UPTAKE OF ORGANIC,COMPOUNDS BY TWO FACULTATIVELY HETEROTROPHIC MARINE CENTRIC DIATOMS1,2

Glucose and galactose uptake ability in Cyclotella cryptica (clone WT-1-8) and glucose uptake ability in Coscinodiscus sp. develop rapidly in the dark. Induction of sugar uptake ability in the dark does not require the presence of sugar in the medium. The sugars are taken up by carrier-mediated systems. In C. cryptica glucose and galactose are probably taken up by the same system. The capacity of glucose uptake in this recently isolated clone of C. cryptica is nearly 5 times that of a previously studied clone (0-3A). Other organic compounds, which by themselves do not support heterotrophic growth, can be taken up and respired by both diatoms at considerable rates compared, to glucose and galactose. Therefore, in nature, these diatoms may be able to utilize a variety of dissolved organic compounds as sources of intermediary metabolites and as respiratory substrates.     

B12 SPECIFICITY OF MARINE CENTRIC DIATOMS1, 2

Twenty-one species (23 isolates) of marine diatoms were examined for their capacity to utilize analogs of cyanocobalamin for growth at the ecologically significant concentration of 4 ng 1-¹. Yields due to the analogs were compared to those produced by B₁₂. Responses of the various clones to the analogs were not all-or-none, but varied continuously; thus, assigning the clones to the conventional B₁₂ specificity types is a convenient but arbitrary classification. The use of 10 and 1% levels of response is suggested for such classification. At the 10% level of response, 11 clones had coliform, 4 lactobacillus, and 8 mammalian specificity patterns. At the 1% response level, 14 had coliform, 5 lactobacillus, and 4 mammalian specificities. All clones exceed the 10% response level on all benzimi-dazole-containine analogs tested. Few clones showed definite enough patterns of response to make them potentially useful for differential bioassay. The clones suggested are clone 675-D (Bidclulphia sp.?), clone F^;-3 (Fragilaria sp.?), and the estuarine clone of Cyclotella nana (3H).     

EFFECT OF POLYCARBONATE CONTAINERS ON THE GROWTH OF TWO SPECIES OF MARINE DIATOMS1

Two species of marine diatoms [Skeletonema costatum (Greville) Cleve and Thalassiosira pseudonana (Hustedt) Hasle and Heimdal] were grown in glass and polyarbonate containers. S. costatum exhibited a signzJicantly lower exponential growth rate and maximal yield and a signajcantly longer lag phase when grown in polycarbonate. Exponential growth rate and maximal yield of T. pseudonana was significantly reduced (P < 0.05 in all cases). This study suggests that a difference in diatom growth between glass and polyarbonate containers might arise in certain cases. However, such a difference may not be detectable with all biomass measurement techniques or with low within-treatment replication.     

SILICATE DEFICIENCY AND LIPID SYNTHESIS OF MARINE DIATOMS1,2

Lipid synthesis of three marine diatoms was studied with a ¹⁴CO₂ incorporation technique in silicate limited batch cultures. Growth rates were independent of the silicate concentration but the cellular yields were proportional to the initial amount of silicate. At the beginning of the stationary growth phase, lipid synthesis rates per unit culture volume increased by 1.7 times for Chaetoceros gracilis, 3.1 times for Hantzschia sp., and 2.8 times for Cyclotella sp., respectively compared to those during the exponential growth phase. Lipid carbon accounted for as much as 57% of the carbon in C. gracilis, 71% in Hantzschia sp., and 65% in Cyclotella sp., respectively. Additional enrichment with silicate during stationary growth phase allowed the cultures to grow further. Lipid synthesis rates were reduced during the subsequent growth phase, and the growth rates themselves were dependent on the level of biomass achieved during the previous stationary phase. However, the cellular yields were similar and probably controlled by light.     

BIOCHEMICAL TAXONOMY OF MARINE PHYTOPLANKTON BY ELECTROPHORESIS OF ENZYMES. I. THE CENTRIC DIATOMS THALASSIOSIRA PSEUDONANA AND T. FLUVIATILIS1,2

Diatom systematics depends almost entirely upon structure of the silica shell. It is not known to what extent the taxonomic species, as defined by shell structure, corresponds to the genetic species—i.e., to the reproductively isolated population. As an approach to this problem, we report here a comparison of enzymes by electrophoresis. We have examined the genetic constitution of a number of clones of (presumably) the same species for each of 2 closely related, centric diatom species: Thalassiosira pseudonana Hasle and Heimdal and T. fluviatilis Hustedt. The 4 clones of T. fluviatilis form a distinct group, clearly separated from all the T. pseudonana clones. Within T. pseudonana, 4 estuarine clones and one reef clone form a group that is distinctly different from 4 oceanic clones. A single clone of T. pseudonana from the Continental Slope waters is intermediate between these 2 groups and probably shares genes with both groups, indicating that the 2 T. pseudonana groups are not genetically isolated. We conclude that i) within groups, isolates are closely related even though they originated from different continents; and, ii) T. pseudonana is subdivided into ecological races.     

ASSAY OPTIMIZATION AND REGULATION OF UREASE ACTIVITY IN TWO MARINE DIATOMS

An in vitro urease enzyme assay was developed for the marine diatoms Thalassiosira pseudonana Hasle et Heimdal (clone 3H) and T. weissflogii (Grunow) Fryxell et Hasle (clone Actin). This assay involves the colorimetric measurement of ammonium following the hydrolysis of urea in crude cell homogenates and it is the first assay to account for the rate of nitrogen assimilation in both species grown on urea as the sole nitrogen source. Urease activity was found to be present regardless of nitrogen source, although activities showed distinctly different patterns depending on the species examined and form of nitrogen supplied. Under nitrogen-replete conditions, urease activity in T. pseudonana was present constitutively when grown on NH₄⁺ and upregulated when grown on NO₃⁻ or urea. In nitrogen-replete T. weissflogii , urease activity was present at high constitutive levels regardless of the nitrogen source and showed no upregulation. Nitrogen starvation did not upregulate activity in either species.     

GROWTH RATE RELATIONSHIPS TO PHYSIOLOGICAL INDICES OF NUTRIENT STATUS IN MARINE DIATOMS

The growth of two species of marine diatom, Thalassiosira weissflogii (Grunow) and Thalassiosira pseudonana (Hustedt), was followed in batch cultures at four concentrations of dissolved inorganic carbon from N- and C-replete lag phase into N- and/or C-deplete stationary phase. Results describe the relationship between carbon-specific growth rate (μ_C) and chl a :carbon (chl a :C) and glutamine:glutamate (gln:glu) ratios with changes in the cells' nutritional status (N:C), during the utilization of either NO₃ ⁻ or NH₄ ⁺ . The use of the gln:glu ratio as an index of N:C requires further clarification. For both species and N sources, N stress resulted in a decrease in μ_C, chl a :C, and N:C relative to μ_Cmax values, whereas C stress resulted in a decrease in μ_C and an increase in chl a :C and N:C relative to μ_Cmax values. Both species attained a chl a :C ratio of approximately 15 μg·g ^{− 1} at μ_Cmax using either N source. However, this value was not necessarily an indicator of maximal growth rate. NC colimitation resulted in decreased μ_C to values less than 20% of μ_Cmax with only minor changes in chl a :C and N:C relative to μ_Cmax values. Chl a :C results suggest a similarity between the light stress and C stress responses of marine diatoms. The potential for C stress in the marine environment needs to be addressed.     

PHOSPHORUS REQUIREMENTS OF TWO PLANKTONIC DIATOMS IN STEADY STATE CULTURE1,2

Minimum phosphorus requirements of 2 marine diatoms, Cylindrotheca (Nitzschia) closterium and Cyclotella nana, were measured in axenic steady state chemostat culture. C. closterium and C. nana required 1.055 and 1.045 × 10^?15 g-at P cell^?1 generation^?1, respectively, with an average cell doubling time of 0.20 divisions/day. The algae were cultured at 15.5 C and exposed to 5380 lux on a light-dark cycle of 19 hr-5 hr. The phosphorus requirement per unit of cell volume was 0.0135 and 0.0129 × 10^?15 g-at P/μ³ of cell volume for C. closterium and C. nana, respectively.     

ROLE OF INORGANIC IONS IN CONTROLLING SEDIMENTATION RATE OF A MARINE CENTRIC DIATOM DITYLUM BRIGHTWELLI1,2

The settling rates and intracellular levels of K⁺, Na⁺, Cl^-, Mg²⁺ and Ca²⁺ were measured in Ditylum bright-welli (West) Grunow, grown axenically in an enriched seawater medium at 20 C at 4,000 lx on an 8:16 LD schedule. Cells at the end of the dark period have high Na⁺ (118 mM), low K⁺ (64 mM) and low Cl^- (117 mM) relative to levels at the end of the light period when K⁺ (126 mM) and Cl^- (154 mM) are high and Na⁺ (101 mM) is low. There is no significant change in Mg²⁺ (16–18 mM) or Ca²⁺ (3–4 mM) with time. The net result of the ion changes during the light period is to increase cell density by about 3.4 mg ml^-1. This change can account for the increase in settling rate of ca. 0.3 day^-1 during the same interval. The density of the cell contents, calculated from observed ion concentrations, is 15–18 mg.ml^-1 less than that of the seawater medium. The ion and settling rate changes are light-dependent and do not persist in the dark or under constant light (ca. 850 lx), but cells do exhibit a free-running circadian rhythm in cell division under continuous dim illumination. The cell vacuole expands during the light period and contracts during the dark, apparently in response to the net ion fluxes. D. brightwelli appears to regulate its density by active ion selectivity accompanied by trans-vacuolar water movement.     

LOCALIZATION OF IRON WITHIN CENTRIC DIATOMS OF THE GENUS THALASSIOSIRA1

The cellular iron (Fe) quota of centric diatoms has been shown to vary in response to the ambient dissolved Fe concentration; however, it is not known how centric diatoms store excess intracellular Fe. Here, we use synchrotron X‐ray fluorescence (SXRF) element mapping to identify Fe storage features in cells of Thalassiosira pseudonana Hasle et Heimdal and Thalassiosira weissflogii G. A. Fryxell et Hasle grown at low and high Fe concentrations. Localized intracellular Fe storage features, defined as anomalously high Fe concentrations in regions of relatively low phosphorus (P), sulfur (S), silicon (Si), and zinc (Zn), were twice as common in T. weissflogii cells grown at high Fe compared to low‐Fe cells. Cellular Fe quotas of this strain increased 2.9‐fold, the spatial extent of the features increased 4.6‐fold, and the Fe content of the features increased 14‐fold under high‐Fe conditions, consistent with a vacuole storage mechanism. The element stoichiometry of the Fe features is consistent with polyphosphate‐bound Fe as a potential vacuolar Fe storage pool. Iron quotas increased 2.5‐fold in T. pseudonana grown at high Fe, but storage features contained only 2‐fold more Fe and did not increase in size compared to low‐Fe cells. The differences in Fe storage observed between T. pseudonana and T. weissflogii may have been due to differences in the growth states of the cultures.     

LIGHT MICROSCOPE OBSERVATIONS ON CYTOLOGICAL MANIFESTATIONS OF NITRATE, PHOSPHATE, AND SILICATE DEFICIENCY IN FOUR MARINE CENTRIC DIATOMS

Cytological changes induced by nitrate or phosphate limitation were reproducible and readily visible by phase contrast microscopy in Coscinodiscus wailesii, Ditylum brightwellii, Rhizosolenia hebetata f. semi-spina, Skeletonema costatum, and Stephanopyxis turris. These effects included changes in chromatophore number and shape, pyrenoid location, and mitochondrial abundance and appearance. In S. costatum, a diatom containing only 1 chromatophore, only nitrogen-deficient cells could be recognized. Silicate deficiency prevented cell division almost entirely. When a few cells did divide in the presence of low silicate, abnormalities in valve structure occurred in some cells. Cytological differences with N and P deficiency may permit a rough assessment of the physiological condition of the same or similar species in natural diatom communities.     

CONTINUOUS CULTURE OF MARINE DIATOMS UNDER SILICATE LIMITATION. II. EFFECT OF LIGHT INTENSITY ON GROWTH AND NUTRIENT UPTAKE OF SKELETONEMA COSTATUM1,2

Two replicate experiments were conducted to investigate the effect of light intensity on the growth and nutrient uptake of Skeletonema costatum (Grev.) Cleve in silicate-limited continuous culture. Each experiment began with 4 identical chemostat cultures of S. costatum growing at the normal laboratory light (0.14 ly · min^?1, continuous illumination) under strong silicate limitation. Screens were placed over 3 cultures reducing them to light intensities of 0.042, 0.021 and 0.0018 ly · min^?1. Based on growth rules, nutrient uptake rates, cell morphology and chemical composition, the cultures receiving 0.021, and 0.0018 ly · min^?1 appeared to he light-limited, whereas the culture receiving 0.14 ly.     

INFLUENCE OF ULTRAVIOLET RADIATION ON GROWTH AND PHOTOSYNTHESIS OF TWO COLD OCEAN DIATOMS1

The influence of chronic exposure to UV-B and UV-A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV-B. Fluorescence induction kinetics of photo-system II (PSII) revealed that UV-B caused lower F_v/F_m ratios and half-rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV-B—exposed populations, which also had the highest chlorophyll a content per cell. The UV-B—exposed cells were, however, more vulnerable to visible light-induced photoinhibition. Exposure to UV-A in the absence of UV-B had little effect on growth, fluorescence induction of PSII, or chlorophyll a contents but did have some inhibitory effects on carbon assimilation per chlorophyll a and per cell. The increased photosynthetic capacity of UV-B-exposed cells suggested some ability to compensate for damage to the photosynthetic apparatus.     

INFLUENCE OF CELL VOLUME ON THE GROWTH AND SIZE REDUCTION OF MARINE AND ESTUARINE DIATOMS1

The maximal growth rate (μ_max) of 19 marine and estuarine diatoms decreased with increasing cell volume (V). The relationship between log μ_max (Y) and log V (X) was calculated. Statistical analyses showed that the slope of the equation was not significantly different from those obtained by other researchers and that the 95% confidence intervals of mean μ_max at cell volumes of 10³–10⁵μm³ were not significantly different from those cited in most studies. A new regression line for diatoms was calculated as follows: log μ_max= 0.47–0.14 log V; r =–0.69. The rate of size reduction per generation of the 19 diatom species ranged from 0.03 to 0.87 μm per generation. The rate increased with increasing cell length and cell volume and with decreasing maximum division rate. Statistical analyses showed that the rate was closely related to the cell volume and to the reciprocal of the growth rate. The relationships between maximal growth rate and cell volume and between rate of size reduction and cell volume showed that a diatom with a large volume had a smaller maximal growth rate and a larger rate of size reduction than a diatom with a small volume. The estimates using the equation for the regression line between the rate of size reduction and the reciprocal of maximum division rate indicated that a diatom with a high maximum division rate would need more generation equivalents for a certain size reduction than a diatom with a low maximum division rate, but the periods required for reduction would be approximately equal irrespective of maximum division rate.     

THRIPS-MEDIATED SELF-POLLINATION OF TWO FACULTATIVELY XENOGAMOUS WETLAND SPECIES

Thrips and/or aphids played an important role in the self-pollination of two facultatively xenogamous herbs that inhabit wetlands in northwestern Iowa. In both Ranunculus sceleratus and Potentilla rivalis the fruit set and/or successful pollination of plants that were sprayed with malathion to kill thrips and aphids and caged to exclude typical flower visitors was substantially and significantly lower than that of open-pollinated and caged plants. We observed pollenbearing thrips and aphids on the flowers. The high fruit sets of emasculated flowers of R. sceleratus showed that insects moved pollen between flowers, and our observations of bees collecting pollen and moving between plants suggest that cross-pollination can occur in both species.