VOLUM OMAGIAL - Facultatea de Ştiinţe ale Naturii şi Ştiinţe Agricole

Dan Răzvan Popoviciu, Ioan Ardelean / Ovidius University Annals, Biology-Ecology Series 14: 139-145 (2010)
V = cell volume (µm 3 ).
Dry biomass was determined for each cell,
calculating then the media for each sample. Total
(wet) biomass can be approximated using a
conventional mean value for bacterial cell density, of
1.1 g/cm 3 [23, 24].
3. Results and Discussions
Bacterial cell abundance. The evolution of cell
density in time (from 0 to 56 days) for each
microcosm is shown in Fig. 1.
Million cells
7
6.5
6
5.5
5
4.5
4
3.5
3
0 14 28 42 56
Time (days)
A
B
C
Fig. 1. Number of bacterial cells (× 10 6 ) per cm 3
of sediment.
For undisturbed sediment cores, bacterial density
ranged between 4.7-6.65 × 10 6 cells/cm 3 sediment
with an average of 5.52 × 10 6 cells/cm 3 .
These values are within the variation limits of
littoral sediment microbial density (although data
found in literature is distributed over a wide range).
For comparison, here are some bacterial densities: 10 9
cells/g dry sand [20], 5 × 10 8 -1.5 × 10 9 cells/g
sediment [9] and 7-9 × 10 7 cells/g [25] on the U.S.A.
East Coast, 1.91-7.32 × 10 7 cells/g dry sediment, in
Eastern Canada, at the waterline [1], 3.6 × 10 8 cells/g
dry sediment, in Florida [26], 6.8-20.3 × 10 8
141
cells/cm 3 , fine sands in a Mexican tropical lagoon, 1.2
m depth [27], 7 × 10 8 -6.7 × 10 9 cells/cm 3 , Baltic Sea
[28], over 5.12 × 10 8 cells/g dry sediment, Western
Mediterranean Sea [29], 1.5 × 10 8 cells/g dry sand
[10], 6-8 × 10 9 cells/g sediment [30] and 3.54-8.08 ×
10 9 cells/g [3] in the Adriatic Sea, at several meters
depth, 0.2-1 × 10 9 cells/g dry sediment, in littoral
sands in the Gulf of Tokyo [14], 2.56-4.46 × 10 6
cells/g, at 2 m depth, in North Sea [31].
The addition of gasoline caused a decrease in cell
abundance to values as low as 3.6 × 10 6 cells/cm 3 . A
return to densities similar to the initial ones was
observed in the last samples. The recovery was faster
in the microcosm supplemented with ammonium
nitrate (28 days).
Direct cuantification of bacteria through
epifluorescence microscopy has some limitations.
Cell masking by sediment particles, background
fluorescence, lack of an efficient method to
distinguish prokaryotes from eukaryotes, the poor
quality of some photographs etc., can cause
overestimation or underestimation of real abundance
[8, 17, 22]. The method used for bacterial dispersion
from sediment grains can also influence the results
[9].
An important factor that can cause
underestimation of bacterial abundance is the
extremely small size of some microorganisms. Many
bacteria have diameters below 0.3 microns, and can
be very difficult or even impossible to visualise,
depending on the optical means employed. Some of
them can even pass through usual filtering
membranes. According to some authors such
ultramicrobacteria constitute up to 72% of the soil
microbiota, and it seems that they have similar
proportions in marine environments [17]. In
conclusion, all data obtained using direct counts
should be regarded as relative.
It should be noted that not all the bacteria
ennumerated with acridine orange are alive. Living
bacteria constitute usually less than one third, rarely
reaching 60% of the total number. The rest are dead
cells, or even cell fragments [10, 32].