1,721,600 research outputs found
Ancestral genetic diversity associated with the rapid spread of stress-tolerant coral symbionts in response to Holocene climate change
No full textCoral communities in the Persian/Arabian Gulf (PAG) withstand unusually high salinity levels and regular summer temperature maxima of up to ?35 °C that kill conspecifics elsewhere. Due to the recent formation of the PAG and its subsequent shift to a hot climate, these corals have had only <6,000 y to adapt to these extreme conditions and can therefore inform on how coral reefs may respond to global warming. One key to coral survival in the world’s warmest reefs are symbioses with a newly discovered alga, Symbiodinium thermophilum. Currently, it is unknown whether this symbiont originated elsewhere or emerged from unexpectedly fast evolution catalyzed by the extreme environment. Analyzing genetic diversity of symbiotic algae across >5,000 km of the PAG, the Gulf of Oman, and the Red Sea coastline, we show that S. thermophilum is a member of a highly diverse, ancient group of symbionts cryptically distributed outside the PAG. We argue that the adjustment to temperature extremes by PAG corals was facilitated by the positive selection of preadapted symbionts. Our findings suggest that maintaining the largest possible pool of potentially stress-tolerant genotypes by protecting existing biodiversity is crucial to promote rapid adaptation to present-day climate change, not only for coral reefs, but for ecosystems in general
A connection between colony biomass and death in Caribbean reef-building corals.
Full text linkIncreased sea-surface temperatures linked to warming climate threaten coral reef ecosystems globally. To better understand how corals and their endosymbiotic dinoflagellates (Symbiodinium spp.) respond to environmental change, tissue biomass and Symbiodinium density of seven coral species were measured on various reefs approximately every four months for up to thirteen years in the Upper Florida Keys, United States (1994-2007), eleven years in the Exuma Cays, Bahamas (1995-2006), and four years in Puerto Morelos, Mexico (2003-2007). For six out of seven coral species, tissue biomass correlated with Symbiodinium density. Within a particular coral species, tissue biomasses and Symbiodinium densities varied regionally according to the following trends: Mexico≥Florida Keys≥Bahamas. Average tissue biomasses and symbiont cell densities were generally higher in shallow habitats (1-4 m) compared to deeper-dwelling conspecifics (12-15 m). Most colonies that were sampled displayed seasonal fluctuations in biomass and endosymbiont density related to annual temperature variations. During the bleaching episodes of 1998 and 2005, five out of seven species that were exposed to unusually high temperatures exhibited significant decreases in symbiotic algae that, in certain cases, preceded further decreases in tissue biomass. Following bleaching, Montastraea spp. colonies with low relative biomass levels died, whereas colonies with higher biomass levels survived. Bleaching- or disease-associated mortality was also observed in Acropora cervicornis colonies; compared to A. palmata, all A. cervicornis colonies experienced low biomass values. Such patterns suggest that Montastraea spp. and possibly other coral species with relatively low biomass experience increased susceptibility to death following bleaching or other stressors than do conspecifics with higher tissue biomass levels
Host-Specific Interactions with Environmental Factors Shape the Distribution of Symbiodinium across the Great Barrier Reef
No full textBackground: The endosymbiotic dinoflagellates (genus Symbiodinium) within coral reef invertebrates are critical to the survival of the holobiont. The genetic variability of Symbiodinium may contribute to the tolerance of the symbiotic association to elevated sea surface temperatures (SST). To assess the importance of factors such as the local environment, host identity and biogeography in driving Symbiodinium distributions on reef-wide scales, data from studies on reef invertebrate-Symbiodinium associations from the Great Barrier Reef (GBR) were compiled
A lipidomic approach to understanding free fatty acid lipogenesis derived from dissolved inorganic carbon within cnidarian-dinoflagellate symbiosis
No full textThe cnidarian-dinoflagellate symbiosis is arguably one of the most important within the marine environment in that it is integral to the formation of coral reefs. However, the regulatory processes that perpetuate this symbiosis remain unresolved. It is essential to understand these processes, if we are to elucidate the mechanisms that support growth and resource accumulation by coral host, and conversely, recently observed reduction and/or mortality of corals in response to rapid environmental change. This study specifically focused on one area of metabolic activity within the symbiosis, that of free fatty acid synthesis within both the dinoflagellate symbionts and cnidarian host. The main model system used was Aiptasia pulchella and Symbiodinium sp. in combination with aposymbiotic A. pulchella, the symbiotic coral Acropora millepora system and dinoflagellate culture. Fatty acids (FAs) were selected because of their multiple essential roles inclusive of energy storage (resource accumulation), membrane structure fluidity and cell signaling. The study addressed free FA lipogenesis by using a new method of enriched stable isotopic (C-13) incorporation from dissolved inorganic carbon ((DIC)-C-13) combined with HPLC-MS. FAs derived from (DIC)-C-13 aligned with a mixture of known lipogenesis pathways with the addition of some unusual FAs. After 120 hr, C-13-enriched FA synthesis rates were attributed to only a complex integration of both n- and n-6 lipogenesis pathways within the dinoflagellate symbionts. Furthermore, there was no detectible evidence of symbiont derived enriched isotope fatty acids, catabolized C-13 derivatives or (DIC)-C-13 being directly utilized, in host late n-6 pathway long-chain FA lipogenesis. These findings do not align with a popular mutualistic translocation model with respect to the use of translocated symbiont photoassimilates in host long-chain FA lipogenesis, which has important connotations for linking nutrient sources with metabolite production and the dynamic regulation of this symbiosis
Bakterielle Reaktion auf erhöhte Gelöst Organische Kohlenstoff in Coral Reef Ökosysteme
No full textCoastal pollution and algal cover are increasing in many coral reefs, resulting in higher dissolved organic carbon (DOC) concentrations. High DOC concentrations are shown to have highly detrimental effects on coral reefs through the stimulation of bacterial growth and the accumulation of suspended particles (i.e., aggregate formation). In this thesis, aggregate formation processes and gene expression of planktonic coral reef microbial populations under elevated concentrations of naturally abundant monosaccharides (glucose, galactose,mannose, and xylose) in algal exudates and sewage inputs, were investigated. The results of this thesis show evidence of the detrimental effects of high DOC concentrations on coral reefs by promoting aggregate formation and microbial activity. Furthermore, this thesis shows that elevated DOC concentrations mediate the expression of virulence factors involved in invasion, iron chelation, accumulation of toxic substances, suggesting to be important for feeding the positive loop of coral reef degradation
Der Korallen-Holobiont im globalen Wandel
No full textThe tropical scleractinian coral holobiont is comprised by the coral animal, dinoflagellate algae of the genus Symbiodinium, and a multitude of other microbes, specifically bacteria, archaea, viruses, and protists. These holobionts are the unit of ecological selection, and remarkably adapted to thrive under oligotrophic (nutrient-poor) conditions. The foundation for this adaptation is provided by the coral-algae symbiosis, a mutualistic nutrient exchange relationship between the coral and Symbiodinium, allowing for the high primary productivity and growth rates of tropical coral holobionts. The coral-algae symbiosis is maintained via nitrogen limitation by the host, and new nitrogen from heterotrophic feeding, dissolved nutrient uptake, or coral-associated nitrogen fixation activity is retained and taken up within the holobiont. This thesis highlights the importance of functional dependencies on nitrogen cycling, particularly the nitrogen fixation pathway, in coral holobiont functioning, and the importance of employing a large set of response parameters covering critical functions of the main holobiont members
Assessment of nitrogen cycle pathways associated with different major benthic organisms in response to environmental changes
No full textNitrogen (N) cycling in coral reefs is of key importance for these oligotrophic ecosystems, but knowledge about its pathways is limited. While dinitrogen (N2) fixation is comparably well studied, the counteracting denitrification pathway is under-investigated. The present thesis shows that both processes occur reef-wide, and that both N2 fixation and denitrificaiton align at least for coral holobionts. Functional groups associated with high N2 fixation show low denitrification activity, and vice versa, with coral holobionts being major denitrifiers. Further, nitrate availability potentially moderates N2 fixation and denitrification in coral reef substrates with substrate-specific and nitrate concentration-depending thresholds that supress or stimulate both N cycle pathways
Auswirkungen von Simulierter Eutrophierung und Überfischung auf Korallenriff Invertebraten, Algen und Mikroben im Roten Meer
No full textBesides the main climate change consequences, ocean warming and acidification, local disturbances such as overfishing and eutrophication are major threats to coral reefs worldwide. Despite its relatively healthy coral reefs that are increasingly faced with growing coastal development, the Red Sea is highly under-investigated, particularly outside the Gulf of Aqaba. This thesis therefore aims to contribute to the understanding of eutrophication and overfishing effects on Red Sea coral reefs by answering the following three key questions: (1) How do different grazer groups contribute to herbivory, and is herbivory therefore susceptible to overfishing? (2) What are the individual and combined effects of eutrophication and overfishing on the development of important reef organisms? (3) What are the potential consequences for reef functioning when local threats increase? The thesis consists of five chapters that are framed between a general introduction and synoptic discussion. At the beginning, a review summarizes the current state of knowledge on marine eutrophication (Chapter 1), an important anthropogenic threat for coral reefs that are highly adapted to very oligotrophic conditions. Further, a series of in situ experiments with settling tiles and coral fragments in the Egyptian and Saudi Arabian Red Sea were used to investigate not only the contribution and influence of herbivory on benthic macroalgae development (Chapters 2 and 3), but also the individual and combined effects of simulated eutrophication and overfishing on settlement of benthic macroalgae (Chapter 3), sessile invertebrates (Chapter 4), and a coral with its associated bacterial community (Chapter 5). Findings revealed that: (a) among the two dominating grazer groups, herbivorous fish were fivefold more effective in reducing algal biomass than sea urchins. (b) the simulation of eutrophication did not affect algal biomass, but decreased coral settlement and caused specific Alphaproteo-, Sphingo-, and Epsilonproteobacteria to emerge in the coral holobiont. (c) the simulation of overfishing exhibited stronger effects than that of eutrophication. It caused algal cover shifting from communities dominated by encrusting algae with low biomass and oxygen consumption in controls to communities containing less calcifying algae, with high-biomass and oxygen consumption. The brown algae Padina sp. and Hydroclathrus clathrathus, along with filamentous algae, benefitted most from this treatment. Coral settlement was absent, while that of polychaetes increased, and specific Deltaproteobacteria were found within the coral holobiont. (d) the combined treatment produced stronger and longer lasting effects on algae than overfishing alone. Settlement of bryozoans and bivalves increased and specific Alphaproteobacteria emerged. In summary, this study underlines the ecological importance of herbivorous fishes, the high susceptibility of herbivory to overfishing, and it provides - for the first time - comprehensive information on how Red Sea coral reefs respond to eutrophication and overfishing. Findings recommend that both stressors, but particularly overfishing, should be prevented in pristine reefs and reduced in already affected reefs to avoid potential phase-shifts from dominance by hard corals to that by brown and filamentous algae, or other invertebrates such as polychaetes, bryozoans, and bivalves. The appearance and composition of algae and invertebrates may be used as bioindicators for local reef monitoring and management measures
Nitrogen cycling associated with corals and other reef organisms under environmental change
No full textNitrogen (N) is a limiting nutrient in highly productive tropical coral reefs, despite its key role for primary production. This requires efficient (re)cycling of N by the dwelling organisms, including the key reef ecosystem engineers, the hard corals. As such, corals evolved symbiotic relationships with eukaryotic and prokaryotic microbes, together called a holobiont, which aid in nutrient acquisition and recycling. The nutrient exchange symbiosis between the coral host and the eukaryotic photosynthetic dinoflagellates of the family Symbiodiniaceae has given corals an ecological advantage over other functional groups such as algae. The Symbiodiniaceae provide the coral host with carbon (C) rich photosynthates, while in return, the Symbiodiniaceae receive N and phosphorus (P). Additionally, diazotrophs, microbes capable of fixing atmospheric dinitrogen (N2), can provide the coral holobiont with bioavailable N. Coral holobionts benefit from low internal availability of N as N-limitation may maintain steady translocation of the photosynthates on which the corals rely. Thus, coral holobionts may be particularly susceptible to increases in (environmental) dissolved inorganic N (DIN) due to e.g. anthropogenic input, or stimulated activity of diazotrophs. As such, corals likely have mechanisms in place for the alleviation of excess N, i.e. denitrification, which may ultimately aid coral functioning.
This thesis aims at extending the current knowledge on biogeochemical cycling of N associated with coral holobionts. Specifically, in addition to N2 fixation, we tested whether the antagonistic N-cycling pathway to N2 fixation, i.e. denitrification, is an active pathway in coral holobionts and whether it is affected by environmental change. In addition, we measured N-cycling pathways associated with other coral reef organisms and substrates under environmental change. This allowed us to make inferences for coral reef functioning when exposed to global and local stressors. We applied a combination of physiological and molecular analyses and used the strong seasonality of the northern and central Red Sea as a natural laboratory.
Our findings reveal that denitrification was actively associated with all investigated coral species. Similar to diazotrophy, denitrification may thus be ubiquitously associated with coral holobionts. Under stable environmental conditions, denitrification and N2 fixation aligned and both N-cycling pathways correlated with Symbiodiniaceae cell densities. Thus, the relationship between denitrification and N2 fixation may be the result of a shared organic C limitation (by translocated photosynthates from the Symbiodiniaceae) within the holobiont. Higher seasonal availability of DIN (leading to higher DIN:dissolved inorganic P [DIP] ratios) dynamically shifted the ratio of denitrifiers and diazotrophs, in favour of the denitrifiers. The proliferation of Symbiodiniaceae suggests incomplete alleviation of excess N by denitrification. Indeed, Symbiodiniaceae cell densities also correlated with environmental DIN availability. In response to moderate in situ eutrophication of DIN and DIP, both N-cycling pathways more than doubled in activity. Surprisingly, the Symbiodiniaceae populations remained stable. In addition, there was no significant incorporation of N originating from the eutrophication event in the Symbiodiniaceae. This suggests that N-limitation was maintained, likely assisted by denitrification. These findings suggest that the dynamic interplay of denitrification and N2 fixation may regulate Symbiodiniaceae populations, but the extent to which they maintain N-limitation may depend on the environmental availability of DIN and DIP.
By comparing coral holobiont associated N-cycling to other functional groups on coral reefs, we postulate that under local and global stress scenarios, coral holobionts may lose the competition for space to algae as they 1) can strongly capitalize on (anthropogenic) nutrient inputs, 2) have high associated N2 fixation rates that increase in response to ocean warming and moderate N/P eutrophication, and/or 3) have low associated denitrification. Turf algae and coral rubble exhibited ∼100-fold higher N2 fixation rates compared to hard corals. Contrastingly, denitrification rates were as low as those associated with hard corals. Therefore, coral reefs in the process of shifting towards algae dominance may get caught in a positive feedback loop where dead coral (coral rubble) is rapidly overgrown by algae which in return naturally provide the reef with bioavailable N. This may facilitate higher growth rates of reef algae.
Collectively, the results described in this thesis suggest that the interplay of N2 fixation and denitrification associated with coral holobionts may indeed aid in coral functioning by maintaining healthy populations of Symbiodiniaceae. Increased activity of diazotrophs induced by thermal stress, both associated with the coral holobiont and other dwelling organisms, as well as eutrophication of N may ultimately shift the coral holobionts’ internal N:P ratios towards P limitation as denitrifiers may be unable to alleviate excess N. Thus, future management efforts should focus strongly on the local prevention of N eutrophication and the mitigation of global warming
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