News and Events

Seminar

Where: Centre for Marine Studies conference room, UQ, videoconference to ARC Centre of Excellence Conference Room Townsville

When: Wednesday 21 May 2008, 2.30pm

Acidification negatively affects dinoflagellate symbionts hosted within reef building corals

Alicia Crawley, graduate student UQ

Rising CO2 levels in the sea threaten to break-down the important relationship between reef-building corals and their symbiotic dinoflagellates. Photorespiration plays a significant role in maintaining dinoflagellate well-being by offering an alternative photochemical-quenching pathway and potentially triggering the Carbon-Concentrating Mechanism. As photorespiration is dependant on the ratio of CO2 to O2, rising CO2 may ultimately lead to reduced capacity for photoprotection and increased occurrence of coral bleaching. Here we present data from a microcosm experiment with controlled levels of CO2 and O2. Physiological data has been derived from classical respiration measurements to produce Photosynthesis-Irradiance (P-E) curves. The P-E curves showed a decline in symbiont production at the highest CO2 level (1100 ppm, pH 7.6). Excitation pressure, measured with a pulse amplitude modulation fluorometer, increased significantly with increasing CO2 and decreasing O2. However, these relationships were dependant on the proximity of growth light to saturating light intensities. We observed calcification at pH 7.8 but decalcification at pH 7.6 using the total alkalinity technique. These findings were closely linked to the gene expression of a key enzyme in the photorespiratory pathway, phosphoglycolate phosphatase (PGPase). mRNA of PGPase declined significantly by 54% at 560 ppm CO2 (pH 7.8) and by 70% at 1100 ppm CO2 (pH 7.6). This decline in photorespiration at high CO2 provides a key insight towards the mechanism of CO2-induced bleaching.

Millennial-scale episodes of reef accretion and degradation determined by U-series dating of coral death assemblages in Moreton Bay, SE Queensland, Australia.

Matthew Lybolt ,graduate student UQ

The sub-tropical marginal reefs of Moreton Bay were characterized by diverse mid-Holocene communities that dramatically declined in the absence of major anthropogenic disturbance. We examined the nature and timing of changes in surficial coral death assemblages from Moreton Bay reefs to gain insight to the interplay of local and global changes. Corals were selected throughout the Bay across a depth range from 1 m above high tide to 5 m below low tide. Uranium-series age determinations (n=57) were made by Uranium-series thermal ionization mass spectrometry. The oldest sample was found atop in situ reef accretions at the highest elevation sampled, so Moreton bay reefs initiated before 6.82 ka and sea level must have reached its Holocene maximum prior to that time. Since the oldest cohort of corals was not buried by younger ones we conclude that coral accretion since 5.5 ka has been minimal. Coral accretion occurred in discrete episodes rather than continuously, and depths of accretion, corrected for palaeo-sea-level, became deeper through the Holocene. Major coral accretion episodes are 0-0.3 ka (19% of samples dated), 1.3-1.7 ka (11%), 4.3-4.7 ka (11%), and 5.5-6.8 ka or older (51%). Conspicuous episodes with no accretion are 0.4-1.1 ka and 4.7-5.5 ka. Sea-level fell from its Holocene high in a series of oscillations which coincide with episodes of accretion (stable/rising) and no accretion (falling). Greater extremes of temperature and salinity would result from a lower volume of water in the Bay, and decreased tidal volume would increase the residence time of river-discharge amplifying the sea-level-driven episodes. The hyposaline wedge associated with increased flooding since ENSO re-establishment ~3.5 ka likely forced younger accretions into deeper water. Because European colonization coincides with an episode of accretion we have an excellent opportunity to isolate historical causes of reef decline from novel anthropogenically-induced causes of decline.