Growth and nitrogen metabolism of marine diatoms in brackish water in response to salinity fluctuation
Rijstenbil, J.W. (1989). Growth and nitrogen metabolism of marine diatoms in brackish water in response to salinity fluctuation. PhD Thesis. Universiteit Amsterdam: Amsterdam. 176 pp.
|
| Available in | Author |
|
Document type: Dissertation
|
| Abstract |
In brackish lakes as well as in estuaries, diatoms are major phytoplankton species. In some small inland waters small species deplete nitrogen at the end of the spring bloom. In a saline sea arm large marine diatoms grow with nitrogen in excess. In both water types species occurred with identical salinity limits for growth. The field study does not give information about the ability of phytoplankton species to adjust to osmotic perturbations. In estuaries pelagic species may prove less adapted to fast salinity changes than phytoplankton subjected to irregular freshwater inflow in small brackish lakes. In competition with Ditylum brightwellii in continuous culture, Skeletonema costatum adjusts faster to osmotic fluctuation. It is unlikely that these diatom species inhibited each other, neither in these selection experiments, nor in the natural environment. However, dense blooms of S. costatum may occasionally inhibit other competing species. Salinity may affect nitrogen uptake as well. For this, both diatoms were grown at constant salinity under ammonium limitation prior to altering salinity. The transient state was studied in cells subjected to a temporal salinity decrease. The species chosen to represent phytoplankton from unstable and stable osmotic environments were respectively S. costatum and D. brightwellii. Cells of S. costatum, adapted to continuous light, were smaller than cells grown in 12:12 light-dark cycles (LDC). In continuous light, the temporal salinity decline ranged from 22.4 to 8.6 promille. Photosynthesis was inhibited as a result of decreasing salinity; the average relative growth rate (µ/µm) became 0.57 (SD=0.23). Cells grown in a 12:12 LDC attained a higher µm (0.062 h-1); photosynthesis was not affected in the range 15-25 01005. Growth (µ) decreased while salinity shifted from 13.5 to 4.8 0100; µ/µm reached an average of 0.47 (SD=0.04). The overcapacity of ammonium uptake disappeared in both light-regimes. Chlorophyll a per cell increased when the salinity minimum was passed. Photosynthesis and ammonium assimilation were resumed immediately. Nitrogen limitation was re-established. Growing in a 12:12 LDC, D. brightwellii was more affected by a tem- poral salinity decrease than S. costatum. Although photosynthesis was inhibited, chlorophyll a increased. Cytoplasm contracted (stress reaction), while µ/µm was reduced to an average of 0.14 (SD=0.09). The ammonium uptake capacity declined. In contrast to S. costatum, the affinity for ammonium decreased in D. brightwellii. Relatively more amino acids accumulated in D. brightwellii, whereas S. costatum excreted amino acids, which may play some role in osmoregulation. Recovery of both species (12:12 LOC) was characterized by the average relative growth rate (µ/µm), determined 24 h after passing salinity minimum. S. costatum reacted with an immediate increase of µ/µm= 0.78 (SO= 0.04), whereas D. brightwellii recovered in an unpredictable way, achieving a µ/µm= 0.32 (SO=0.25). While carbon fixation and nitrogen assimilation increased, cell division processes may be delayed until cytoplasm has been restored. Accumulated ammonium was rapidly assimilated, causing a temporal increase of the amino acid pools; alanine increased in D. brightwellii, glutamic acid in S. costatum. Finally, the results of the experiments were extrapolated to the phytoplankton-groups represented by the two test-species. Small species in mesohaline lakes and estuaries were regarded as the "Skeletonema"-type; the large marine diatoms of the polyhaline waters, as the "Ditylum"-type. This may explain why a group of small, "estuarine" species wins the competition wherever salinity is unstable. Nitrogen will be assimilated rapidly by such species in periods of increasing salinity. The group dominated by large, "coastal" species may win the competitio |
|
