The effects of Symbiodinium (Pyrrhophyta) identity on growth, survivorship, and thermal tolerance of newly settled coral recruits

Nutrient dynamics in coral symbiosis depend on both the relative and absolute abundance of Symbiodiniaceae species

BACKGROUND

Symbionts provide a variety of reproductive, nutritional, and defensive resources to their hosts, but those resources can vary depending on symbiont community composition. As genetic techniques open our eyes to the breadth of symbiont diversity within myriad microbiomes, symbiosis research has begun to consider what ecological mechanisms affect the identity and relative abundance of symbiont species and how this community structure impacts resource exchange among partners. Here, we manipulated the in hospite density and relative ratio of two species of coral endosymbionts (Symbiodinium microadriaticum and Breviolum minutum) and used stable isotope enrichment to trace nutrient exchange with the host, Briareum asbestinum.

RESULTS

The patterns of uptake and translocation of carbon and nitrogen varied with both density and ratio of symbionts. Once a density threshold was reached, carbon acquisition decreased with increasing proportions of S. microadriaticum. In hosts dominated by B. minutum, nitrogen uptake was density independent and intermediate. Conversely, for those corals dominated by S. microadriaticum, nitrogen uptake decreased as densities increased, and as a result, these hosts had the overall highest (at low density) and lowest (at high density) nitrogen enrichment.

CONCLUSIONS

Our findings show that the uptake and sharing of nutrients was strongly dependent on both the density of symbionts within the host, as well as which symbiont species was dominant. Together, these complex interactive effects suggest that host regulation and the repression of in hospite symbiont competition can ultimately lead to a more productive mutualism. Video Abstract.

Community composition of coral-associated Symbiodiniaceae differs across fine-scale environmental gradients in Kāne'ohe Bay

The survival of most reef-building corals is dependent upon a symbiosis between the coral and the community of Symbiodiniaceae. Montipora capitata, one of the main reef-building coral species in Hawai'i, is known to host a diversity of symbionts, but it remains unclear how they change spatially and whether environmental factors drive those changes. Here, we surveyed the Symbiodiniaceae community in 600 M. capitata colonies from 30 sites across Kāne'ohe Bay and tested for host specificity and environmental gradients driving spatial patterns of algal symbiont distribution. We found that the Symbiodiniaceae community differed markedly across sites, with M. capitata in the most open-ocean (northern) site hosting few or none of the genus Durusdinium, whereas individuals at other sites had a mix of Durusdinium and Cladocopium. Our study shows that the algal symbiont community composition responds to fine-scale differences in environmental gradients; depth and temperature variability were the most significant predictor of Symbiodiniaceae community, although environmental factors measured in the study explained only about 20% of observed variation. Identifying and mapping Symbiodiniaceae community distribution at multiple scales is an important step in advancing our understanding of algal symbiont diversity, distribution and evolution and the potential responses of corals to future environmental change.

Genetic structure of dinoflagellate symbionts in coral recruits differs from that of parental or local adults

The symbiotic relationship between dinoflagellate algae in the family Symbiodiniaceae and scleractinian corals forms the base of the tropical reef ecosystem. In scleractinian corals, recruits acquire symbionts either "vertically" from the maternal colony or initially lack symbionts and acquire them "horizontally" from the environment. Regardless of the mode of acquisition, coral species and individual colonies harbor only a subset of the highly diverse complex of species/taxa within the Symbiodiniaceae. This suggests a genetic basis for specificity, but local environmental conditions and/or symbiont availability may also play a role in determining which symbionts within the Symbiodiniaceae are initially taken up by the host. To address the relative importance of genetic and environmental drivers of symbiont uptake/establishment, we examined the acquisition of these dinoflagellate symbionts in one to three-month-old recruits of Orbicella faveolata to compare symbiont types present in recruits to those of parental populations versus co-occurring adults in their destination reef. Variation in chloroplast 23S ribosomal DNA and in three polymorphic microsatellite loci was examined. We found that, in general, symbiont communities within adult colonies differed between reefs, suggesting that endemism is common among symbiont populations of O. faveolata on a local scale. Among recruits, initial symbiont acquisition was selective. O. faveolata recruits only acquired a subset of locally available symbionts, and these generally did not reflect symbiont populations in adults at either the parental or the outplant reef. Instead, symbiont communities within new recruits at a given outplant site and region tended to be similar to each other, regardless of parental source population. These results suggest temporal variation in the local symbiont source pool, although other possible drivers behind the distinct difference between symbionts within O. faveolata adults and new generations of recruits may include different ontogenetic requirements and/or reduced host selectivity in early ontogeny.

Fish predation on corals promotes the dispersal of coral symbionts

BACKGROUND

The microbiomes of foundation (habitat-forming) species such as corals and sponges underpin the biodiversity, productivity, and stability of ecosystems. Consumers shape communities of foundation species through trophic interactions, but the role of consumers in dispersing the microbiomes of such species is rarely examined. For example, stony corals rely on a nutritional symbiosis with single-celled endosymbiotic dinoflagellates (family Symbiodiniaceae) to construct reefs. Most corals acquire Symbiodiniaceae from the environment, but the processes that make Symbiodiniaceae available for uptake are not resolved. Here, we provide the first comprehensive, reef-scale demonstration that predation by diverse coral-eating (corallivorous) fish species promotes the dispersal of Symbiodiniaceae, based on symbiont cell densities and community compositions from the feces of four obligate corallivores, three facultative corallivores, two grazer/detritivores as well as samples of reef sediment and water.

RESULTS

Obligate corallivore feces are environmental hotspots of Symbiodiniaceae cells: live symbiont cell concentrations in such feces are 5-7 orders of magnitude higher than sediment and water environmental reservoirs. Symbiodiniaceae community compositions in the feces of obligate corallivores are similar to those in two locally abundant coral genera (Pocillopora and Porites), but differ from Symbiodiniaceae communities in the feces of facultative corallivores and grazer/detritivores as well as sediment and water. Combining our data on live Symbiodiniaceae cell densities in feces with in situ observations of fish, we estimate that some obligate corallivorous fish species release over 100 million Symbiodiniaceae cells per 100 m² of reef per day. Released corallivore feces came in direct contact with coral colonies in the fore reef zone following 91% of observed egestion events, providing a potential mechanism for the transfer of live Symbiodiniaceae cells among coral colonies.

CONCLUSIONS

Taken together, our findings show that fish predation on corals may support the maintenance of coral cover on reefs in an unexpected way: through the dispersal of beneficial coral symbionts in corallivore feces. Few studies examine the processes that make symbionts available to foundation species, or how environmental reservoirs of such symbionts are replenished. This work sets the stage for parallel studies of consumer-mediated microbiome dispersal and assembly in other sessile, habitat-forming species.

Unfamiliar partnerships limit cnidarian holobiont acclimation to warming

Enhancing the resilience of corals to rising temperatures is now a matter of urgency, leading to growing efforts to explore the use of heat tolerant symbiont species to improve their thermal resilience. The notion that adaptive traits can be retained by transferring the symbionts alone, however, challenges the holobiont concept, a fundamental paradigm in coral research. Holobiont traits are products of a specific community (holobiont) and all its co-evolutionary and local adaptations, which might limit the retention or transference of holobiont traits by exchanging only one partner. Here we evaluate how interchanging partners affect the short- and long-term performance of holobionts under heat stress using clonal lineages of the cnidarian model system Aiptasia (host and Symbiodiniaceae strains) originating from distinct thermal environments. Our results show that holobionts from more thermally variable environments have higher plasticity to heat stress, but this resilience could not be transferred to other host genotypes through the exchange of symbionts. Importantly, our findings highlight the role of the host in determining holobiont productivity in response to thermal stress and indicate that local adaptations of holobionts will likely limit the efficacy of interchanging unfamiliar compartments to enhance thermal tolerance.

Assessing the role of historical temperature regime and algal symbionts on the heat tolerance of coral juveniles

The rate of coral reef degradation from climate change is accelerating and, as a consequence, a number of interventions to increase coral resilience and accelerate recovery are under consideration. Acropora spathulata coral colonies that survived mass bleaching in 2016 and 2017 were sourced from a bleaching-impacted and warmer northern reef on the Great Barrier Reef (GBR). These individuals were reproductively crossed with colonies collected from a recently bleached but historically cooler central GBR reef to produce pure and crossbred offspring groups (warm-warm, warm-cool and cool-warm). We tested whether corals from the warmer reef produced more thermally tolerant hybrid and purebred offspring compared with crosses produced with colonies sourced from the cooler reef and whether different symbiont taxa affect heat tolerance. Juveniles were infected with Symbiodinium tridacnidorum, Cladocopium goreaui and Durusdinium trenchii and survival, bleaching and growth were assessed at 27.5°C and 31°C. The contribution of host genetic background and symbiont identity varied across fitness traits. Offspring with either both or one parent from the northern population exhibited a 13- to 26-fold increase in survival odds relative to all other treatments where survival probability was significantly influenced by familial cross identity at 31°C but not 27.5°C (Kaplan-Meier P=0.001 versus 0.2). If in symbiosis with D. trenchii, a warm sire and cool dam provided the best odds of juvenile survival. Bleaching was predominantly driven by Symbiodiniaceae treatment, where juveniles hosting D. trenchii bleached significantly less than the other treatments at 31°C. The greatest overall fold-benefits in growth and survival at 31°C occurred in having at least one warm dam and in symbiosis with D. trenchii Juveniles associated with D. trenchii grew the most at 31°C, but at 27.5°C, growth was fastest in juveniles associated with C. goreaui In conclusion, selective breeding with warmer GBR corals in combination with algal symbiont manipulation can assist in increasing thermal tolerance on cooler but warming reefs. Such interventions have the potential to improve coral fitness in warming oceans.This article has an associated First Person interview with the first author of the paper.

Acquisition of obligate mutualist symbionts during the larval stage is not beneficial for a coral host

Theory suggests that the direct transmission of beneficial endosymbionts (mutualists) from parents to offspring (vertical transmission) in animal hosts is advantageous and evolutionarily stable, yet many host species instead acquire their symbionts from the environment (horizontal acquisition). An outstanding question in marine biology is why some scleractinian corals do not provision their eggs and larvae with the endosymbiotic dinoflagellates that are necessary for a juvenile's ultimate survival. We tested whether the acquisition of photosynthetic endosymbionts (family Symbiodiniaceae) during the planktonic larval stage was advantageous, as is widely assumed, in the ecologically important and threatened Caribbean reef-building coral Orbicella faveolata. Following larval acquisition, similar changes occurred in host energetic lipid use and gene expression regardless of whether their symbionts were photosynthesizing, suggesting the symbionts did not provide the energetic benefit characteristic of the mutualism in adults. Larvae that acquired photosymbionts isolated from conspecific adults on their natal reef exhibited a reduction in swimming, which may interfere with their ability to find suitable settlement substrate, and also a decrease in survival. Larvae exposed to two cultured algal species did not exhibit differences in survival, but decreased their swimming activity in response to one species. We conclude that acquiring photosymbionts during the larval stage confers no advantages and can in fact be disadvantageous to this coral host. The timing of symbiont acquisition appears to be a critical component of a host's life history strategy and overall reproductive fitness, and this timing itself appears to be under selective pressure.

Intraspecific variation in growth rate is a poor predictor of fitness for reef corals

Genetic variation underlying differences in organism performance is subject to natural selection, and organisms with high values of genetically determined phenotypic measures of fitness should perform better than those that do not. Using small scleractinian corals (i.e., ≤40-mm diameter), this principle was tested with 20 yr of census data from St. John, US Virgin Islands. Using growth rate (change in diameter) as a measure of fitness, growth in one year was tested for association with growth and survivorship in the following two years, and this process was repeated over 20 yr using a 3-yr sliding window. Virtually all variation in growth was independent of colony size, and growth among pairs of years was highly variable, with corals that grew fast in one year rarely growing fast in the next 2 yr. While growth in some pairs of years was positively correlated, ≤4% of the growth variance was explained by growth in the preceding 2 yr. Survivorship was related positively to growth in the preceding year, but the association was weak, it did not extend over 3 yr, and was inconsistent over the study. These results demonstrate the importance of the environment in translating phenotypic measures of fitness into future performance, and for small Caribbean corals, they suggest that environmental conditions may preempt genotype in determining short-term success.