Merinda C. Nash

A short-term in situ CO 2 enrichment experiment on Heron Island (GBR)

Ocean acidification poses multiple challenges for coral reefs on molecular to ecological scales, yet previous experimental studies of the impact of projected CO2 concentrations have mostly been done in aquarium systems with corals removed from their natural ecosystem and placed under artificial light and seawater conditions. The Coral–Proto Free Ocean Carbon Enrichment System (CP-FOCE) uses a network of sensors to monitor conditions within each flume and maintain experimental pH as an offset from environmental pH using feedback control on the injection of low pH seawater. Carbonate chemistry conditions maintained in the −0.06 and −0.22 pH offset treatments were significantly different than environmental conditions. The results from this short-term experiment suggest that the CP-FOCE is an important new experimental system to study in situ impacts of ocean acidification on coral reef ecosystems.

Greenhouse conditions induce mineralogical changes and dolomite accumulation in coralline algae on tropical reefs

Human-induced ocean acidification and warming alter seawater carbonate chemistry reducing the calcification of reef-building crustose coralline algae (CCA), which has implications for reef stability. However, due to the presence of multiple carbonate minerals with different solubilities in seawater, the algal mineralogical responses to changes in carbonate chemistry are poorly understood. Here we demonstrate a 200% increase in dolomite concentration in living CCA under greenhouse conditions of high pCO2 (1,225 μatm) and warming (30 °C). Aragonite, in contrast, increases with lower pCO2 (296 μatm) and low temperature (28 °C). Mineral changes in the surface pigmented skeleton are minor and dolomite and aragonite formation largely occurs in the white crust beneath. Dissolution of high-Mg-calcite and particularly the erosive activities of endolithic algae living inside skeletons play key roles in concentrating dolomite in greenhouse treatments. As oceans acidify and warm in the future, the relative abundance of dolomite in CCA will increase.

Dolomite rich coral reef coralline algae resist dissolution in acidified conditions

Wave-resistant algal rims—chiefly composed of carbonate from crustose coralline algae—form critical structures for the survival of many shallow coral reefs, raising concerns about the susceptibility of these protective structures to ocean acidification. Research now shows that dolomite-rich frameworks—common in shallow coral reefs globally—are likely to persist as carbon dioxide increases.

Ocean acidification: assessing the vulnerability of socioeconomic systems in Small Island Developing States

Ocean acidification poses an increasing threat to marine ecosystems and also interacts with other anthropogenic environmental drivers. A planned response strategy could minimize exposure of socioeconomic systems to potential hazards and may even offer wider advantages. Response strategies can be informed by understanding the hazards, assessing exposure and assessing risks and opportunities. This paper assesses exposure of key socioeconomic systems to the hazards of ocean acidification and analyzes the risks and opportunities of this exposure from Small Island Developing States (SIDS) perspectives. Key socioeconomic systems that are likely to be affected by ocean acidification are identified. A risk analysis matrix is developed to evaluate the risks or opportunities arising from ocean acidification. Analysis of the matrix reveals similarities and differences in potential adaptive responses at global and regional levels. For example, while ocean acidification poses significant threats to SIDS from more frequent toxic wild-caught seafood events and, potentially destruction of coral reef structure and habitat, SIDS may have a relative advantage in aquaculture and an important role to play in global marine ecosystem conservation.

Spatial variation in mechanical properties of coral reef substrate and implications for coral colony integrity

The physical structure of coral reefs plays a critical role as a barrier to storm waves and tsunamis and as a habitat for living reef-building and reef-associated organisms. However, the mechanical properties of reef substrate (i.e. the non-living benthos) are largely unknown, despite the fact that substrate properties may ultimately determine where organisms can persist. We used a geo-mechanical technique to measure substrate material density and strength over a reef hydrodynamic gradient. Contrary to expectation, we found a weak relationship between substrate strength and wave-induced water flow: flow rates decline sharply at the reef crest, whereas substrate properties are relatively constant over much of the reef before declining by almost an order of magnitude at the reef back. These gradients generate a novel hump-shaped pattern in resistance to mechanical disturbances for live corals, where colonies closer to the back reef are prone to dislodgement because of poorly cemented substrate. Our results help explain an intermediate zone of higher taxonomic and morphological diversity bounded by lower diversity exposed reef crest and unstable reef back zones.

Phymatolithon (Melobesioideae, Hapalidiales) in the Boreal–Subarctic Transition Zone of the North Atlantic

Species of the coralline algal genus Phymatolithon are the dominant algal calcifiers in the rocky intertidal and shallow photic sublittoral zone of the Boreal–Subarctic transition zone that stretches across the North Atlantic from the Gulf of Maine and the southern Canadian Maritimes to southwestern Iceland and the Norwegian outer coast. In this paper, we use extensive field and laboratory data on the biology, physiology, and ecology of Phymatolithon species, supported by statistical analysis and DNA sequence data, to develop a multiscale view of this key genus of the ecosystems of this region. We demonstrate that species of Phymatolithon that occur in the Boreal–Subarctic transition zone in the North Atlantic can be segregated systematically by a statistical/developmental analysis of their morpho-anatomical characters. We show these results to be congruent with DNA sequence-based methods. Six species are recognized: Phymatolithon laevigatum , P. rugulosum ( P. lamii ), P. squamulosum ( P. lenormandii ), P. investiens , P. borealis sp. nov. ( P. polymorphum ), and P. nantuckensis sp. nov. Based on paraffin section, compound microscope, and EDS-SEM analysis, we show that coralline anatomy comprises a diversity of both tissue types and high magnesium carbonate wall structure. Variations in vegetative tissue morphology, particularly with respect to cell division and elongation patterns, as well as variation in conceptacle (reproductive structure) location and development, are the result of a complex of genetic and environmental factors. Some of these factors can be linked to adaptation to environmental and biogeographical niches, providing a basis for experimental analysis of the mechanisms of adaptation. We have analyzed conceptacle development in Phymatolithon and demonstrated the linkage between genetic control and concurrent vegetative growth; these parameters interact to produce considerable variation in some characters of final gross morphology but not in others. These results strongly demonstrate that morpho-anatomical research in the broader Corallinophycideae requires greater biological sophistication, with utilization of quantitative population-level data, if success in correlating it with DNA sequence data is to be generally achieved. Smithsonian Contributions to the Marine Sciences , number 41 , viii + 90 pages, 21 figures, 11 plates, 5 tables, 1 appendix, 2018.

Presence of skeletal banding in a reef-building tropical crustose coralline alga.

The presence of banding in the skeleton of coralline algae has been reported in many species, primarily from temperate and polar regions. Similar to tree rings, skeletal banding can provide information on growth rate, age, and longevity; as well as records of past environmental conditions and the coralline alga’s growth responses to such changes. The aim of this study was to explore the presence and characterise the nature of banding in the tropical coralline alga Porolithon onkodes, an abundant and key reef-building species on the Great Barrier Reef (GBR) Australia, and the Indo-Pacific in general. To achieve this we employed various methods including X-ray diffraction (XRD) to determine seasonal mol% magnesium (Mg), mineralogy mapping to investigate changes in dominant mineral phases, scanning electron microscopy–electron dispersive spectroscopy (SEM-EDS), and micro-computed tomography (micro-CT) scanning to examine changes in cell size and density banding, and UV light to examine reproductive (conceptacle) banding. Seasonal variation in the Mg content of the skeleton did occur and followed previously recorded variations with the highest mol% MgCO3 in summer and lowest in winter, confirming the positive relationship between seawater temperature and mol% MgCO3. Rows of conceptacles viewed under UV light provided easily distinguishable bands that could be used to measure vertical growth rate (1.4 mm year-1) and age of the organism. Micro-CT scanning showed obvious banding patterns in relation to skeletal density, and mineralogical mapping revealed patterns of banding created by changes in Mg content. Thus, we present new evidence for seasonal banding patterns in the tropical coralline alga P. onkodes. This banding in the P. onkodes skeleton can provide valuable information into the present and past life history of this important reef-building species, and is essential to assess and predict the response of these organisms to future climate and environmental changes.

Discovery of the mineral brucite (magnesium hydroxide) in the tropical calcifying alga Polystrata dura (Peyssonneliales, Rhodophyta)

Red algae of the family Peyssonneliaceae typically form thin crusts impregnated with aragonite. Here, we report the first discovery of brucite in a thick red algal crust (~1 cm) formed by the peyssonnelioid species Polystrata dura from Papua New Guinea. Cells of P. dura were found to be infilled by the magnesium‐rich mineral brucite [Mg(OH)2]; minor amounts of magnesite and calcite were also detected. We propose that cell infill may be associated with the development of thick (> ~5 mm) calcified red algal crusts, integral components of tropical biotic reefs. If brucite infill within the P. dura crust enhances resistance to dissolution similarly to crustose coralline algae that infill with dolomite, then these crusts would be more resilient to future ocean acidification than crusts without infill.