Production and fate of dimethylsulfide and dimethylsulfoniopropionate in pelagic mesocosms:the role of sedimentation

Abstract

Elevated concentrations of dlmethylsulf~de (DMS) sometimes occur In sea water dunng phytoplankton blooms. To determine the factors controlling the concentration of DMS in sea water, the development and fate of a bloom dominated by Phaeocystis sp. was studied in relation to the production and fate of DMS and its precursor P-dimethylsulfoniopropionate (DMSP) in a pelagic mesocosm expenment. The part of this study described here focused on the role of sedimentation of algae as a loss factor for DMSP and as a trigger mechanism for DMS production A hypothesis tested was that high DMS concentrations occur after a mass release of DMSP from algae due to mass sedimentat~on followed by cell lysis on the mesocosm floor. This was studied by monitoring Phaeocystis cell numbers, chlorophyll a, particulate DMSP, dissolved DMSP and DMS In the water column, and by daily measurements of the sedimentation of Phaeocystis cells. We evaluated a technique for measuring DMSP indirectly as acrylate by high-performance liquid chromatography (HPLC). This method seemed to be hampered by DMSP-lyase activity of Phaeocystis sp during sample processing, but gave good results when Phaeocy~tis~as not dominant. Sedinlentation rates of Phaeocystis sp. were found to be high and constantly related to the standing stock in the water column. The average sinking velocity of Phaeocystis cells was 1.4 m d-' At the decline of the bloom, sedimentation accounted for approximately 50% of the observed loss of Phaeocystis biomass, which indicates that cell lysis may have been important as well at this time The decline of the bloom did not result in an elevated DMS concentration. However, a significant peak in DMS was observed at the end of the exponential growth phase of the Phaeocystis bloom It was concluded that the decline of a Phaeocystis bloom does not lead to an elevated DMS concentration in the water column. The continuously high sedimentation of living Phaeocystis cells suggests that, in these mesocosms, lysis after sedimentation was an important mechanism for release of DMSP from cells and, hence, production of DMS. Nevertheless, this mechanism in itself can not explain the strong fluctuations in the DMS concentration observed in this experiment

High acrylate concentrations in the mucus of Phaeocystis globosa colonies

“…Maximum concentrations of total acrylate reported for Phaeocystis in the field range between 0.51 and 0.7 pm01 1-' (Yang et al 1994, Osinga et al 1996. These values are comparable to the total acrylate concentrations measured in the medium of axenic cultures during exponential growth of cells in the present study (0.1 to 1.0 pm01 1-l).…”

“…Total acrylate concentrations rose above the detection limit (0.1 pM) on Day 6, and continued to increase to 1.0 pM at the onset of the stationary phase and to 4.3 pM during the senescent phase. This was in contrast to measurements that had been taken in the field where the total acrylate concentration detected in the senescent phase of blooms quickly dropped to zero (Yang et al 1994, Osinga et al 1996. The non-linear regression curve (r2 = 0.98) of the total concentration of acrylate versus time was used to calculate acrylate production rates (Fig.…”

“…Total acrylate concentrations in the open ocean, however, only appear to be in the nM range (Gibson et al 1996) but may reach values as high as 1.21 pM (Gibson et al 1996) and 0.51 to 0.7 pM for Phaeocystis blooms (Yang et al 1994, Osinga et al 1996. Thus, the maximum total concentrations of acrylate measured in the marine environment are far too low to expect inhibition of bacteria.…”

“…In field studies, acrylate is detected in the beginning of the bloom, but as the bloom reaches the senescent phase, acrylate concentrations quickly drop to zero (Yang et al 1994, Osinga et al 1996. Davidson & Marchant (1987) have suggested that bacteria are able to invade old and senescent colonies because acrylate would only be produced in the exponential phase of growth.…”

Aerobic degradation of phytoplankton debris dominated by Phaeocystis sp. in different physiological stages of growth

“…The concentration of acrylate in the algal debris was measured on a Waters 600E high performance liquid chromatography (HPLC) system (Osinga et al 1995).…”