AME 47:91-98 (2007)  -  doi:10.3354/ame047091

ABSTRACT: Studies of microbes on Scirpus lacustris submerged in river (oligotrophic) and wetland (eutrophic) waters were conducted to determine how nutrient status affected colonization and growth on the same substrate. The antibiotic tetracycline was used to repress bacteria to determine if they inhibited fungal growth. The use of tetracycline was also relevant in a broader context as antibiotics are now being detected in aquatic systems. Dead S. lacustris stems were submerged for 33 d in natural and tetracycline amended (500 and 4000 µg l^–1) river and wetland waters. Confocal laser scanning microscopy was used to scan the biofilm while image analysis was used to determine microbial (algal, bacterial, fungal) cell volume and fungal biomass by measuring the length of fungal hyphae. In all treatments, microbial cell volume peaked on Day 10 and was greater (ANOVA, all p < 0.05) than on all other sampling days. Microbial biovolume was higher (p = 0.04) in the wetland vs. river water, possibly because nutrients were not limiting in the wetland. Biofilm thickness was not different between the 2 waters, between treatments or over time (p = 0.7, 0.8 and 0.07, respectively). Fungal biomass was greater (p = 0.01) in the river water compared to the wetland, indicating that the same plant in different aquatic systems will vary in the ratio of bacterial/fungal constituents that colonize it after death. Though there seemed to be a trend of increased fungal biomass in the tetracycline treatments, suggesting bacterial inhibition of fungi, the differences were not statistically significant (p > 0.05). On Day 10, the controls of both water treatments had significantly greater microbial biovolume than both the 500 and 4000 µg l^–1 antibiotic treatments (both p = 0.02), indicating that tetracycline had a negative effect on microbes colonizing Scirpus.

KEY WORDS: Macrophyte · Biofilm · Litter decomposition · Confocal scanning laser microscopy · Fungal biomass · Aquatic plant decay · Tetracycline · Carbon sequestration