Original articleEffect of copper on algal-host interactions in the symbiotic coral Plesiastrea versipora
Introduction
Many marine invertebrates have symbiotic associations with dinoflagellates (algae), in which the algae are contained within host cells. Such symbioses occur in many cnidarians, including scleractinian corals, anemones and zoanthids. Since the algae are intracellular, all compounds must first pass through the animal cells to reach the algae. Thus, all algal physiology and biochemistry are potentially under the control of the host. In the coral Plesiastrea versipora, host mechanisms for regulating algal carbon metabolism include the stimulation of photosynthate release by host release factor (HRF) [13], [14], [18] and regulation of photosynthetic carbon fixation by photosynthesis inhibiting factor (PIF) [15]. HRF activity in marine symbioses, provides a mechanism by which the animal host receives nutrients derived from the photosynthesis of its symbiotic algae [18]. We have suggested that the partial inhibition of photosynthesis by PIF in P. versipora, may serve to reduce the growth rate of algae and thus be an efficient means of controlling their numbers [15]. PIF is a recent discovery and we do not yet know whether the production of PIF is a common phenomenon in corals. However, a stable symbiosis depends upon the maintenance of host cell-signalling molecules such as HRF and PIF, that regulate the metabolism of intracellular algal symbionts.
When corals lose large numbers of symbiotic algae or algal photosynthetic pigments, they appear to be bleached [20], [24]. Coral bleaching is primarily thought to be a stress response to abnormal environmental conditions such as elevated temperatures. Oxidative stress caused by high levels of UV light has also been implicated in coral bleaching [24]. For example, in the coral, Agaricia tenuifolia, the rate of algal photosynthesis was reduced by exposure to a combination of elevated temperature and increased UV light. As this effect was reversed by the addition of antioxidants, it was concluded that the reduction in photosynthesis was due to oxidative damage [24]. Because of the close relationship between the host and algal symbiont, coral bleaching may be caused by oxidative stress to either the host or the symbiont, or to both [27].
Pollution has been implicated in coral bleaching events [11], although the exact mechanisms involved are not fully understood. Copper is a common pollutant of the marine environment. Sources of copper include anti-fouling paints (containing up to 500 g copper l–1), sewage, and stormwater run-off from agricultural land where copper is used as a herbicide [6]. Copper may also reach the sea as a result of activities related to industry [10]. While copper is an essential component of many proteins [3], in excess, it can cause damage through autooxidation in aerobic solutions with the formation of hydroxyl radicals [17]. Despite the existence of a number of detoxifying and storage systems for copper in organisms, it is the most toxic metal after mercury and silver for a wide spectrum of marine life, which accounts for its value in anti-fouling preparations [6].
There have been several reports of the damaging effects of excess copper on higher plants and freshwater algae [9], particularly to mechanisms involved in photosynthesis [3]. Short-term exposure to copper has also been reported to damage free-living marine algae, such as Dunaliella tertiolecta (48 h exposure to 8–16 mg l–1 [1]) and the diatom Phaeodactylum tricornutum (50 μg l–1 for 24 h [5]).
Although the reasons for damage by copper in algae and plants are not fully understood [3], there is some evidence that many of the harmful effects of copper on plants are actually due to the generation of reactive oxygen species [25], [30], [33]which can damage biological molecules [17].
Several studies have shown that copper causes oxidative damage to marine invertebrates. Exposure of mussels to copper (50 μg l–1 for 6 d), increased the concentration of protein carbonyls [23], an indicator of oxidative damage. In another study, sublethal exposure of the clam Sunetta scripta and the mussel Perna viridis to copper for 48 h caused an increase in the levels of total carotenoids which was linked to the accumulation of lipofuscin, another marker of oxidative damage [26]. In fact, copper in excess of an organism’s requirements, has the potential to increase oxidative damage to most biological molecules [17].
Oxidative damage may not be readily apparent, but may nevertheless lead to a loss of physiological function [12] and in the case of marine symbioses, such damage may in turn lead to a disruption of the symbiosis.
Two studies have investigated the short-term effects of exposure of symbiotic corals to copper. Jones [22] found that exposure of branch tips of the coral Acropora formosa to copper (40 μg l–1) resulted in the loss of algae (bleaching) and the death of four out of five tips after 24 h. However, in a second experiment, using tips from a different colony and similar experimental conditions, no deaths occurred [22]. In another study, when tips of Montipora verrucosa were exposed to copper (30 μg l–1) for 1 h, then returned to fresh seawater, bleaching was evident within 3 h and continued to increase over the following 24 h [19]. After 96 h of exposure to copper (48 μg l–1), 50% of the tips had died, but no clear conclusions could be drawn because of a large variation in the data [19]. It appears that even within a single species [19], [22], some coral colonies may be more susceptible to damage by copper than others.
Cnidarian genera may also vary in their responses to copper. In pilot experiments we exposed the anemone Aiptasia pulchella and the zoanthid Zoanthus robustus to daily doses of copper (40 μg l–1) for 2 and 3 d, respectively. After exposure to copper, algae were expelled from A. pulchella and Z. robustus became pale, although there was no apparent expulsion of algae. We isolated the algae from Z. robustus and found that algal photosynthesis was reduced by 32% with respect to seawater controls. On the other hand, following exposure of the coral P. versipora to copper (40 μg l–1) for 4 d, algae were not expelled, there were no visible changes to the corals and there was no decrease in algal photosynthesis.
Algal expulsion represents an extreme response to stress as it involves the disruption of a stable symbiosis. Our aim was to find methods that would detect signs of damage by copper before this stage was reached. We used the coral P. versipora as our model and examined the effects of continual exposure to low levels of copper on several physiological parameters that involved both algae and the host. The algal parameters measured were photosynthesis and the response of algae to the host signalling compounds, PIF and HRF, in P. versipora homogenate. In the coral host, we determined whether exposure to copper reduced the activities of PIF and HRF. We also looked for signs of oxidative damage by copper in algae as changes in the levels of chlorophylls and total carotenoids, and in the coral host as changes in the levels of fluorescent compounds.
This is the first study to examine the effects of copper on physiological interactions between symbiotic algae and their host, particularly with regard to two host signalling compounds that control algal carbon metabolism.
Section snippets
Effect of copper on photosynthetic carbon fixation
Photosynthesis in algae isolated from blue and green morphs of the coral P. versipora (Fig. 1) did not decrease following the exposure of corals to copper. When incubated in seawater, algae isolated from seawater control corals (SA), fixed 52.5 ± 5.3 nmol carbon per 105 cells (Expt. 1), and 51.3 ± 4.3 nmol carbon per 105 cells (Expt. 3) which were similar to the amounts fixed by algae isolated from corals exposed to copper (CA) for 12 d (Expt. 1, 61.3 ± 5.9 nmol carbon per 105 cells), and for
Discussion
The aim of this study was to find methods that would detect early signs of pollutant damage to corals, before such damage became lethal.
Since the symbiotic algae provide a large proportion of the carbon requirements of symbiotic corals [18], it is vital for the coral to maintain an integrated symbiosis. We chose to use the symbiotic coral, P. versipora because we have studied the physiology of this coral for some years [13], [14], [15], [28], [35], particularly, the interactions between algae
Methods
The aim of this study was to find markers that would detect pollution damage to symbiotic corals before the damage became lethal. Therefore, we needed both conditions and a model system that avoided extreme stress effects such as algal expulsion. Within the laboratory, it is not possible to simulate exact field conditions such as substratum, wave action, tidal variations and supply of specific concentrations of pollutant. However, under field conditions, it is feasible that copper may
Acknowledgements
The authors wish to thank Mr. M. Ricketts for photography, Mr. M. Ahern for preliminary copper analyses and Mr. L. Edwards and Mr. H. Giragossyan for collection of corals.
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