Total dissolved carbohydrates in an enclosure experiment with unialgal Skeletonema costatum culture

In a southern Norwegian fjord (Rosfjord) during POSER a plastic enclosure (1 m Ø, 20 m depth) was filled with filtered fjord water enriched with inorganic nutrients and inoculated with a monoculture of Skeletonema costatum. The enclosure was exposed to fjord conditions from March 18 to April 5, 1979. Pre-filtration had removed ca. 80 % of the bacterial population in the fjord water. Following a short lag phase, bacteria grew exponentially. After consumption of nitrogen-containing organic substances, dissolved carbohydrates were the main energy source; however, bacterial numbers did not increase further, probably due to grazing by nanoflagellates. In spite of sub-optimal light and temperature conditions, S. costatum reached cell densities as high as 35 · 10⁶ dm⁻³ at 3 m water depth, since competing algae and grazing zooplankton were missing in the enclosure. The diatoms grew exponentially, with a division rate of μ = 0.6 d⁻¹ and reached the stationary phase after 16 d, when inorganic nutrients were exhausted. During growth of S. costatum relatively high amounts of carbohydrates were released. Maximum concentrations followed the exponential phase, but highest release rates per cell – 2.0 pmoles glucose equivalents cell⁻¹ d⁻¹ – were reached immediately after inoculation. Measurements at 3 h intervals at the end of the experiment showed that concentrations and therefore release activities varied abruptly within a few hours. Main release occurred from noon to midnight. Not all dissolved carbohydrates were immediately taken up by heterotrophic organisms, part of them could not be utilized, others only after adaptation or succession of bacteria. During the experiment, the proportion of the labile fraction to the total dissolved carbohydrates varied greatly.