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The ecological and evolutionary consequences of partner fidelity and flexibility among coral–dinoflagellate mutualisms are widely debated. Resident symbionts can modulate the resilience of their hosts to environmental stressors, which explains, in part, why host–symbiont combinations differ over broad geographic ranges and across physical–environmental gradients in light and temperature. Therefore, flexibility in these mutualisms may influence the longevity of coral populations and communities subjected to ocean warming. However, despite decades of research, basic knowledge about these mutualisms remains incomplete, hindering the development of predictive ecological theory. In particular, few studies have investigated the long-term composition of symbiont populations within individual colonies. To further examine the extent to which coral colonies have stable relationships with specific symbionts over multiple years, diverse coral taxa (Scleractinia) from a West Indo-Pacific fore reef (Palau) were tagged and sampled at various intervals—ranging from six months to several years—over nine years' time. Symbiont identity was examined using multiple genetic markers that resolved symbiont diversity to species and individual genotypes (i.e., clonal strains). Members of the genus Cladocopium (formerly Symbiodinium Clade C) were prevalent across the host community. Generally, corals with open modes of symbiont acquisition harbored a host–generalist symbiont, while corals with vertical symbiont transmission were associated with co-evolved host-specific symbionts. Consistent with previous colony monitoring studies, symbiont populations in a majority of colonies were dominated by one species and one strain (based on multilocus genotyping) over multiple years. Thus, the distribution of symbiont diversity at the genus, species and clone level, comprising specific and stable partner combinations, scale predictably to reef habitat, host taxon, and individual colony. Recognizing these fundamental ecological patterns establishes a more comprehensive understanding of the population and community structure of these mutualisms. [ABSTRACT FROM AUTHOR]

Coral reefs are declining worldwide, yet some coral populations are better adapted to withstand reductions in pH and the rising frequency of marine heatwaves. The nearshore reef habitats of Palau, Micronesia are a proxy for a future of warmer, more acidic oceans. Coral populations in these habitats can resist, and recover from, episodes of thermal stress better than offshore conspecifics. To explore the physiological basis of this tolerance, we compared tissue biomass (ash-free dry weight cm−2), energy reserves (i.e., protein, total lipid, carbohydrate content), and several important lipid classes in six coral species living in both offshore and nearshore environments. In contrast to expectations, a trend emerged of many nearshore colonies exhibiting lower biomass and energy reserves than colonies from offshore sites, which may be explained by the increased metabolic demand of living in a warmer, acidic, environment. Despite hosting different dinoflagellate symbiont species and having access to contrasting prey abundances, total lipid and lipid class compositions were similar in colonies from each habitat. Ultimately, while the regulation of colony biomass and energy reserves may be influenced by factors, including the identity of the resident symbiont, kind of food consumed, and host genetic attributes, these independent processes converged to a similar homeostatic set point under different environmental conditions. [ABSTRACT FROM AUTHOR]

Under predicted future ocean conditions, reefs exposed to elevated nutrients will simultaneously experience ocean acidification and elevated temperature. We evaluated if moderate nutrients mitigate, minimize, or exacerbate negative effects of predicted future ocean conditions on coral physiology. For 30 days, Acropora millepora and Turbinaria reniformis were exposed to a fully factorial experiment of eight treatments including two seawater temperatures (26.4 °C and 29.8 °C), pCO2 levels (401 μatm pCO2 and 760 μatm pCO2), and nutrient concentrations (ambient: 0.40 μmol L−1 NO3− and 0.22 μmol L−1 PO43−, and moderate: 3.56 μmol L−1 NO3− and 0.31 μmol L−1 PO43−). Added nitrate was taken up by the algal endosymbionts and transferred to the coral hosts in both species, though to a much higher degree in A. millepora. When exposed to elevated temperature, elevated pCO2, or both, effects observed for chlorophyll a, calcification, biomass, and energy reserves were not compounded by the moderate nutrient concentrations in either species. Moderate nutrients enabled A. millepora to continue to meet daily metabolic demand via photosynthesis under predicted future ocean conditions and T. reniformis to greatly exceed daily metabolic demand via photosynthesis and heterotrophy. Our results suggest that balanced moderate nutrients are not detrimental to corals under predicted future ocean conditions and may even provide some benefits. [ABSTRACT FROM AUTHOR]

Harmful algal blooms (HABs) affect both freshwater and marine systems. Laboratory experiments suggest an exudate produced by the bacterium Shewanella sp. IRI-160 could be used to prevent or mitigate dinoflagellate blooms; however, effects on non-target organisms are unknown. The algicide (IRI-160AA) was tested on various ontogenetic stages of the copepod Acartia tonsa (nauplii and adult copepodites), the blue crab Callinectes sapidus (zoea larvae and megalopa postlarvae), and the eastern oyster Crassostrea virginica (pediveliger larvae and adults). Mortality experiments with A. tonsa revealed that the 24-h LC50 was 13.4% v/v algicide for adult females and 5.96% for early-stage nauplii. For C. sapidus, the 24-h LC50 for first-stage zoeae was 16.8%; results were not significant for megalopae or oysters. Respiration rates for copepod nauplii increased in the 11% concentration, and in the 11% and 17% concentrations for crab zoeae; rates of later stages and oysters were unaffected. Activity level was affected for crab zoeae in the 1%, 11%, and 17% treatments, and for oyster pediveliger larvae at the 17% level. Activity of later stages and of adult copepods was unaffected. Smaller, non-target biota with higher surface to volume could be negatively impacted from IRI-160AA dosing, but overall the taxa and stages assayed were tolerant to the algicide at concentrations required for dinoflagellate mortality (EC50 = ~ 1%). [ABSTRACT FROM AUTHOR]

Corals with high levels of total lipids are known to have increased resilience potential to bleaching, and lipid class management may shed further light on why some species are more resilient to, or are able to acclimatize to, annual bleaching stress. Here, we measured the lipid class composition of three species of Caribbean corals (Porites astreoides, Porites divaricata, and Orbicella faveolata) collected in July 2009 near Puerto Morelos, Mexico (20° 50′ N, 86° 52′ W) that were experimentally bleached 2 years in a row. Our results show that single bleaching can significantly alter lipid class composition in all species, while repeated bleaching can result in stable (i.e., acclimatized) or even more altered (i.e., not acclimatized) lipid class composition depending on the species. Specifically, P. divaricata and O. faveolata both had altered lipid class composition with losses in storage lipids following single bleaching, but maintained lipid class composition following repeated bleaching stress. However, both single and repeated bleaching altered the lipid class composition in P. astreoides, with changes persisting for the 6 weeks after repeated bleaching stress. This study provides evidence that lipid class management is part of the suite of variables associated with coral resilience, that P. divaricata and O. faveolata acclimatize their lipid class management in response to repeated bleaching stress, but that P. astreoides does not. Corals like P. divaricata and O. faveolata may, therefore, be more suitable for coral restoration efforts since they are more likely to persist under chronic repeat bleaching scenarios predicted for later this century. [ABSTRACT FROM AUTHOR]

Dinoflagellates in the genus Symbiodinium are among the most abundant and important group of eukaryotic microbes found in coral reef ecosystems. Recent analyses conducted on various host cnidarians indicated that Symbiodinium assemblages in the Caribbean Sea are genetically and ecologically diverse. In order to further characterize this diversity and identify processes important to its origins, samples from six orders of Cnidaria comprising 45 genera were collected from reef habitats around Barbados (eastern Caribbean) and from the Mesoamerican barrier reef off the coast of Belize (western Caribbean). Fingerprinting of the ribosomal internal transcribed spacer 2 identified 62 genetically different Symbiodinium. Additional analyses of clade B Symbiodinium using microsatellite flanker sequences unequivocally characterized divergent lineages, or “species,” within what was previously thought to be a single entity ( B1 or B184). In contrast to the Indo-Pacific where host-generalist symbionts dominate many coral communities, partner specificity in the Caribbean is relatively high and is influenced little by the host’s apparent mode of symbiont acquisition. Habitat depth (ambient light) and geographic isolation appeared to influence the bathymetric zonation and regional distribution for most of the Symbiodinium spp. characterized. Approximately 80% of Symbiodinium types were endemic to either the eastern or western Caribbean and 40–50% were distributed to compatible hosts living in shallow, high-irradiance, or deep, low-irradiance environments. These ecologic, geographic, and phylogenetic patterns indicate that most of the present Symbiodinium diversity probably originated from adaptive radiations driven by ecological specialization in separate Caribbean regions during the Pliocene and Pleistocene periods. [ABSTRACT FROM AUTHOR]

In the original article, the light cycle was indicated as 12:12 h light:dark. [ABSTRACT FROM AUTHOR]

The author would like to correct the error in the Funding Section of the original publication of the article. The corrected Funding section is given below: [ABSTRACT FROM AUTHOR]

High sea surface temperatures often lead to coral bleaching wherein reef-building corals lose significant numbers of their endosymbiotic dinoflagellates (Symbiodiniaceae). These increasingly frequent bleaching events often result in large scale coral mortality, thereby devasting reef systems throughout the world. The reef habitats surrounding Palau are ideal for investigating coral responses to climate perturbation, where many inshore bays are subject to higher water temperature as compared with offshore barrier reefs. We examined fourteen physiological traits in response to high temperature across various symbiotic dinoflagellates in four common Pacific coral species, Acropora muricata, Coelastrea aspera, Cyphastrea chalcidicum and Pachyseris rugosa found in both offshore and inshore habitats. Inshore corals were dominated by a single homogenous population of the stress tolerant symbiont Durusdinium trenchii, yet symbiont thermal response and physiology differed significantly across coral species. In contrast, offshore corals harbored specific species of Cladocopium spp. (ITS2 rDNA type-C) yet all experienced similar patterns of photoinactivation and symbiont loss when heated. Additionally, cell volume and light absorption properties increased in heated Cladocopium spp., leading to a greater loss in photo-regulation. While inshore coral temperature response was consistently muted relative to their offshore counterparts, high physiological variability in D. trenchii across inshore corals suggests that bleaching resilience among even the most stress tolerant symbionts is still heavily influenced by their host environment. [ABSTRACT FROM AUTHOR]

Bacteria may play a role in regulating harmful algal blooms, but little is known about the biochemical and physiological changes associated with cell death induced by algicidal bacteria. Previous work characterized an algicidal exudate (IRI-160AA) produced by Shewanella sp. IRI-160 that is effective against dinoflagellates, while having little to no effect on other phytoplankton species in laboratory culture experiments. The objective of this study was to evaluate biochemical changes associated with cell death and impacts on the cell cycle in three dinoflagellate species (Prorocentrum minimum, Karlodinium veneficum and Gyrodinium instriatum) after exposure to IRI-160AA. In this study, IRI-160AA induced cell cycle arrest in all dinoflagellates examined. Several indicators for programmed cell death (PCD) that are often observed in phytoplankton in response to a variety of stressors were also evaluated. Cell death was accompanied by significant increases in DNA degradation, intra- and extracellular ROS concentrations and DEVDase (caspase-3 like) protease activity, which have been associated with PCD in other phytoplankton species. Overall, results of this investigation provide strong evidence that treatment with the bacterial algicide, IRI-160AA results in cell cycle arrest and induces biochemical changes consistent with stress-related cell death responses observed in other phytoplankton. [ABSTRACT FROM AUTHOR]