Chemoautotrophic Symbionts in the Bivalve Lucina floridana from Seagrass Beds

Enzymatic and histological evidence suggest that the eulamellibranch bivalve Lucina floridana possesses bacterial endosymbionts capable of a chemoautotrophic metabolism. Dense populations of L. floridana (83± 11 per m2; 95% CI, n = 33) are found closely associated with the O2-releasing root systems of seagrasses in sulfiderich sediments; the sandy sediments of both Thalassia and Ruppia beds contain 1.67 ± 0.31 mM (95% CI, n = 13) and 2.49 ± 0.55 mM (95% CI, n = 13) sulfide, respectively. Both transmission electron microscopy of gill tissue and scanning electron microscopy of freeze-fractured gills reveal numerous rod-shaped procaryotic inclusions in vacuoles of large, eucaryotic cells ("bacteriocytes") located deeply within demibranch cross sections; no such inclusions are seen in the ciliated gill epithelium which is rich in mitochondria. Activities of ribulose 1,5-bisphosphate carboxylase (RuBPCase), phosphoribulokinase, APS reductase, ATP sulfurylase, and nitrite reductase have been measured and partially characterized in homogenates of fresh gill tissue. Light microscopy reveals numerous aggregations of pigmented granules localized to the interior of the gill in association with the bacteriocytes. Histochemical staining demonstrates the presence of iron in these granules, consistent with the idea that their composition, in part, may be respiratory pigment and/or iron-containing cytochromes. Energy dispersive X-ray analysis reveals sulfur as a dominant inorganic element in the gill tissue. Based on abundance data of L. floridana and in vitro levels of RuBPCase (half-maximal velocity) this bivalve could potentially contribute 336± 96 g C/ m2/year (95% CI) to the gross carbon fixation of seagrass beds.