Photosynthetic impact of hypoxia on in hospite zooxanthellae in the scleractinian coral Pocillopora damicornis

Shallow water coral reefs may experience hypoxia under conditions of calm weather doldrums. Anaerobic responses of endosymbionts (i.e. zooxanthellae) within Pocillopora damicornis coral colonies were tested using both slow and fast chlorophyll a fluorescence induction kinetics. Zooxanthellae were examined in hospite when exposed to control conditions (26°C, 200 μmol photons m-2 s-1, 100% air-saturation, 4 cm s-1 flow) and to 2 treatments of reduced air content (40 and 0%), achieved by controlling the N2:O2 ratio in water circulating at 2 cm s -1. Furthermore, the impact of water flow on photosynthesis was examined at 0% air saturation by turning off the flow entirely (0 cm s -1), thereby mimicking the environmental conditions of calm weather doldrums. Corals exposed to depleted air content (0 % with and without flow) showed a significant decrease (p < 0.001) in effective quantum yield (φPSII) in comparison with controls. Maximum quantum yield was significantly reduced when gas exchange was inhibited (0% without flow), whereas non-photochemical quenching (NPQ) was not affected. Fast polyphasic fluorescence transients of chlorophyll a fluorescence showed a significant increase in minimum dark-adapted fluorescence, F0, when corals were exposed to anaerobic conditions. Furthermore, an increase in the J peak (2 ms) corresponding to the reduction of the primary electron acceptor, QA, was observed in 0% air-saturation with flow. We found that the most sensitive parameters for detecting physiological change associated with hypoxia were φPSII using slow (pulse-amplitude modulation) fluorescence kinetics, as well as an increase in the O peak, φPo(electron transport efficiency before QA), and an elevation of the J peak on a double-normalised transient using fast (Plant Efficiency Analyser) induction kinetics. © Inter-Research 2005

Variation in bleaching sensitivity of two coral species across a latitudinal gradient on the Great Barrier Reef: The role of zooxanthellae

The ability of corals to cope with environmental change, such as increased temperature, relies on the physiological mechanisms of acclimatisation and long-term genetic adaptation. We experimentally examined the bleaching sensitivity exhibited by 2 species of coral, Pocillopora damicornis and Turbinaria reniformis, at 3 locations across a latitudinal gradient of almost 6 degrees on the Great Barrier Reef (GBR). Target bleaching temperature was reached by using a ramping rate of 0.2°C/h. We found that the bleaching sensitivity and recovery of both species differed between corals with clade D symbionts and those with clade C. However, in P. damicornis bleaching susceptibility corresponded more strongly with latitude than with zooxanthella type and hence, temperature history, suggesting that local adaptation has occurred. The observed bleaching sensitivity was shown by a decrease in photochemical efficiency (Fv/Fm) in both species of coral. The rate of recovery in T. reniformis was highest in explants containing clade D symbionts. The occurrence of clade D in the northern section of the GBR may reflect a long-term response to high sea water temperatures, while the presence of clade D in low abundance in T. reniformis at Heralds Prong Reef and Percy Island may be a result of recent bleaching events. © Inter-Research 2006

Seasonal variation in the photo-physiology of homogeneous and heterogeneous Symbiodinium consortia in two scleractinian corals

Seasonal variation in the composition of the algal endosymbiont community and photophysiology was determined in the corals Pocillopora damicornis, which show high local fidelity to one symbiont type (Symbiodinium C1), and Acropora valida, with a mixed Symbiodinium symbiont community, comprising members of both clades A and C. The relative abundances of Symbiodinium types varied over time. A significant decline in symbiont densities in both coral species during the summer of 2005 coincided with a NOAA 'hotspot' warning for Heron Island. This also coincided with a relative increase in the presence and dominance of clade A in A. valida, particularly in sun-adapted surfaces. The effective quantum yield of Photosystem II (ΦPSII) suggested that sun-adapted surfaces of P. damicornis are more sensitive than shade-adapted surfaces to combined effects of higher temperature and irradiance in summer. Xanthophyll cycling was greater in P. damicornis than A. valida, irrespective of branch position and sampling time; this may be a mechanism by which P. damicornis compensates for its fidelity to Symbiodinium C1. Furthermore, xanthophyll de-epoxidation in P. damicornis symbionts was greater in sun-adapted than shade-adapted surfaces, correlating with non-photochemical quenching (NPQRLC). No variation was found in A. valida, indicating that resident symbiont communities may not have been physiologically compromised, perhaps as a result of changes in the composition of the Symbiodinium community consortia. © Inter-Research 2008

Coral community response to bleaching on a highly disturbed reef

While many studies of coral bleaching report on broad, regional scale responses, fewer examine variation in susceptibility among coral taxa and changes in community structure, before, during and after bleaching on individual reefs. Here we report in detail on the response to bleaching by a coral community on a highly disturbed reef site south of mainland Singapore before, during and after a major thermal anomaly in 2010. To estimate the capacity for resistance to thermal stress, we report on: a) overall bleaching severity during and after the event, b) differences in bleaching susceptibility among taxa during the event, and c) changes in coral community structure one year before and after bleaching. Approximately two thirds of colonies bleached, however, post-bleaching recovery was quite rapid and, importantly, coral taxa that are usually highly susceptible were relatively unaffected. Although total coral cover declined, there was no significant change in coral taxonomic community structure before and after bleaching. Several factors may have contributed to the overall high resistance of corals at this site including Symbiodinium affiliation, turbidity and heterotrophy. Our results suggest that, despite experiencing chronic anthropogenic disturbances, turbid shallow reef communities may be remarkably resilient to acute thermal stress

Environmental Factors Controlling the Distribution of Symbiodinium Harboured by the Coral Acropora millepora on the Great Barrier Reef

Background: The Symbiodinium community associated with scleractinian corals is widely considered to be shaped by seawater temperature, as the coral's upper temperature tolerance is largely contingent on the Symbiodinium types harboured. Few studies have challenged this paradigm as knowledge of other environmental drivers on the distribution of Symbiodinium is limited. Here, we examine the influence of a range of environmental variables on the distribution of Symbiodinium associated with Acropora millepora collected from 47 coral reefs spanning 1,400 km on the Great Barrier Reef (GBR), Australia

Symbiodinium Genotypic and Environmental Controls on Lipids in Reef Building Corals

BACKGROUND: Lipids in reef building corals can be divided into two classes; non-polar storage lipids, e.g. wax esters and triglycerides, and polar structural lipids, e.g. phospholipids and cholesterol. Differences among algal endosymbiont types are known to have important influences on processes including growth and the photobiology of scleractinian corals yet very little is known about the role of symbiont types on lipid energy reserves. METHODOLOGY/PRINCIPAL FINDINGS: The ratio of storage lipid and structural lipid fractions of Scott Reef corals were determined by thin layer chromatography. The lipid fraction ratio varied with depth and depended on symbiont type harboured by two corals (Seriatopora hystrix and Pachyseris speciosa). S. hystrix colonies associated with Symbiodinium C1 or C1/C# at deep depths (>23 m) had lower lipid fraction ratios (i.e. approximately equal parts of storage and structural lipids) than those with Symbiodinium D1 in shallow depths (<23 m), which had higher lipid fraction ratios (i.e. approximately double amounts of storage relative to structural lipid). Further, there was a non-linear relationship between the lipid fraction ratio and depth for S. hystrix with a modal peak at ∼23 m coinciding with the same depth as the shift from clade D to C types. In contrast, the proportional relationship between the lipid fraction ratio and depth for P. speciosa, which exhibited high specificity for Symbiodinium C3 like across the depth gradient, was indicative of greater amounts of storage lipids contained in the deep colonies. CONCLUSIONS/SIGNIFICANCE: This study has demonstrated that Symbiodinium exert significant controls over the quality of coral energy reserves over a large-scale depth gradient. We conclude that the competitive advantages and metabolic costs that arise from flexible associations with divergent symbiont types are offset by energetic trade-offs for the coral host

Impacts of Sediments on Coral Energetics: Partitioning the Effects of Turbidity and Settling Particles

Sediment loads have long been known to be deleterious to corals, but the effects of turbidity and settling particles have not previously been partitioned. This study provides a novel approach using inert silicon carbide powder to partition and quantify the mechanical effects of sediment settling versus reduced light under a chronically high sedimentary regime on two turbid water corals commonly found in Singapore (Galaxea fascicularis and Goniopora somaliensis). Coral fragmentswere evenly distributed among three treatments: an open control (30% ambient PAR), a shaded control (15% ambient PAR) and sediment treatment (15% ambient PAR; 26.4 mg cm22 day21). The rate of photosynthesis and respiration, and the dark-adapted quantum yield were measured once a week for four weeks. By week four, the photosynthesis to respiration ratio (P/R ratio) and the photosynthetic yield (Fv/Fm) had fallen by 14% and 3–17% respectively in the shaded control,contrasting with corals exposed to sediments whose P/R ratio and yield had declined by 21% and 18–34% respectively. The differences in rates between the shaded control and the sediment treatment were attributed to the mechanical effects of sediment deposition. The physiological response to sediment stress differed between species with G. fascicularis experiencing a greater decline in the net photosynthetic yield (13%) than G. somaliensis (9.5%), but a smaller increase in the respiration rates (G. fascicularis = 9.9%, G. somaliensis = 14.2%). These different physiological responses were attributed, in part, to coral morphology and highlighted key physiological processes that drive species distribution along high to low turbidity and depositional gradients

Microbial contributions to the persistence of coral reefs

On contemplating the adaptive capacity of reef organisms to a rapidly changing environment, the microbiome offers significant and greatly unrecognised potential. Microbial symbionts contribute to the physiology, development, immunity and behaviour of their hosts, and can respond very rapidly to changing environmental conditions, providing a powerful mechanism for acclimatisation and also possibly rapid evolution of coral reef holobionts. Environmentally acquired fluctuations in the microbiome can have significant functional consequences for the holobiont phenotype upon which selection can act. Environmentally induced changes in microbial abundance may be analogous to host gene duplication, symbiont switching / shuffling as a result of environmental change can either remove or introduce raw genetic material into the holobiont; and horizontal gene transfer can facilitate rapid evolution within microbial strains. Vertical transmission of symbionts is a key feature of many reef holobionts and this would enable environmentally acquired microbial traits to be faithfully passed to future generations, ultimately facilitating microbiome-mediated transgenerational acclimatisation (MMTA) and potentially even adaptation of reef species in a rapidly changing climate. In this commentary, we highlight the capacity and mechanisms for MMTA in reef species, propose a modified Price equation as a framework for assessing MMTA and recommend future areas of research to better understand how microorganisms contribute to the transgenerational acclimatisation of reef organisms, which is essential if we are to reliably predict the consequences of global change for reef ecosystems

The Roles and Interactions of Symbiont, Host and Environment in Defining Coral Fitness

Background: Reef-building corals live in symbiosis with a diverse range of dinoflagellate algae (genus Symbiodinium) that differentially influence the fitness of the coral holobiont. The comparative role of symbiont type in holobiont fitness in relation to host genotype or the environment, however, is largely unknown. We addressed this knowledge gap by manipulating host-symbiont combinations and comparing growth, survival and thermal tolerance among the resultant holobionts in different environments.\ud Methodology/Principal Findings: Offspring of the coral, Acropora millepora, from two thermally contrasting locations, were experimentally infected with one of six Symbiodinium types, which spanned three phylogenetic clades (A, C and D), and then outplanted to the two parental field locations (central and southern inshore Great Barrier Reef, Australia). Growth and survival of juvenile corals were monitored for 31–35 weeks, after which their thermo-tolerance was experimentally assessed. Our results showed that: (1) Symbiodinium type was the most important predictor of holobiont fitness, as measured by growth, survival, and thermo-tolerance; (2) growth and survival, but not heat-tolerance, were also affected by local environmental conditions; and (3) host population had little to no effect on holobiont fitness. Furthermore, coral-algal associations were established with symbiont types belonging to clades A, C and D, but three out of four symbiont types belonging to clade C failed to establish a symbiosis. Associations with clade A had the lowest fitness and were unstable in the field. Lastly, Symbiodinium types C1 and D were found to be relatively thermo-tolerant, with type D conferring the highest tolerance in A. millepora.\ud Conclusions/Significance: These results highlight the complex interactions that occur between the coral host, the algal symbiont, and the environment to shape the fitness of the coral holobiont. An improved understanding of the factors affecting coral holobiont fitness will assist in predicting the responses of corals to global climate change