Sinking velocities of phytoplankton measured on a stable density gradient by laser scanning

Two particular difficulties in measuring the sinking velocities of phytoplankton cells are preventing convection within the sedimenting medium and determining the changing depth of the cells. These problems are overcome by using a density-stabilized sedimentation column scanned by a laser. For freshwater species, a suspension of phytoplankton is layered over a vertical density gradient of Percoll solution; as the cells sink down the column their relative concentration is measured by the forward scattering of light from a laser beam that repeatedly scans up and down the column. The Percoll gradient stabilizes the column, preventing vertical mixing by convection, radiation or perturbation of density by the descending cells. Measurements were made on suspensions of 15 μm polystyrene microspheres with a density of 1050 kg m(−3); the mean velocity was 6.28 μm s(−1), within 1.5% of that calculated by the Stokes equation, 6.36 μm s(−1). Measurements made on the filamentous cyanobacterium Planktothrix rubescens gave mean velocities within the theoretical range of values based on the range of size, shape, orientation and density of the particles in a modified Stokes equation. Measurements on marine phytoplankton may require density gradients prepared with other substances

The daily integral of nitrogen fixation by planktonic cyanobacteria in the Baltic Sea

Measurements were made of the rates of nitrogenase activity (acetylene reduction) by cyanobacteria collected from the Baltic Sea at 2-h intervals, over a period of 24 h, and incubated under natural light. By relating the chlorophyll- specific rate of N-2 fixation (P-N) to the mean photon irradiance (I) at different periods, a P-N/I curve was constructed. A mathematical description of this relationship was used in the calculation of rates of N-2 fixation at different depths and times of day from continuous measurements of surface irradiance and light attenuation. By relating these calculations to the vertical distribution of chlorophyll due to Aphanizomenon flos-aquae, an estimate of the daily integral of N-2 fixation by the population of this cyanobacterium was obtained: it varied from 0.39 to 0.71 mmol m(-2) with a mean value of 0.53 mmol m(-2) over 9 d. Comparisons with similar calculations of the daily integral of photosynthesis over the same period indicated the atomic ratio of N/C fixed is about 0.22, not very different from the N/C ratio expected for the elemental composition of these cyanobacteria. It is demonstrated that when buoyant gas-vacuolate colonies float up during calm periods the increased irradiance that they experience supports an increased rate of N-2 fixation. This increase is less than that seen for carbon fixation because photosynthesis is negated by respiration at low irradiances and is less inhibited at high irradiances. [KEYWORDS: Nitrogen fixation; phytoplankton buoyancy; gas vesicles; cyanobacteria; Aphanizomenon sp.; Baltic Sea Numerical-model; lake valencia; gas vesicles; phytoplankton; communities; venezuela; buoyancy; surface; mats]

Effects of macronutrients upon buoyancy regulation by metalimnetic Oscillatoria agardhii in Deming Lake, Minnesota

Gas-vacuolate filaments of Oscillatoria agardhii form a metalimnetic layer in Deming Lake, Minnesota. The environmental factors which affect buoyancy and the physiological processes which mediate changes in buoyancy were determined. Buoyant filaments lost their buoyancy in a few hours when incubated at light intensities above those found in situ (approximately 15 mumol photons m-2 s-1, or 1% of the surface value). The rate of buoyancy loss was accelerated by the addition of 10 muM phosphate at irradiances >200 mumol photons m-2 s-1. The effect of nutrient additions on buoyancy was also investigated over a longer time period by incubating metalimnetic samples in situ. The samples were deployed for 6 days at a depth where the irradiance was 8% of the surface value. As found in short-term experiments, the addition of phosphate resulted in the largest decrease in buoyancy. However, the addition of ammonia in addition to phosphate attenuated the buoyancy loss on day 2, and on day 6 the filaments in these treatments were almost completely buoyant. The physiological status of the filaments in these treatments was assayed by analysis of elemental ratios of C, N and P, and by measurement of cellular chlorophyll, polysaccharide and protein. In addition, the cellular content of gas vesicles was determined. The construction of ballast balance sheets from these data indicated that changes in buoyancy were primarily due to differences in the amount of polysaccharide ballast in the cells. However, in another set of in situ experiments, the increase in measured ballast molecules did not explain the observed loss of buoyancy. We hypothesized that another, undetected ballast-providing molecule had accumulated in the cells

The selective advantage of buoyancy provided by gas vesicles for planktonic cyanobacteria in the Baltic Sea

Observations were made on the vertical distribution of colonies of Aphanizomenon flos-aquae for 9 d at a drift-station east of Bornholm Island in the Baltic Sea. The buoyant colonies were dispersed in the upper layers of the water column during periods of wind-induced mixing but floated up during calm periods. From measurements of the vertical light extinction, surface irradiance and the photosynthesis versus irradiance curve, calculations were made of the changes in the daily integral of photosynthesis with respect to time and depth throughout the water column. From these calculations it is demonstrated that net photosynthesis by the population of Aphanizomenon flos-aquae increased nearly threefold by floating up after a deep mixing event. It is estimated that, averaged over alternating periods of calm and mixing, the buoyancy provided by gas vesicles in this organism will result in a nearly twofold increase in photosynthesis. A quantitative analysis has been made of the relationship of the daily integral of photosynthesis by the Aphanizomenon population with the mean depth of the population in the water column and the daily insolation. The analysis shows that the integral decreases linearly with respect to mean depth. [KEYWORDS: phytoplankton buoyancy, gas vesicles, cyanobacteria, Aphanizomenon sp., Baltic Sea

The selective advantage of buoyancy provided by gas vesicles for planktonic cyanobacteria in the Baltic Sea

Observations were made on the vertical distribution of colonies of Aphanizomenon flos-aquae for 9 d at a drift-station east of Bornholm Island in the Baltic Sea. The buoyant colonies were dispersed in the upper layers of the water column during periods of wind-induced mixing but floated up during calm periods. From measurements of the vertical light extinction, surface irradiance and the photosynthesis versus irradiance curve, calculations were made of the changes in the daily integral of photosynthesis with respect to time and depth throughout the water column. From these calculations it is demonstrated that net photosynthesis by the population of Aphanizomenon flos-aquae increased nearly threefold by floating up after a deep mixing event. It is estimated that, averaged over alternating periods of calm and mixing, the buoyancy provided by gas vesicles in this organism will result in a nearly twofold increase in photosynthesis. A quantitative analysis has been made of the relationship of the daily integral of photosynthesis by the Aphanizomenon population with the mean depth of the population in the water column and the daily insolation. The analysis shows that the integral decreases linearly with respect to mean depth. [KEYWORDS: phytoplankton buoyancy, gas vesicles, cyanobacteria, Aphanizomenon sp., Baltic Sea]