CHEMOAUTOTROPHIC SYMBIONTS IN THE BIVALVE LUCINA FLORIDANA FROM SEAGRASS BEDS

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Abstract
Enzymatic and histological evidence suggest that the eulamellibranch bivalve L. 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 sulfide-rich sediments; the sandy sediments of both Thalassia and Ruppia beds contain 1.67 .+-. 0.31 mM (95% confidence interval (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 deep 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, adenosine 5''phosphosulfate reductase, ATP sulfurylase and nitrite reductase were 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 Fe in these granules, consistent with the idea that their composition, in part, may be respiratory pigment and/or Fe-containing cytochromes. Energy dispersive X-ray analysis reveals S 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 per yr (95% CI) to the gross C fixation of seagrass beds.