engleski

Production of particulate and dissolved organic carbon by marine phytoplankton in the light and in the dark

Phytoplankton photosynthesis of cellular materials is accompanied by release of organic molecules from the algal cells into the water. Laboratory and in situ experiments were conducted to investigate controls on carbon flow from the inorganic carbon pool into marine phytoplankton cellular material and from phytoplankton to dissolved organic carbon. Production of particulate organic carbon (POC) and rates of extracellular organic carbon (EOC) were measured by uptake of 14C-sodium bicarbonate and its subsequent incorporation and release into organic compounds by employing traditional 14C radiotracer protocols. Results indicated that phytoplankton cells are capable of channelling significantly more inorganic carbon into an extracellular organic carbon (EOC) fraction than into particulate organic carbon (POC), both in situ and in culture. Release/incorporation ratios ranged from 10-2 to as much as 1.2*103 (EOC/POC). The cell specific rate at which the phytoplankton community released radiolabelled organic compounds declined hyperbolically as the concentration of cells increased, i.e. the release rate was found primarily as a function of cell density. For both laboratory and in situ experiments, cell-specific extracellular organic carbon release rates were elevated following dilution, suggesting an "environmental stress" response. We believe that the autotrophic community keeps the level of dissolved organic carbon constant by producing reduced carbon compounds that are released to the environment. Bacterial activity was not statistically correlated with EOC release rates, suggesting that quality, as opposed to the quantity of the released materials regulated bacterial growth. Cell-specific release rates in the light and dark were stastically correlated, whereas uptake into POC in light and dark were not correlated, owing to the more complex nature of mechanisms controlling the production of cellular material. Channeling of inorganic carbon into particulate and dissolved organic pool in darkness is due to the activity of autotrophs. The influence of bacteria is negligible. We suggest that the dark bottle (DB) 14C uptake rates should not be substracted from the light bottle (LB) rates within the courses of primary production measurements. The relation between the LB cell-specific values and DB values may be an indication of the relative activity of light-independent CO2 fixation pathways of particulate algal community. Our results emphasize the importance of EOC in phytoplankton synthesis of organic matter, i.e. primary production. There is an active component of 14CO2fixation in the dark, among which EOC contributes significantly to the light-independent production of organic matter.

organic carbon; marine phytoplankton; extracellular release

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