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Physiological control of diatom sedimentation Waite, Anya Mary


The specific physiological mechanisms governing the process of diatom sedimentation were not well understood. This study investigated the possibility of a short term energy requirement for buoyancy maintenance in marine diatoms which varied predictably with physiological state. The maximum sinking rate (MSR) of Ditylum brightwellii was measured under progressive energy limitation which allowed the determination of the energydependent potential for a cell to reduce its MSR. There was systematic experimental evidence that this diatom's ability to gain energy from storage products via respiration is the principal determinant of its sinking rate. This study provides the first quantification of the negative exponential relationship between respiration rate and sinking rates. Preliminary studies with two other species indicate that a spectrum of energy-dependence may exist. Diatom blooms are the annual periods of highest "new" production and carbon sedimentation. Results from a 5-year study in Auke Bay, Alaska showed that termination of the spring bloom consistently occurred at limiting nitrate concentrations. The sinking response of diatoms to ambient nutrients influenced both species succession during the spring bloom and the subsequent sedimentation of new production. Threshold nitrate concentrations approximating Ks values of the species present, were found to signal initiation of increased sedimentation. Results suggested genus-specific differences in sinking-rate sensitivity to nitrate exhaustion. Overall, sinking rates of the three principal genera ranked (high to low) Thalassiosira spp.--> S. costatum --> Chaetoceros spp., while the nitrate sensitivities of the sinking rates of the genera ranked (high to low) Thalassiosira spp. --> Chaetoceros spp. --> S. costatum. For Thalassiosira aestivalis, sinking rates over all five years did not vary significantly with physical water properties such as ambient temperature, salinity or sigma-t. However daily irradiance showed a significant negative correlation with sinking rates, as did ambient nutrient concentrations. At low daily irradiances (< 3 x 10^21 quanta m^-2 d^-1), ambient nutrient concentration was a better predictor of sinking rate than at high irradiances. The fastest-sinking, most nutrient-sensitive diatoms, the Thalassiosira species, constituted the major source of vertical carbon flux in this embayment during the spring bloom and, considering their cosmopolitan distribution, probably do so in many other such coastal ecosystems. In general, the tendency to sink to the benthos during and/or after a bloom was highly dependent on species-specific cell physiology. Sexuality was observed in Ditylum brightwellii, both in the laboratory and during a fall bloom of this species off Jericho Beach, Vancouver B.C. The field observations indicated that a fraction (ca. 3 %) of the natural population blooming in the upper 3 m became sexual, about a week after the species was first observed to dominate the community. In the laboratory, when sexuality was induced through NO3 limitation and an increase in irradiance, sinking rates increased over 100 times. Maximum sinking rates occurred for zygotes and early post-auxospore cells. Post auxospore cells quickly reduced their sinking rates, and a large fraction of post-auxospore cells began to float at substantial rates (up to 0.4 m d^-1) within one week of formation. Sexuality may have a special role in the life cycle of diatoms such as D. brightwellii, including a mechanism for rapid escape from the photic zone, vertical partitioning of a physiologically stressed population into sexual and non-sexual fractions, and subsequent recolonization of the surface waters by positively buoyant postauxospore cells. The timing and magnitude of diatoms' sinking response to physiological stress may be important in all life history stages of diatoms, and may ultimately affect the balance of selective forces in their evolution.

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