Stable carbon isotope evidence for coupling between sedimentary bacteria and seagrasses in a sub-tropical lagoon

We measured stable carbon isotope ratios (δ 13 C) 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 20:5∆3 PLFA, averaged -20.5 ± 0.6‰. The δ 13 C of total organic carbon (TO 13 C) 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 δ 13 C of the ubiquitous 16:0 PLFA indicated more complexity in surface vegetated sedi- ments and at depth (ca. 19 cm) in bare and transitional habitats. In turn, the δ 13 C of branched, iso- and anteiso-15:0 (i&a15:0) PLFAs found only in bacteria were within ±3‰ of TO 13 C in all habitats and at all depths. Our work confirmed coupling between sedimentary bacteria and seagrasses occurs in oligotrophic systems with few allochthonous inputs.