Photosynthetic symbionts within aquatic invertebrates

Hydra and Chlorella

Algae are found within the tissues of a variety of animals, particularly in the phylum Cnidaria. In freshwater symbioses the algal symbiont is usually Chlorella. For example, in Hydra viridis, cells of Chlorella are present in large numbers (1.5 X 105 per hydroid) within the digestive cells of the endoderm. In the light, a Hydra receives photosynthates from the algae and 50-100% of its oxygen needs. It can also use organic food. Yet when a Hydra is maintained in darkness and fed daily with organic food, a reduced symbiotic population of algae is maintained for at least 6 months that can return to normal within 2 days of exposure to light (Muscatine & Pool, 1979). Thus, armed with its symbionts, and depending on local conditions and resources, Hydra can behave both as an autotroph and a heterotroph. There must then be regulatory processes harmonizing the growth of the endosymbiont and its host (Douglas & Smith, 1984), as there must presumably be in all such symbioses. If this were not the case, the symbionts would either overgrow and kill the host or fail to keep pace and become diluted as the host grew.

There are many records of close associations between algae and protozoa marine plankton in the marine plankton. For example, in the ciliate Mesodinium rubrum, 'chloroplasts' are present that appear to be symbiotic algae. The mutualistic consortium of protists and algae can fix carbon dioxide and take up mineral nutrients, and often forms dense populations known as 'red tides' (e.g. Crawford et al., 1997). Extraordinarily high production rates have been recorded from such populations (in excess of 2 g m-3 h-4 of carbon) - apparently the highest levels of primary productivity ever recorded for populations of aquatic microorganisms.

13.7.1 Reef-building corals and coral bleaching

We have already noted that mutualists dominate environments around the world in terms of their biomass. Coral reefs provide an important example: reef-building corals (another dramatic example of autogenic ecosystem engineering - see Section 13.1) are in fact mutualistic associations between heterotrophic Cnidaria and phototrophic dinoflagellate algae from the genus Symbiodinium. Coral reefs provide an illustration, too, of the potential vulnerability of even the most dominant of 'engineered' habitat features. There have been repeated reports of 'coral bleaching' since it was first described in 1984: the whitening of corals as a result of the loss of the endosymbionts and/or their photosynthetic pigments (Brown, 1997). Bleaching occurs mainly in response to unusually elevated temperatures (as seen at the Phuket study site, Thailand; Figure 13.13a), but also in response to high intensities of solar radiation and even disease. Thus, episodes of bleaching seem likely to become increasingly frequent as global temperatures rise (Figure 13.13a; see Section 2.8.2), which is a particular cause for concern, since some bleaching episodes have been followed by mass mortality of corals. This was apparent at Phuket, for example, associated with the bleaching episodes of 1991 and 1995 (Figure 13.13b). (On the other hand, a more catastrophic loss had occurred in 1987 as a result, not of bleaching, but of dredging activity, and the decline in cover in the early 1990s appeared to result from an interaction between bleaching and a variety of local human disturbances.)

We clearly cannot be complacent bleaching and global about the effects of global warming on warming coral reefs - and there are likely always

Figure 13.13 (a) Monthly mean sea surface temperatures (SSTs) for sea areas off Phuket, Thailand, from 1945 to 1995. The regression line for all points is shown (P < 0.001). The dashed line drawn at 30.11°C represents a tentative bleaching threshold. The years exceeding this are shown: bleaching was observed in 1991 and 1995 but not monitored prior to that. (b) Mean percentage coral cover (± SE)

(-) reef flats at Phuket, Thailand, over the period 1979 -95. (After Brown, 1997.)

Figure 13.13 (a) Monthly mean sea surface temperatures (SSTs) for sea areas off Phuket, Thailand, from 1945 to 1995. The regression line for all points is shown (P < 0.001). The dashed line drawn at 30.11°C represents a tentative bleaching threshold. The years exceeding this are shown: bleaching was observed in 1991 and 1995 but not monitored prior to that. (b) Mean percentage coral cover (± SE)

(-) reef flats at Phuket, Thailand, over the period 1979 -95. (After Brown, 1997.)

to be human disturbances with which bleaching effects can interact - but it is also apparent that reef corals are able to acclimate to the changed conditions that may induce bleaching and to recover from bleaching episodes. Their adaptability is illustrated by another study at Phuket. During the 1995 episode, it had been observed that bleaching in the coral Goniastrea aspera occurred predominantly on east- rather than west-facing surfaces. The latter normally suffer greater exposure to solar radiation, which also has a tendency to cause bleaching. This therefore suggests that tolerance to bleaching had been built up in the west-facing corals. Such a difference in tolerance was confirmed experimentally (Figure 13.14): there was little or no bleaching on the 'adapted' west-facing surfaces at high temperatures.

Meanwhile, another study of coral bleaching adds to the growing realization that seemingly simple two-species mutualisms may be more complex and subtler than might be imagined. The ecologically dominant Caribbean corals Montastraea annularis and M. faveolata both host three quite separate 'species' or 'phylotypes' of Symbiodinium (denoted A, B and C and distinguishable only by genetic methods). Phylotypes A and B are common in shallower, high-irradiance habitats, whereas C predominates in deeper, lower irradiance sites - illustrated both by comparisons of colonies from different depths and of samples from different depths within a colony (Figure 13.14b). In the fall of 1995, following a prolonged period above the mean maximum summer temperature, bleaching occurred in M. annularis and M. faveolata in the reefs off Panama and elsewhere. Bleaching, however, was rare at the shallowest and the deepest sites, but was most apparent in shallower colonies at shaded sites and in deeper colonies at more exposed sites. A comparison of adjacent samples before and after bleaching provides an explanation (Figure 13.14c). The bleaching resulted from the selective loss of Symbiodinium C. It appears to have occurred at locations supporting C and one or both of the other two species, near the irradiance limit of C under non-

bleaching conditions. At shaded deep-water sites, dominated by C, the high temperatures in 1995 were not sufficient to push C into bleaching conditions. The shallowest sites were occupied by the species A and B, which were not susceptible to bleaching at these temperatures. Bleaching occurred, however, where C was initially present but was pushed beyond its limit by the increased temperature. At these sites, the loss of C was typically close to 100%, B decreased by around 14%, but A more than doubled in three of five instances.

It seems, therefore, first, that the coral-Symbiodinium mutualism involves a range of endosymbionts that allows the corals to thrive in a wider range of habitats than would otherwise be possible. Second, looking at the mutualism from the algal side, the endosymbionts must constantly be engaged in a competitive battle, the balance of which alters over space and time (see Section 8.5). Finally, bleaching (and subsequent recovery), and possibly also 'adaptation' of the type described above, may be seen as manifestations of this competitive battle: not breakdowns and reconstructions in a simple two-species association, but shifts in a complex symbiotic community.

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