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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.

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Exposure of coral reefs to river plumes carrying increasing loads of nutrients and sediments is a pressing issue for coral reefs around the world including the Great Barrier Reef (GBR). Laboratory experiments were conducted to investigate the eVects of changes in inorganic nutrients (nitrate, ammonium and phosphate), salinity and various types of suspended sediments in isolation and in combination on rates of fertilisation and early embryonic development of the scleractinian coral Acropora millepora. Dose-response experiments showed that fertilisation declined signiWcantly with increasing sediments and decreasing salinity, while inorganic nutrients at up to 20 M nitrate or ammonium and 4 M phosphate had no signiWcant eVect on fertilisation. Suspended sediments of ¸100 mg l ¡1 and salinity of 30 ppt reduced fertilisation by >50%. Developmental abnormality occurred in 100% of embryos at 30 ppt salinity, and no fertilisation occurred at ·28 ppt. Another experiment tested interactions between sediment, salinity and nutrients and showed that fertilisation was signiWcantly reduced when nutrients and low concentrations of sediments co-occurred, although both on their own had no eVect on fertilisation rates. Similarly, while slightly reduced salinity on its own had no eVect, fertilisation was reduced when it coincided with elevated levels of sediments or nutrients. Both these interactions were synergistic. A third experiment showed that sediments with diVerent geophysical and nutrient properties had diVerential eVects on fertilisation, possibly related to sediment and nutrient properties. The Wndings highlight the complex nature of the eVects of changing water quality on coral health, particularly stressing the signiWcance of water quality during coral spawning time.

No abstract