster and Bourne, 2012). These kinds of experiments would complement long-term data collection. In addition, continuous monitoring of seawater samples would provide valuable baseline data for observing climate trends into the future.
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