Certain lectins may possibly be involved in the recognition process, but their role has not been critically investigated in the case of most algae-invertebrate systems. In fact, several hosts are capable of discriminating between different algae and they can recognize some algae as compatible and others as incompatible.
In general, symbiotic algae resist host destruction by avoiding digestion either by counteracting the fusion of lysosomes with vacuoles containing the algae, or by possessing cell walls which resist hydrolysis by the host (Trench, 1979).
Sometime structural modifications occur following establishment of symbiosis; thus in the case of the green alga Platymonas convolutae symbiotic within the flatworm Convoluta roscoffensis, cells of the free-living P. convulutae possess a distinct cell wall, flagella and eyespot, but all those structures are lost when the algae enter within the host cell.
While within the host cells, the algal metabolites seem to move freely to the animals, but the reverse movement of organic metabolites from the animal to the alga is not so well known or studied. Most symbiotic green algae release glucose or maltose but Platymonas convolutae mainly releases amino acids.
The specificity in intracellular association between plant and animal cells depends on the genetic identity of two interacting genomes. However, several cases are known where one algal species can live in the cytoplasms of different invertebrates; this may possibly be due to the fact that the hosts possess poorly developed immune systems. At the same time, it is also significant that the examples of animal hosts which are simultaneously infected by more than one algal species are few and far between.
According to Trench (1979), “the aspect of symbiosis that is fundamental to both interacting organisms is the selective advantage of the association. Although it has been postulated that autotroph-heterotroph symbiosis permits exploitation of nutrient-depleted environment, documentation of selective advantage to individual hosts in terms of growth or reproductive potential is lacking in most instances with the exception of Hydra, Convoluta, and Amphiscolops”.
The geographical range of important symbiont species (e.g., Gymnodinium microadriaticum and Chlorella spp.) is quite extensive. They occur very frequently in warm tropical seas, where symbiosis with invertebrates is a dominant feature.
In these areas, associations involving herma- typic corals and G. microadriaticum have evolved as a powerful geological phenomenon that has served to structure the biological activities of associated benthic ecosystems on a worldwide basis.
Compatibility with the environment provided by host tissues may not require special adaptations on the part of the algal symbionts. Most free-living microalgae are nutritionally versatile, and a generalized capacity which can be utilized for symbiosis may preexist in many species.
Some recent researchers have suggested that the possession of surface enzyme systems for polysaccharide synthesis, transglycosylation or other processes might be preadaptive properties which in symbiosis can be altered for the production and excretion of simple carbohydrates (see Taylor, 1973). The presence of algae often causes nutritional and metabolic enhancement in the host. Some specialized relationships require the development of specific symbiont functions.
Uptake of glycine and release of nucleoside triphosphates by symbionts has been reported as possible mechanism for metabolic regulation in the association between G. microadriaticum and Zoanthus and it is quite likely that other similar examples may exist. Some recent studies of terpenoid biosynthesis in gorgonian species strongly indicate that certain synthetic steps may be shared between algal and animal cells.
It is also known that rates of calcification in hermatypic corals are significantly influenced by the presence of algal symbionts and by their photosynthetic activity and that this influence may well go beyond the effects of gas exchange and carbonate equilibria but is based on mechanisms of energy transfer and ion transport established within the functional unit.
Coral reefs are probably the oldest known living communities in existence. They provide valuable insights into the functioning of natural systems at maturity. Their foundations lie in the fundamental cellular interactions arising from the joining together of an autotroph and a heterotroph as a functional unit. These interactions permit the successful exploitation and definition of major environments and ecosystems.
Some of the interactions critical to the success of the coral/algal symbiosis include mechanisms for the entrapment, conservation and cycling of nutrients and energy, as well as the enhancement of CaCo3 deposition in skeletal elements through the photosynthesis of the autotrophic partner.
As a result of these, corals can operate at every trophic level, and that is why coral reefs are some of the most highly productive ecosystems known (Taylor, 1981). They show a shortened food web which provides a nutritional and energetic surplus that may be diverted to support non-metabolic requirements such as clacification, and also a basis for the conservation of nitrogen and other limiting nutrients. These two characteristics are central to the success of symbiotically-based ecosystems in nutrient-deficient water (Taylor, 1981). They constitute a major factor that influences the success and evolution of reef ecosystems.