Stable carbon isotopic compositions of bacterial fatty acids in a seagrass dominated system

Abstract

The major source of carbon to the bacterial community in a seagrass (Thalassia testudinum) dominated region of Lower Laguna Madre, Texas was determined with the isotopic composition of bacterial phospholipid fatty acids. Rough estimates of bacterial abundance were also obtained from total phospholipid fatty acid concentrations. Core samples came from three differing habitat types consisting of a bare area, a transitional area, and a vegetated area. Five depth intervals of 0-0.5 cm, 0.5-2.5 cm, 4.5-6.5 cm, 8.5-10.5 cm, and 18.5-20.5 cm from each core were used for analyses. Bacterial abundance was significantly higher in the vegetated habitat compared with bare or transitional habitats, which showed little difference from one another. The stable carbon isotope ratios ([ð]¹³C) of branched chain fatty acids, iso- and anteiso-15:0 (i&a15:0) found only in bacteria were used to assess carbon utilization. The [ð]¹³C of total organic carbon (TO¹³C) and the ubiquitous fatty acid 16:0 were used as a proxy of organic carbon sources to the sediment. T. testudinum above ground tissues averaged -11.8±0.3[0/00] and benthic microalgae, as represented by the fatty acid 20:5[]3, averaged -20.5±0.6[0/00]. The TO¹³C from all habitats and depths were within ±2[0/00] of T. testudinum above ground tissues suggesting the majority of organic carbon was derived from this source. The [ð]¹³C of i&a15:0 in all habitats and depths were within ±3[0/00] of TO¹³C. In bare and transitional habitats, ¹³C-enriched values of -9[0/00] were observed at the surface, possibly indicating the influence of degradation. These data suggest that seagrass carbon is entering the microbial loop, thereby making this carbon available to higher trophic levels.