Transfer of photosynthetically fixed carbon between the prokaryotic green alga Prochloron and its ascidian host

Abstract

In the symbiotic association between the prokaryotic green alga Prochloron and three didemnid host species (Diplosoma similis, Lissoclinum bistratum, Trididemnum cyclops), between 6 and 51 % of the total carbon fixed during exposure for 1 h to H¹⁴CO₃^- in the light (150 µE m^-2 s^-1) becomes associated with the host tissue. Dark fixation of ¹⁴CO₂ in these ascidian species and in Lissoclinum punctatum never exceeds 6% of photosynthetic fixation at saturating light intensity. The corresponding values for dark fixation of ¹⁴CO₂ in isolated Prochloron cells fall within the same range. There is very little excretion of photosynthate from whole colonies of the above ascidian species nor from Didemnum molle, Lissoclinum voeltzkowi and Trididemnum miniatum (usually less than 1 % of total photosynthate at saturation light intensity), suggesting an efficient transfer mechanism from Prochloron to host. Evidence from pulse-chase experiments suggests that transfer probably involves the early products of photosynthesis. The extent of transfer of photosynthate between Prochloron and T. cyclops varies with the rate of photosynthetic ¹⁴CO₂ fixation into the whole colony but there is some transfer even at low light intensities, which strongly limit photosynthesis.