MEPS 255:15-25 (2003)  -  doi:10.3354/meps255015

ABSTRACT: We measured stable carbon isotope ratios (δ¹³C) in phospholipid fatty acids (PLFAs) to identify the primary carbon source utilized by sedimentary bacteria in Lower Laguna Madre, Texas, which is a seagrass dominated lagoon. Comparisons were made between 3 differing habitat types consisting of a bare area, a transitional area, and a vegetated area. Using PLFA concentrations, we estimated that bacterial abundance was significantly higher in the vegetated habitat compared with bare or transitional habitats. Seagrass Thalassia testudinum above-ground tissues averaged -10.8 ± 0.3‰ and benthic microalgae, based on 20:5Δ3 PLFA, averaged -20.5 ± 0.6‰. The δ¹³C of total organic carbon (TO¹³C) from all habitats and depths were within ±2‰ of T. testudinum above-ground tissues, suggesting that the majority of sedimentary organic carbon originated from this source. The δ¹³C of the ubiquitous 16:0 PLFA indicated more complexity in surface vegetated sediments and at depth (ca. 19 cm) in bare and transitional habitats. In turn, the δ¹³C of branched, iso- and anteiso-15:0 (i&a15:0) PLFAs found only in bacteria were within ±3‰ of TO¹³C in all habitats and at all depths. Our work confirmed coupling between sedimentary bacteria and seagrasses occurs in oligotrophic systems with few allochthonous inputs.

KEY WORDS: Seagrasses · Carbon cycling · Bacteria · Phospholipid fatty acids · Stable carbon isotope ratio