Enumeration of Protozoa and Bacteria in Muddy Sediment

Abstract

Improved enumeration methods were developed for marine bacteria and protozoa in muddy sediment containing large amounts of organic detritus. In sediment from polluted and nonpolluted sites, total bacterial counts ranged from 6.2 × 10⁹ to 15 × 10⁹ g⁻¹ dry sediment and were significantly higher in nonpolluted sediment. Ciliates were restricted to the top 1 cm where they reached abundances of ≈1000 cm⁻³. Nanoflagellate populations were high to a depth of 5 cm and reached higher populations in nonpolluted sediment (1.7 × 10⁷ cm⁻³) than in polluted sediment (0.5 × 10⁷ cm⁻³). For enumeration of protozoa, direct counts of fixed, unstained cells by phase-contrast microscopy was more accurate than counting using epifluorescent microscopy of fixed, DAPI-stained cells. The Percoll-sorbitol method for separating protists from sediment was effective for sandy sediment but much less effective (35–64% recovery efficiency) for muddy, detritus-rich sediment.