Extracellular organic carbon from Eichhornia crassipes (Mart.) Solms: does water hyacinth leak organic carbon?

Abstract

Aquatic macrophytes leak photosynthetically-fixed carbon as dissolved organic molecules. This leakage of organic carbon appears to be an energetically inefficient process. However macrophyte extracellular organic carbon (EOC) may be evidence of a symbiotic association between macrophytes and epiphytic bacteria. Bacteria colonize macrophytes, and EOC is a potential energy source for heterotrophic bacteria. I investigated extracellular release of organic carbon from F, i chhornia eras sipes (Mart.) Solms (water hyacinth), and utilization of that EOC by epiphytic bacteria. I reduced the number of bacteria on water hyacinth roots using a combination of sodium hypochlorite (bleach) and chloramphenicol (antibiotic). Using l^C-labelled CO2, I then compared the percentage of photosynthetically-fixed carbon released from plants having intact epiphytic communities with the percentage released from plants having artificially-reduced epiphytic communities. Water hyacinth roots supported a large and active microbial community. The mean density of epiphytic bacteria was 1.4 x 10 cm in the spring and 1.8 x 10 cm in late summer. Approximately 24% of the bacteria on water hyacinth roots were metabolically active. Water hyacinth released 0.02-0.15% of photoassimilated carbon during a 12-hour light period. Bacterial uptake of EOC did not appear to mask true EOC release, because more EOC was not recovered from plant-epiphyte complexes having reduced epiphytic communities. Leakage of organic carbon from water hyacinth was an insignificant portion of the plant's carbon budget. Thus leakage from water hyacinth is unlikely to represent a significant portion of an aquatic system's total carbon budget. However water hyacinth EOC may have enhanced the development of the plant's epiphytic community. The bacterial population on treated roots grew quickly, almost doubling in 12 hours. In turn, bacteria on water hyacinth roots appeared to stimulate EOC production. When more bacteria were present on water hyacinth roots, more EOC was released from the plant-epiphyte complex.