Growth and Reproduction of Convoluta Roscoffensis Containing Different Naturally Occurring Algal Symbionts

INTRODUCTIONA common characteristic of alga-invertebrate symbioses is their specificity; in general, a given symbiotic invertebrate species forms a stable association with just one species or genus of alga. One approach to examine the basis and significance of the specificity is to compare the performance (i.e. survival, growth and reproduction) of invertebrates experimentally infected with a range of algae. To date, such investigations have been made almost exclusively with algae which do not naturally form an association with the host species (e.g. Schoenberg & Trench, 1980; Mews & Smith, 1982; Douglas, 1983), and the relevance of the conclusions to the natural situation is not always clear.

Establishment of the symbiosis in Convoluta roscoffensis

Of a wide range of algae tested, juvenile Convoluta roscoffensis ingested only Platymonas convolutae, the natural symbiont; related species of the genera Platymonas, Prasinocladus and Tetraselmis; and Chlamydomonas coccoides. Platymonas convolutae was not ingested to a greater extent than Prasinocladus marinus, Tetraselmis tetrathele and Tetraselmis verrucosa, or taken up in preference to T. verrucosa when animals were exposed to a choice between the two species. Convoluta ingested fewer cells of C. coccoides than P. convolutae and related species. Uptake of P. convolutae was not affected by pretreatment of the cells with lectins or proteases, incubation in media of pH 5·0–9·0 or inhibition of algal photosynthesis, but was substantially reduced if the algae were killed.Cells of P. convolutae, Pr. marinus, T. tetrathele and T. verrucosa persisted and divided in juvenile Convoluta. The algal population in the worms started to increase 2–3 d after ingestion and within 15–20 d the animals were uniformly green. These algae formed a viable symbiosis with Convoluta and promoted the growth of the animals. In contrast, C. coccoides cells did not persist in Convoluta for more than 12–24 h a nd were probably disrupted.P. convolutae cells lost their thecae within a few days of ingestion and before migration from the central to sub-epidermal region of the animal. Animal vacuoles surrounded recently ingested thecate algae. Structural studies of the adult symbiosis suggest that the algae were also intracellular and enclosed in vacuoles.It is proposed that Convoluta discriminates against algae unrelated to P. convolutae on initial contact and in the central region of the host. The nature of the recognition mechanism(s) has not been established.

Uric acid utilization inPlatymonas convolutaeand symbioticConvoluta roscoffensis

Uric acid is an excellent nitrogen source for the growth of cultures ofPlatymonas convolutae, the symbiotic alga fromConvoluta roscoffensis, and relatedPlatymonasandTetraselmisspecies. Nitrate-grown cells ofP. convolutaeandT. tetrathelehave two uptake systems for uric acid, which conform to Michaelis–Menten kinetics; a high-affinity system operating in the concentration range 0·2–4·5 μM a nd a low-affinity system operating at higher concentrations of uric acid. Uric acid uptake byP. convolutaeis abolished by uncouplers of phosphorylation. In darkness, intact cells ofP. convolutaemetabolize [2-14C]uric acid to [14C]carbon dioxide. These results are consistent with the proposal that the algal symbionts ofC. roscoffensisutilize uric acid, received from the host, as a nitrogen source.Aposymbiotic juvenile and symbiotic adultC. roscoffensisunder standard culture conditions contain uric acid. The solid uric acid content of juveniles declines on establishment of symbiosis withP. convolutaeand the endogenous uric acid is utilized in the adult symbiosis under conditions of nitrogen demand. However, adultC. roscoffensisdo not utilize exogenous uric acid. The growth of adult but not juvenileC. roscoffensisis dependent on nitrogen enrichment of the medium, and it is proposed that uric acid utilization is of significance to the growth of the developing symbiosis in a nitrogen-poor environment.

Two Species of Algal Symbiont in Naturally Occurrin G Populations of Convoluta Roscoffensis

Convoluta roscoffensis (Graff) has long been known for its symbiosis with a green flagellate definitively described from French stock as Platymonas convolutae by Parke and Manton (Parke & Manton, 1967). Provasoli subsequently discovered a species of Prasinocladus in a culture isolated from C. roscoffensis, but concluded that it was a surface contaminant of the animal (Provasoli, Yamasu & Manton, 1968).Populations of C. roscoffensis from Aberthaw, South Wales (Mettam, 1979), and eighteen sites on Guernsey have been studied. In most patches of the Aberthaw colony, and in two of the Guernsey sites, two algal symbionts have been found. One of the Guernsey, and one of the Aberthaw symbionts appear identical to Platymonas convolutae.They can be distinguished at the ultrastructural level by the presence of canaliculi which penetrate the pyrenoid, and are lined with the double-layered plastid envelope. In both cases the second flagellate shows the nucleus-pyrenoid complex typical of the genus Prasinocladus, where the nucleus penetrates the pyrenoid, and a pair of thylakoid membranes separate the pyrenoid from the starch shell (Parke & Manton, 1965). Neither symbiont has been identified specifically.It is also possible to distinguish the two symbionts in the living animals, since the cup-shaped pyrenoid of the Prasinocladus type is easily distinguished from the spherical condition in Platymonas using the light microscope. A mixed population of symbionts has never been found in an individual worm. For each patch of animals in a colony it was possible to establish the percentage of animals containing each symbiont. This ratio varies from patch to patch in the colony, but has remained remarkably constant within each patch over the six months it has been studied. The Prasinocladus type always predominates at Aberthaw, the Platymonas type being absent in places.