The growth of bacteria and the fungus Phaeosphaeria typharum (Desm.) Holm (Eumycota: Ascomycotina) in salt-marsh microcosms in the presence and absence of mercury

Marine microcosms were used to test the effect of mercury on the growth and respiration of microbes associated with dying Spartina alterniflora Loisel. leaves. All microcosms, except controls, received a bacterial inoculum and one-half also received an inoculum of Phaeosphaeria typharum (Desm.) Holm, a salt-marsh fungus. Experimental microcosms received 0.5 mg of mercuric chloride (0.74 mg Hg/l). Leaves from microcosms representing each mercury treatment (2) and community type (2) were sampled weekly for 21 days. Muramic acid and glucosamine concentrations, bacterial and fungal direct-counts, and carbon dioxide production were measured. Concentrations of muramic acid and glucosamine and direct-counts were converted to microbial biomasses (mg/g leaf). The addition of mercury did not produce a significant difference (P > 0.05) in any measured parameter for any mercury treatment or community type, although there were indications that the mercury addition might have caused microbial biomasses to increase and bacterial respiration to decrease. Glucosamine analysis could be used to differentiate the two communities (P < 0.01). Microbial community type and age could be distinguished by carbon dioxide production (P < 0.01). Bacterial biomasses calculated from direct-counts and muramic acid concentrations gave similar values (Y = 0.93X - 1.39, r = 0.90, P < 0.01). The direct-count method yielded high values for fungal biomass relative to the glucosamine method (Y = 2.51X + 0.93, r = 0.88, P < 0.01). Microbial growth and respiration on dying S. alterniflora leaves appear to be rather insensitive to the mercury levels recorded for contaminated salt marshes in Georgia.