Peter J. Ralph

Coral bleaching: the role of the host.

Coral bleaching caused by global warming is one of the major threats to coral reefs. Very recently, research has focused on the possibility of corals switching symbionts as a means of adjusting to accelerating increases in sea surface temperature. Although symbionts are clearly of fundamental importance, many aspects of coral bleaching cannot be readily explained by differences in symbionts among coral species. Here we outline several potential mechanisms by which the host might influence the bleaching response, and conclude that predicting the fate of corals in response to climate change requires both members of the symbiosis to be considered equally.

Photosynthetic response of Halophila ovalis to heavy metal stress

Abstract This paper deals with the photosynthetic effects of a range of heavy metals on the seagrass Halophila ovalis . In this study, the photosynthetic response of laboratory-cultured H. ovalis to four heavy metals (Cu, Cd, Pb, Zn) was investigated. The results indicated clearly that chlorophyll a fluorescence was effective in monitoring the onset and development of stress, and occasional recovery, of H. ovalis when exposed to a wide range of heavy metals. Heavy metals in concentrations from 1 to 10 mg litre −1 produced several acute toxic responses. They had a variety of effects on the photosynthetic processes of this seagrass, with Cu and Zn having substantially greater effects than Pb and Cd. Quantum yield was the most sensitive measure of the photosynthetic processes affected by all heavy metals tested. With some exceptions, photosynthetic pigment content generally confirmed the chlorophyll a fluorescence responses.

Ecology: A niche for cyanobacteria containing chlorophyll d

The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.

Operation of the xanthophyll cycle in the seagrass Zostera marina in response to variable irradiance

Changes in the photobiology and photosynthetic pigments of the seagrass Zostera marina from Chesapeake Bay (USA) were examined under a range of natural and manipulated irradiance regimes. Photosynthetic activity was assessed using chlorophyll-a fluorescence, and photosynthetic pigments were measured by HPLC. Large changes in the violaxanthin, zeaxanthin, and antheraxanthin content were concomitant with the modulation of non-photochemical quenching (NPQ). Photokinetics (Fv/Fm, rapid light curves (RLC), and non-photochemical quenching) varied as a result of oscillating irradiance and were highly correlated to xanthophyll pigment content. Zeaxanthin and antheraxanthin concentrations increased under elevated light conditions, while violaxanthin increased in darkened conditions. Unusually high concentrations of antheraxanthin were found in Z. marina under a wide range of light conditions, and this was associated with the partial conversion of violaxanthin to zeaxanthin. These results support the idea that xanthophyll intermediate pigments induce a photoprotective response during exposure to high irradiances in this seagrass.

Quantifying and modelling the carbon sequestration capacity of seagrass meadows – A critical assessment

Seagrasses are among the planets most effective natural ecosystems for sequestering (capturing and storing) carbon (C); but if degraded, they could leak stored C into the atmosphere and accelerate global warming. Quantifying and modelling the C sequestration capacity is therefore critical for successfully managing seagrass ecosystems to maintain their substantial abatement potential. At present, there is no mechanism to support carbon financing linked to seagrass. For seagrasses to be recognised by the IPCC and the voluntary C market, standard stock assessment methodologies and inventories of seagrass C stocks are required. Developing accurate C budgets for seagrass meadows is indeed complex; we discuss these complexities, and, in addition, we review techniques and methodologies that will aid development of C budgets. We also consider a simple process-based data assimilation model for predicting how seagrasses will respond to future change, accompanied by a practical list of research priorities.

Loss of Functional Photosystem II Reaction Centres in Zooxanthellae of Corals Exposed to Bleaching Conditions: Using Fluorescence Rise Kinetics

Mass coral bleaching is linked to elevated sea surface temperatures, 1–2 °C above average, during periods of intense light. These conditions induce the expulsion of zooxanthellae from the coral host in response to photosynthetic damage in the algal symbionts. The mechanism that triggers this release has not been clearly established and to further our knowledge of this process, fluorescence rise kinetics have been studied for the first time. Corals that were exposed to elevated temperature (33 °C) and light (280 μmol photons m−2 s−1), showed distinct changes in the fast polyphasic induction of chlorophyll-a fluorescence, indicating biophysical changes in the photochemical processes. The fluorescence rise over the first 2000ms was monitored in three species of corals for up to 8 h, with a PEA fluorometer and an imaging-PAM. Pocillopora damicornis showed the least impact on photosynthetic apparatus, while Acropora nobilis was the most sensitive, with Cyphastrea serailia intermediate between the other two species. A. nobilis showed a remarkable capacity for recovery from bleaching conditions. For all three species, a steady decline in the slope of the initial rise and the height of the J-transient was observed, indicating the loss of functional Photosystem II (PS II) centres under elevated-temperature conditions. A significant loss of PS II centres was confirmed by a decline in photochemical quenching when exposed to bleaching stress. Non-photochemical quenching was identified as a significant mechanism for dissipating excess energy as heat under the bleaching conditions. Photophosphorylation could explain this decline in PS II activity. State transitions, a component of non-photochemical quenching, was a probable cause of the high non-photochemical quenching during bleaching and this mechanism is associated with the phosphorylation-induced dissociation of the light harvesting complexes from the PS II reaction centres. This reversible process may account for the coral recovery, particularly in A. nobilis.

CORAL PHOTOBIOLOGY STUDIED WITH A NEW IMAGING PULSE AMPLITUDE MODULATED FLUOROMETER

A new high‐resolution imaging fluorometer (Imaging‐PAM) was used to identify heterogeneity of photosynthetic activity across the surface of corals. Three species were examined: Acropora nobilis Dana (branching), Goniastrea australiensis Edwards & Haime (massive), and Pavona decussata Dana (plate). Images of fluorescence parameters (F, Fm′, effective quantum yield, optimal quantum yield, electron transport rate, relative photosynthetic rate, and non‐photochemical quenching) allowed heterogeneity to be detected in terms of position on colony and indicated that the photosynthetic activity of polyp and coenosarc tissues responded differently to changing light for all three species. The Imaging‐PAM offers a special routine, with which images of PAR absorption (absorptivity) are obtained. In this way, for the first time it has become possible to derive images of the relative photosynthesis rate. Polyps had a lower PAR absorptivity than coenosarc tissue for A. nobilis and P. decussata, whereas G. australiensis showed the opposite pattern. Acropora nobilis showed heterogeneity along the longitudinal axis of the branch, which could be differentiated from the effect of variations in illumination across the rugose and curved surface. Diel changes were apparent and influenced the longitudinal heterogeneity along the A. nobilis branch. Images were also obtained showing the degree of photoinhibition caused by high‐light stress across a coral surface at a hitherto unobtainable level of resolution.

Use of fluorescence-based ecotoxicological bioassays in monitoring toxicants and pollution in aquatic systems: Review

Chlorophyll a fluorescence has the potential to become a valuable ecotoxicological endpoint, which could be used with a range of aquatic phototrophs. Chlorophyll a fluorescence bioassays have been applied in the assessment of heavy metals, herbicides, petrochemicals and nutrients. The strengths of this endpoint are that it is rapid, non-invasive and non-destructive, while the major weakness is the lack of clear ecological relevance. We provide an overview of chlorophyll a fluorescence applications in ecotoxicology. We reviewed test conditions, parameters and protocols used to date and found standardised protocols to be lacking. The most favoured fluorescence parameters were maximum quantum yield (F v /F m) and effective quantum yield (ΦPSII); microalgae were the most widely used tested organism, herbicides the most commonly tested toxicant, while most studies lacked a summary statistic (such as EC50). We recommend that future research in aquatic chlorophyll a fluorescence ecotoxicology focus on standardisation of test protocols and statistical techniques.

Photosynthetic responses of the seagrass Halophila ovalis (R. Br.) Hook. f. to high irradiance stress, using chlorophyll a fluorescence

Abstract With the increasing threat of destruction to diminishing seagrass resources, there is a need for an accurate stress monitoring procedure to be developed for species of this community. In this study chlorophyll a fluorescence (20°C) was used to monitor the stress responses of the seagrass Halophila ovalis (R. Br.) Hook. f. when exposed to increased irradiance. Leaf tissue was exposed to the following light treatments: 100, 500 and 1000 μmol m −2 s −1 for 10, 20, 40, 60 and 120 min. Exposure to the higher irradiances resulted in significant photoinhibitory responses for both fluorescence and oxygen evolution measurements. There was evidence of both photoinhibitory responses; photoprotection and photodamage. Photoprotective processes appear to be operating in all 500 μmol m −2 s −1 treatments, and in the 1000 μmol m −2 s −1 treatments for at least the first 60 min. Responses included an elevated initial fluorescence, accompanied by a reduction in the variable/maximum fluorescence ratio, maximum fluorescence and variable fluorescence. Photodamage appeared to be involved after 120 min exposure at 1000 μmol m −2 s −1 , where all fluorescence parameters including initial fluorescence were quenched and the maximum oxygen evolution rate saturated at a lower irradiance. A direct linear relationship between variable/maximum fluorescence ratio and the light saturated photosynthetic rate was demonstrated for H. ovalis. The oxygen evolution data corroborate the evidence of the stress responses indicated with the fluorescence results. Generally, the longer the exposure period or higher the irradiance, the greater the damage to the tissue and the associated stress symptoms. These results demonstrate the potential of chlorophyll a fluorescence to assess the relative photoinhibitory response of H. ovalis to short-term exposure to increased irradiance.

Photosynthetic response of Halophila ovalis (R. Br.) Hook. f. to combined environmental stress

Combinations of stresses showed an additive effect in comparison to the individual stress responses. It is apparent from these results that thermal, elevated-light or osmotic stress increases the sensitivity of Halophila ovalis to any of the other stress factors. Photosynthetic stress was detected using chlorophyll a fluorescence. Quantum yield was consistently the most effective measure of photosynthetic stress from combination stress exposures. Chlorophyll pigment analysis supported the general decline in photosynthetic capacity as indicated by the chlorophyll fluorescence; however, several anomalies did occur.