Environmental stressors such as increased sea surface temperatures are well-known for contributing to coral bleaching; however, the effect of increased temperatures and subsequent bleaching on coral-associated microbial communities is poorly understood. Colonies of the hard coral Acropora millepora were tagged on a reef flat off Magnetic Island (Great Barrier Reef) and surveyed over 2.5 years, which included a severe bleaching event in January/February 2002. Daily average water temperatures exceeded the previous 10-year average by more than 1 °C for extended periods with field-based visual surveys recording all tagged colonies displaying signs of bleaching. During the bleaching period, direct counts of coral zooxanthellae densities decreased by ~64%, before recovery to pre-bleaching levels after the thermal stress event. A subset of three tagged coral colonies were sampled through the bleaching event and changes in the microbial community elucidated. Denaturing gradient gel electrophoresis (DGGE) analysis demonstrated conserved bacterial banding profiles between the three coral colonies, confirming previous studies highlighting specific microbial associations. As coral colonies bleached, the microbial community shifted and redundancy analysis (RDA) of DGGE banding patterns revealed a correlation of increasing temperature with the appearance of Vibrio-affiliated sequences. Interestingly, this shift to a Vibrio-dominated community commenced prior to visual signs of bleaching. Clone libraries hybridized with Vibrio-specific oligonucleotide probes confirmed an increase in the fraction of Vibrio-affiliated clones during the bleaching period. Post bleaching, the coral microbial associations again shifted, returning to a profile similar to the fingerprints prior to bleaching. This provided further evidence for corals selecting and shaping their microbial partners. For non-bleached samples, a close association with Spongiobacter-related sequences were revealed by both clone libraries and DGGE profiling. Despite Vibrio species being previously implicated in bleaching of specific coral species, it is unsure if the relative increase in retrieved Vibrio sequences is due to bacterial infection or an opportunistic response to compromised health and changing environmental parameters of the coral host. This study provides the first molecular-based study demonstrating changes in coral-associated bacterial assemblages during a bleaching event on a natural reef system.

Echinoderms play a key role in structuring many marine ecosystems and are notorious for large population density variations in so‐called “outbreak” or “dieoff” events. In a review of this phenomenon, we assess the causal factors and ecological and evolutionary consequences. We identified 28 species (6 Asteroidea, 8 Echinoidea, 10 Holothuroidea, 4 Ophiuroidea) that exhibit large (more than two population doublings or halvings) population density changes. Three generalized patterns were identified and named for exemplary species: (1) rapid decreases followed by no or slow recovery (Diadema–Paracentrotus Model), (2), rapid increase and apparent stability at a new population density (Amperima–Amphiura Model), and (3) population density fluctuations (Acanthaster–Asterias Model). Echinoderms identified were distributed from the shallow intertidal to the deep sea, and from tropical to temperate regions. In most cases, significant impacts on the respective ecosystems were observed. The most striking similarity among all species identified was possession of the ancestral‐type planktotrophic larva. This larval type was significantly overrepresented in species identified within the Asteroidea, Echinoidea, Holothuroidea, and for the combined data set. We suggest three main factors that render a life history with planktotrophic larvae a high‐risk–high‐gain strategy: (1) a strong nonlinear dependency of larval production on adult densities (Allee effects), (2) a low potential for compensatory feedback mechanisms, and (3) an uncoupling of larval and adult ecology. The alternative (derived) lecithotrophic larva occurs in 68% of recent echinoderm species, suggesting an evolutionary trend toward this larval type. Lecithotrophic development represents a more buffered life history because compensatory feedback between adult densities and larval output is likely to be more efficient. For lecithotrophic developers, direct nutritive coupling from adult to larva to the early benthic juvenile provides a buffer against starvation. Lecithotrophic larvae are independent of the vagaries of planktonic food supply, and their short planktonic duration may promote local recruitment. Anthropogenic influences contributed to the population density variations in most cases, including increased primary productivity through eutrophication or global change, disease, overfishing, and species introductions. We suggest that anthropogenic disturbance, through its influence on the frequency and/or amplitude of echinoderm population density changes, may go beyond present ecosystem impacts and alter future evolutionary trends.

Atmospheric carbon dioxide (CO₂) levels are rapidly rising causing an increase in the partial pressure of CO₂ (pCO₂) in the ocean and a reduction in pH known as ocean acidification (OA). Natural volcanic seeps in Papua New Guinea expel 99% pure CO₂ and thereby offer a unique opportunity to explore the effects of OA in situ. The corals Acropora millepora and Porites cylindrica were less abundant and hosted significantly different microbial communities at the CO₂ seep than at nearby control sites <500 m away. A primary driver of microbial differences in A. millepora was a 50% reduction of symbiotic Endozoicomonas. This loss of symbiotic taxa from corals at the CO₂ seep highlights a potential hurdle for corals to overcome if they are to adapt to and survive OA. In contrast, the two sponges Coelocarteria singaporensis and Cinachyra sp. were ∼40-fold more abundant at the seep and hosted a significantly higher relative abundance of Synechococcus than sponges at control sites. The increase in photosynthetic microbes at the seep potentially provides these species with a nutritional benefit and enhanced scope for growth under future climate scenarios (thus, flexibility in symbiosis may lead to a larger niche breadth). The microbial community in the apparently pCO₂-sensitive sponge species S. massa was not significantly different between sites. These data show that responses to elevated pCO₂ are species-specific and that the stability and flexibility of microbial partnerships may have an important role in shaping and contributing to the fitness and success of some hosts.

The ecological effects of ocean acidification (OA) from rising atmospheric carbon dioxide (CO ₂ ) on benthic marine communities are largely unknown. We investigated in situ the consequences of long-term exposure to high CO ₂ on coral-reef-associated macroinvertebrate communities around three shallow volcanic CO ₂ seeps in Papua New Guinea. The densities of many groups and the number of taxa (classes and phyla) of macroinvertebrates were significantly reduced at elevated CO ₂ (425–1100 µatm) compared with control sites. However, sensitivities of some groups, including decapod crustaceans, ascidians and several echinoderms, contrasted with predictions of their physiological CO ₂ tolerances derived from laboratory experiments. High CO ₂ reduced the availability of structurally complex corals that are essential refugia for many reef-associated macroinvertebrates. This loss of habitat complexity was also associated with losses in many macroinvertebrate groups, especially predation-prone mobile taxa, including crustaceans and crinoids. The transition from living to dead coral as substratum and habitat further altered macroinvertebrate communities, with far more taxa losing than gaining in numbers. Our study shows that indirect ecological effects of OA (reduced habitat complexity) will complement its direct physiological effects and together with the loss of coral cover through climate change will severely affect macroinvertebrate communities in coral reefs.