K. Stoderegger et Gj. Herndl, PRODUCTION AND RELEASE OF BACTERIAL CAPSULAR MATERIAL AND ITS SUBSEQUENT UTILIZATION BY MARINE BACTERIOPLANKTON, Limnology and oceanography, 43(5), 1998, pp. 877-884
Bacterioplankton not only take up dissolved organic carbon (DOC) and c
onvert it into biomass and CO2, but also release DOC into the water co
lumn. Because bacterioplankton represent the largest living surface in
the world's oceans, and most intact bacterioplankton have a capsular
envelope, we tested the hypothesis that most of the DOC released by ba
cterioplankton into the water column is derived from capsular material
. Therefore, the bacterial uptake of radiolabeled glucose was differen
tiated between incorporation into intracellular and capsular pools to
obtain production estimates for capsular material. Release of the radi
olabeled material into ambient water was followed and its potential as
a carbon source for bacterioplankton determined. Of the radiolabed or
ganic carbon detected in bacterioplankton, similar to 55% was incorpor
ated into intracellular material and 45% into capsular material. No si
gnificant difference was found between seawater cultures grown under P
-limited and balanced nutrient conditions. After transfer of the radio
labeled bacteria into aged seawater, bacterial-derived, radiolabeled D
OC was released into the ambient water at a rate of similar to 15 amol
C cell(-1) h(-1), which corresponds to similar to 25% of the respired
C. Incorporation and respiration rates of this bacterial-derived DOC
by bacterioplankton were at least three orders of magnitude lower than
the corresponding rates for glucose uptake. Incorporation of bacteria
l-derived DOC was only detectable when additional inorganic nutrients
were added. Thus, we have evidence that bacterioplankton are constantl
y renewing parts of the capsule by releasing this material into ambien
t water. The release rate of capsular material represents similar to 2
5% of the bacterial respiration rate and suggests that a considerable
portion of the oceanic DOC pool should consist of ''semi-labile'' bact
erial-derived DOC.