Carbon and nitrogen budgets of two Ascidians and their symbiont, Prochloron, in a tropical seagrass meadow

Abstract

Two species of ascidian, Didemnum molle Herdman and Lissoclinum voeltzkowi Michaelsen, were collected from a Fijian seagrass meadow. The primary production of their symbiont (Prochloron), the inorganic nitrogen metabolism and the filtration rate were measured to assess the nutritional coupling between the symbiont and the host animal. The loss of organic carbon due to the respiration of D. molle (1.1 µg at. C (mg dry wt)^-1 day^-1) was greater than that supplied through photosynthesis of the Prochloron (0.69 µg at. C (mg dry wt)^-1 day,^-1). The carbon supplied through filter-feeding appeared to supplement the ascidian's carbon budget. In contrast, organic carbon from the Prochloron of L. voeltzkowi appeared to meet the colony's respiration needs.

The nitrogen budgets of both ascidian colonies were estimated from their respiration rates, the nitrogen requirement of the Prochloron, and the uptake of inorganic nitrogen and particulate organic nitrogen uptake from the water column. The nitrogen incorporated from the surrounding environment could contribute to the net nitrogen gain of the colony. However, our estimate of the nitrogen needed by the Prochloron was much greater than that which could be supplied externally. The amount of nitrogen released by the ascidians was also greater than that which could be supplied externally. This suggests that nitrogen is efficiently recycled within the symbiotic Prochloron-ascidian relationship.