Vibrio alginolyticus,a tetrodotoxin-producing bacterium,in the intestines of the fish Fugu vermicularis vermicularis

The influence of nutrient deprivation on cell-cycle progression was examined in two phytoplankton species, the diatom Thalassiosira weissflogii (actin) and the coccolithophorid Hymenomonas carterae (cocco II). The diatom was starved for nitrogen, silicon or both, whereas only nitrogen limitation was examined in H. carterae. In both species, nitrogen-starved cells were arrested in the early part of the cell cycle (G₁ phase). In the diatom, silicon-starvation arrested cells in late G₁ phase and also in the last part of the cell cycle (G₂+M). In all cases, cell-cycle arrest could be reversed by addition of fresh medium, but cell-cycling times during the first generation were increased in comparison to those in nutrient replete, steady-state growth conditions. These results supply evidence for simultaneous dual-nutrient limitation of population growth and provide a mechanistic interpretation for the division patterns observed in cultures where nutrients are supplied periodically.     

Role of sinking in diatom life-history cycles: ecological,evolutionary and geological significance

Rapid mass sinking of cells following diatom blooms, observed in lakes and the sea, is argued here to represent the transition from a growing to a resting stage in the life histories of these algae. Mass sinking is of survival value in those bloom diatoms that retain viability over long periods in cold, dark water but not in warm, nutrient-depleted surface water. Mechanisms for accelerating sinking speed of populations entering a resting or seeding mode are proposed. Previously unexplained features of diatom form and behaviour take on a new meaning in this context of diatom seeding strategies. Diatoms have physiological control over buoyancy as declining growth is accompanied by increasing sinking rates, where the frustule acts as ballast. Increased mucous secretion in conjunction with the cell protuberances characteristic of bloom diatoms leads to entanglement and aggregate formation during sinking; the sticky aggregates scavenge mineral and other particles during descent which further accelerates the sinking rate. Such diatom flocs will have sinking rates of 100 m d^-1 or more. This is corroborated by recent observations of mass phytoplankton sedimentation to the deep sea. This mechanism would explain the origin of marine snow flocs containing diatoms in high productivity areas and also the well-known presence of a viable deep sea flora. That mortality is high in such a seeding strategy is not surprising. A number of species-specific variables pertaining to size, morphology, physiology, spore formation and frustule dissolution rate will determine the sinking behaviour and thus control positioning of resting stages in the water column or on the bottom. It is argued that sinking behaviour patterns will be environmentally selected and that some baffling aspects of diatom form and distribution can be explained in this light. Rapid diatom sedimentation is currently believed to be mediated by zooplankton faecal pellets, particularly those of copepods. This view is not supported by recently published observations. I speculate that copepod grazing actually retards rather than accelerates vertical flux, because faecal pellets tend to be recycled within the surface layer by the common herbivorous copepods. Egestion of undigested food by copepods during blooms acts as a storage mechanism, as ungrazed cells are likely to initiate mass precipitation and depletion of the surface layer in essential elements. Unique features of diatoms are discussed in the light of their possible evolution from resting spores of other algae. An evolutionary ecology of pelagic bloom diatoms is deduced from behavioural and morphological characteristics of meroplanktonic and tychopelagic forms. Other shell-bearing protistan plankters share common features with diatoms. Similar life-history patterns are likely to be present in species from all these groups. The geological significance of mass diatom sinking in rapidly affecting transfer of biogenic and mineral particles to the sea floor is pointed out.     

Responses of marine fouling communities to sewage abatement in Kaneohe Bay,Oahu, Hawaii

The development of marine fouling communities was followed as part of a sewage abatement study in Kaneohe Bay (Oahu, Hawaii). Panels immersed near the outfall during sewage discharge were dominated by dense mats of blue-green algae and diatoms. This mat was significantly lighter at this station when sewage did not flow into the bay, and it never developed on panels within the same sector of the bay but away from the influence of the outfall. Taxonomic diversity, and the frequency of filamentous algae and the serpulidHydroides elegans were reduced near the outfall during discharge. It is suggested that this reduction in diversity and abundance of certain key foulers is due to competitive exclusion by the dense blue-green and diatom mats, and is not a direct effect of the sewage effluent.     

Persistent blooms of surf diatoms along the Pacific coast,USA

Productivity was studied in two diatom species, Chaetoceros armatum T. West and Asterionella socialis Lewin and Norris, which form persistent dense blooms in the surf zone along the Pacific coast of Washington and Oregon, USA. Past observations have shown that surf-diatom standing stock usually declines in summer along with concentrations of nitrate and ammonium. Using the ¹⁴C method, photosynthetic rates in natural surf samples were measured monthly for one year (October 1981 through September 1982) at a study site on the Washington coast. Also measured were temperature, salinity, dissolved nutrients, particulate carbon and nitrogen (used as estimates of phytoplankton C and N), and chlorophyll a. Assimilation numbers (P _max) were higher in summer (5 to 8 g C g^-1 chl a h^-1) than in winter (3 to 4gC). Specific carbon incorporation rates (µ_max) showed no obvious seasonality, mostly falling within the range of 0.09 to 0.13 g C g^-1 C(POC) h^-1. The discrepancy between the seasonal trends for chlorophyll-specific and carbon-specific rates reflects a change in the carbon-to-chlorophyll ratio. Because of seasonal differences in daylength and light intensity, daily specific growth rates () are thought to be higher in summer than in winter. Neither ammonium enrichment assays nor particulate carbon-to-nitrogen ratios provided convincing evidence for nitrogen limitation during summer, and the observed changes in diatom abundance cannot be explained on this basis. Both the high diatom concentrations and their seasonal variations probably are due mainly to factors other than growth rates; two factors considered important are diatom flotation and seasonal changes in wind-driven water transport. C. armatum usually dominates the phytoplankton biomass in the surf zone, and evidence suggests that this species is strongly dominant in terms of primary production.Contribution No. 1391 of the School of Oceanography, University of Washington, Seattle, Washington, USA     

“Good” and “bad” diatoms: development,growth and juvenile mortality of the copepod <Emphasis Type="Italic">Temora longicornis</Emphasis> on diatom diets

We measured development, growth and juvenile mortality of the common copepod Temora longicornis on 11 different monospecific diatom diets in order to estimate (1) how common the negative effects of diatoms are on the development of this copepod and (2) whether the arrested development is connected to deleterious polyunsaturated aldehydes (PUA) or food nutritional quality. Four diatom species (Thalassiosira weissflogii, Thalassiosira rotula CCMP1647, Leptocylindricus danicus CCPM469 and Skeletonema costatum CCMP1281) supported complete development, whereas development failed in or before metamorphosis on seven diatom species/strains (Chaetoceros affinis CCMP158, C. decipiens CCMP173, C. socialis, T. rotula CCMP1018, Thalassiosira pseudonana CCMP1010 and CCMP1335). However, four out of these seven species were not ingested by nauplii, either due to morphology (Chaetoceros spp.) or large size (T. pseudonana CCMP1010). The growth rate did not correlate with the ingestion rate of PUA, neither with ingestion of food mineral (nitrogen) nor with biochemical (polyunsaturated fatty acids, sterols) components. We show that, although some diatoms are of inferior food quality, this is unlikely to be connected to toxicity or due to a direct limitation by a single food nutritional compound.     

甘河着生藻类群落结构及其与环境因子的关系

2018年8月(丰水期),在嫩江支流甘河,调查了19个点位的着生藻类群落结构和水环境特征。并运用聚类分析、Mcnaughton优势度分析和IBD(biological diatom index,硅藻生物指数)3种方法研究了甘河着生硅藻群落结构和优势种清洁状况,使用主成分分析、Spearman相关分析和冗余分析等统计分析方法研究了甘河着生硅藻群落和环境因子的响应关系。结果表明:甘河共鉴定出着生藻类90种(属),以硅藻门、绿藻门和蓝藻门为主,生物量丰富。丰水期甘河水生态环境质量状况较为健康,着生硅藻主要由清洁种构成,其中Achnanthidium minutissimum优势度0.48,为绝对优势种。聚类分析结果表明,甘河采样点位可分为3组;其中组1和组2点位主要位于甘河上、中游,上、中游水质、生境质量较好,健康程度较高,优势种均为清洁种,上游Achnanthidium minutissimum优势度达到了0.64;组3点位均位于甘河下游,下游人为干扰较强,健康程度较其余两组偏低,Nitzschia palea和Nitzschia paleaeformis等耐污种在组3的优势度较高。导致甘河着生硅藻群落清洁度由上游到下游逐渐变差的主要水环境因子为电导率、总磷和高锰酸盐指数,其中影响中游硅藻群落结构变化的主要水环境因子为高锰酸盐指数,影响下游硅藻群落结构变化的主要水环境因子为电导率和总磷。由此可见,着生硅藻群落可以较好的指示甘河流域水体有机污染程度和营养富集程度。     

Historical processes constrain patterns in global diatom diversity

There is a long-standing belief that microbial organisms have unlimited dispersal capabilities, are therefore ubiquitous, and show weak or absent latitudinal diversity gradients. In contrast, using a global freshwater diatom data set, we show that latitudinal gradients in local and regional genus richness are present and highly asymmetric between both hemispheres. Patterns in regional richness are explained by the degree of isolation of lake districts, while the number of locally coexisting diatom genera is highly constrained by the size of the regional diatom pool, habitat availability, and the connectivity between habitats within lake districts. At regional to global scales, historical factors explain significantly more of the observed geographic patterns in genus richness than do contemporary environmental conditions. Together, these results stress the importance of dispersal and migration in structuring diatom communities at regional to global scales. Our results are consistent with predictions from the theory of island biogeography and metacommunity concepts and likely underlie the strong provinciality and endemism observed in the relatively isolated diatom floras in the Southern Hemisphere.     

新疆博斯腾湖表层沉积物中硅藻分布特征及影响因素分析

通过采集新疆博斯腾湖不同水深、矿化度、营养盐浓度及pH环境下的湖泊表层沉积物,提取硅藻并进行种属组合分析,研究了中国西北干旱区半封闭淡水湖泊中硅藻的分布特征,并且运用降维对应分析（DCA）,探讨了影响博斯腾湖现生硅藻组合特征的主要水质环境因子及硅藻种属对水体理化性质变化的响应。研究结果表明,在博斯腾湖硅藻组合受湖泊水深、矿化度影响明显分区：河口区受水体扰动作用影响明显,硅藻浓度变化大,以Fragilaria属为主;黄水湾区硅藻分布受矿化度及pH影响显著,以Mastogloia smithii、Navicula anglica、Gomphonema parvulum等底栖、喜碱性、微咸水种为主,淡水种Fragilaria属的含量明显低于其它区域,其含量的高低指示了水体矿化度的变化;在湖心区硅藻组合主要响应于水深变化,当水深增加时,Cyclotella radiosa、C.meneghiniana、Fragilaria bidens等浮游种含量显著增加,Mastogloria smithii、Gomphonema parvulum等底栖种含量明显降低或消失,且发现Fragilaria属在5.4～7.8m水深处生长繁盛。研究结果为建立中国西北干旱区的硅藻-环境因子关系数据库、重建本地区过去环境提供了理论依据。     

Inhibition of diatom photosynthesis by germanic acid: Separation of diatom productivity from total marine primary productivity

Germanic acid was shown to inhibit photosynthetic ¹⁴CO₂ uptake in marine diatoms. Inhibition was not complete even at concentrations of 20 mg Ge/1 nor in cultures incubated for extended periods of time (up to 24 h). The decrease in photosynthesis due to Ge(OH)₄ was independent of the stage of growth of the diatom culture. At 0.5 and 1.0 mg Ge/1, the degree of inhibition was dependent on the concentration of Si(OH)₄ in the medium. At 5 and 10 mg Ge/1, inhibition was not affected by Si(OH)₄ concentrations as high as those found in the sea-120 g-at Si/1. The effect of Ge(OH)₄ on photosynthesis is specific for diatoms; other marine phytoplankton were not inhibited. In mixed cultures of diatoms and marine flagellates, the reduction in ¹⁴CO₂ fixation upon addition of Ge(OH)₄ was used to calculated the proportion of diatom photosynthesis to total photosynthesis, and calculated proportions agreed well with actual proportions. Inhibition by Ge(OH)₄ was also used to estimate the percent of diatom photosynthesis in a natural marine community, and this was compared with the diatom portion of the crop. Diatom photosynthesis was higher than one might expect from crop figures, although both diatom photosynthesis and diatom numbers in the crop were low.     

A comprehensive evaluation of the potential chemical defenses of antarctic ascidians against sympatric fouling microorganisms

The present study analyzed the bioactivity of whole body extracts from six solitary and eight colonial ascidian taxa against 20 sympatric bacterial isolates and one sympatric diatom species from the Western Antarctic Peninsula. Ascidians had crude lipophilic and hydrophilic extracts assayed against 20 bacterial strains. The lipophilic extract of one ascidian caused growth inhibition in all bacterial isolates at 3× tissue-level concentrations. The lipophilic and hydrophilic extracts were fractionated into seawater-soluble and insoluble fractions and assayed at three concentrations against a sympatric diatom species. Significant diatom mortality was detected at 3× and 1× concentrations in all but one ascidian taxon. Lipophilic fractions caused higher diatom mortality than hydrophilic extracts. The specificity of secondary metabolites against diatom fouling and the lack of activity against bacteria suggest high selective pressure for chemical defenses against diatom fouling or the potential that bacterial pathogens are controlled by the ascidian immune system.     

Mosaicism of microplankton communities in the northern Strait of Georgia,British Columbia

The northern Strait of Georgia, British Columbia, Canada, was sampled five times between March and September 1986, employing a grid of stations spanning the strait. The spring diatom bloom was not observed and may have been suppressed by a combination of wind exposure and grazing by microzooplankton, notably non-loricate oligotrichs and the dinoflagellateGyrodinium spirale. Macrozooplankton were not studied. By June, the ecosystem was dominated by flagellates, and ciliate biomass was three times greater than that of the nanoflagellates. The photosynthetic dominantHeterosigma akashiwo was possibly inhibiting diatom growth, as environmental conditions were conducive to the latter's growth. During late summer (August–September), surface waters became nutrient-depleted and a mosaic of organismal types was formed. The pattern included diatoms on the more turbulent west side, nanoflagellates on the more stable east side, and dinoflagellates in the north associated with frontal boundaries. The driving force behind the mosaic appeared to be tidal turbulence and was most effective at times of advanced water-column stratification. Other points of interest were subsurface concentrations ofChaetoceros species, which were perhaps maintained at the pycnocline by entrainment in areas of high tidal turbulence, and a large ratio of heterotrophic to photoautotrophic biomass, possibly due to mixotrophy in ciliates.Please address all correspondence and requests for reprints to Dr. Taylor at the Department of Botany     

Specific features and origin of the Caspian Sea diatom flora

The systematic composition of the diatom flora of the Caspian Sea is original, but poor, especially in marine species. At present, 286 species, varieties and forms, belonging to 46 genera and 4 orders are known to exist in the Caspian Sea. Of this number, 77 species and varieties (27%) are planktonic and 209 (63%) benthonic. The phytoplankton of the Caspian Sea is wholly neritic. In regard to salinity, a characteristic of the Caspian diatom flora is that it is composed of three different groups: marine, brackish, and fresh-water species. The brackish water group is of great interest, as it includes many diatoms which are known only from this water body (endemisms). The poor species-composition of the diatom flora is accounted for by variations in the hydrogical conditions of the Caspian Sea in the geological past. The formation of the flora is based on such sources as the ancient flora remains of the Upper-Tertiary Caspian-Black Sea, the younger elements of the Pliocene Sea during its isolation period, and the immigrants entering the Caspian Sea during the short period of its communication with the Black Sea.     

Blooms of surf-zone diatoms along the coast of the Olympic Peninsula,Washington. X. Chemical composition of the surf diatom Chaetoceros armatum and its major herbivore,the Pacific razor clam Siliqua patula

The surf diatom Chaetoceros armatum T. West, collected from its natural habitat along the Washington coast (USA), had a large inorganic component in the form of a clay coat (consisting of clay minerals, illite and montmorillonite) surrounding the cell chains: 63% of the dry weight of the natural material collected in November was inorganic. The organic fraction was composed of 67.6% lipid, 29.7% protein, and 1.3% carbohydrate. Culture cells were likewise high in protein and lipid and low in carbohydrate. Traces of chitin found in field samples were probably a contaminant, since chitin was absent from cultured cells. This diatom species serves as a major food source throughout the year for the Pacific razor clam Siliqua patula Dixon, which inhabits these same beaches. Protein constituted 47% and lipid 42% of the dry weight of razor clam tissue. The fatty acid distribution in the diatom lipid resembled that previously reported for other diatom species; similarly, the fatty acid distribution of the clam lipid was similar to that previously reported for other bivalve molluscs. The clam fatty acids differed in chain length and degree of saturation from those extracted from its food source, indicating an active fatty acid metabolism in the clam.Contribution No. 995 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Larval metamorphosis of the sea urchins,<Emphasis Type="Italic"> Pseudocentrotus depressus</Emphasis> and<Emphasis Type="Italic"> Anthocidaris crassispina</Emphasis> in response to microbial films

In Japan, mass-production of sea urchin juveniles involves the culture of periphytic diatom films on plastic plates in 5- to 15-tonne tanks for the induction of larval metamorphosis. This study focused on the larval response of sea urchins, Pseudocentrotus depressus and Anthocidaris crassispina, to natural microbial films in the sea and diatom-based films formed in the tanks. The effect of diatoms and bacteria on larval metamorphosis was also examined using laboratory-cultured diatom-based films in the presence of germanium dioxide and antibiotics during film culture. Furthermore, the nature of the cue of the cultured diatom-based film was also investigated. Results showed that P. depressus and A. crassispina metamorphosed both on natural microbial films and diatom-based films in a tank. In the sea, the metamorphosis (%) of P. depressus increased gradually in accordance with the immersion period of film formed on glass slides, whereas the larval metamorphosis of A. crassispina had a bell-shaped response curve. In the tank, although the diatom-based films showed a low inducing activity for larval metamorphosis of A. crassispina, the metamorphosis of P. depressus larvae increased linearly in accordance with the diatom density. These results suggest that diatom-based films could promote the larval metamorphosis of P. depressus, but are less important in A. crassispina. In a simultaneous larval assay (May), P. depressus showed a higher percentage of metamorphosis than A. crassispina. We concluded that the former is more sensitive to diatom-based film than the latter and that this is due to differences in their natural habitats. For laboratory-cultured diatom-based film, both species of sea urchins showed a similar response, in which reduction in diatom and bacteria density resulted in a decrease in the original inducing activity. There seems to be a synergistic effect between diatom and bacteria in inducing larval metamorphosis. Films subjected to treatment with 0.1 N HCl were no longer inductive for either sea urchin, while those films treated with 40^°C heat or EtOH (5% and 10% EtOH) showed a significant reduction in the inducing activity. Thus the surface-associated cues may be highly susceptible to the above treatments.Communicated by T. Ikeda, Hakodate     

Phytoplankton carbon dynamics during a winter-spring diatom bloom in an enclosed marine ecosystem: primary production,biomass and loss rates

Phytoplankton production, standing crop, and loss processes (respiration, sedimentation, grazing by zooplankton, and excretion) were measured on a daily basis during the growth, dormancy and decline of a winter-spring diatom bloom in a large-scale (13 m³) marine mesocosm in 1987. Carbonspecific rates of production and biomass change were highly correlated whereas production and loss rates were unrelated over the experimental period when the significant changes in algal biomass characteristic of phytoplankton blooms were occurring. The observed decline in diatom growth rates was caused by nutrient limitation. Daily phytoplankton production rates calculated from the phytoplankton continuity equation were in excellent agreement with rates independently determined using standard ¹⁴C techniques. A carbon budget for the winter bloom indicated that 82.4% of the net daytime primary production was accounted for by measured loss processes, 1.3% was present as standing crop at the end of the experiment, and 16.3% was unexplained. Losses via sedimentation (44.8%) and nighttime phytoplankton respiration (24.1%) predominated, while losses due to zooplankton grazing (10.7%) and nighttime phytoplankton excretion (2.8%) were of lesser importance. A model simulating daily phytoplankton biomass was developed to demonstrate the relative importance of the individual loss processes.     

Photosynthetic performance,oxidative damage and antioxidants in <Emphasis Type="Italic">Cylindrotheca closterium</Emphasis> in response to high irradiance,UVB radiation and salinity

Cylindrotheca closterium is a common marine diatom living in intertidal environments where it can be present both in the water column and on sediments, depending on the tidal regime. In the present work this diatom was employed to investigate the responses to desiccation and to increase in PAR and UVB intensity, as occurs during emersion. Under these circumstances, the production of active oxygen species (AOS) may be enhanced resulting in an oxidative stress. Stress responses in this species were measured by exposing it to normal (30) and double salinity (60), supplying light of low or high intensity for 12 h, in the latter case either without or with moderate dose rates of UVB. Pulse amplitude modulated fluorometry was used to measure Chl a autofluorescence (F ₀), an index of photosynthetic efficiency of PSII (F _v/F _m) and the relative electron transfer rate (rETR). The oxidative stress was evaluated by analysing GSH pools and SOD activity. It was observed that at double salinity and under low light, intracellular pools of reduced glutathione (GSH) were higher than under the two conditions of high light without and with UVB at both salinities. The antioxidative defence activity of superoxide dismutase (SOD) was far higher under hypersaline conditions. The oxidative damage was evaluated as protein and lipid damage. The results showed that it expressed itself mainly through protein peroxidation: at normal salinity relative protein carbonyl content was (a) twice as high as in cells grown at double salinity, and (b) three times as high under UVB. Total unsaturated lipid contents doubled under hypersalinity conditions. The lipid peroxidation marker malondialdehyde showed the strongest response to low light and UVB at salinity value of 60. Lipid peroxide content was significantly higher at salinity of 60 compared to normal salinity and was the highest under low light and high light with UVB. The simulated emersion condition of the diatom seems to lead to the establishment of a balance between damage and repair, expressed mainly as (a) oxidative protein damage at normal salinity, in particular due to UV radiation, (b) sufficient protection by SOD activity mainly under hypersaline conditions.     

Adaptive significance of phytoplankton stickiness with emphasis on the diatom Skeletonema costatum

Diatom aggregate formation was analyzed using coagulation theory. Population dynamics models show that coagulation has an important impact on species succession during diatom blooms. When different species collide and form mixed aggregates this process causes interspecific interference competition within the diatom community. The outcome can be predicted by a set of simple differential equations. For a twospecies system the equations reduce to the Lotka-Volterra two-species competition model. The outcome of this interference competition depends on species-specific growth rates, cell sizes, stickiness and on the species composition of the seeding populations of a bloom. Due to mutual flocculation some species may disappear from the environment. Small and fast growing diatoms are favoured by high stickiness coefficients. The impact of stickiness on species succession was found to be most pronounced in eutrophic and hydrographically isolated environments. The sticking properties of the diatom Skeletonema costatum are discussed in an evolutionary context; we suggest that mutual coagulation increases the abundance of S. costatum relative to other diatom species in coastal areas. The model was tested on field data, and the predicted dynamics of a spring bloom was very similar to that observed.     

Learning habitat models for the diatom community in Lake Prespa

Habitat suitability modelling studies the influence of abiotic factors on the abundance or diversity of a given taxonomic group of organisms. In this work, we investigate the effect of the environmental conditions of Lake Prespa (Republic of Macedonia) on diatom communities. The data contain measurements of physical and chemical properties of the environment as well as the relative abundances of 116 diatom taxa. In addition, we create a separate dataset that contains information only about the top 10 most abundant diatoms. We use two machine learning techniques to model the data: regression trees and multi-target regression trees. We learn a regression tree for each taxon separately (from the top 10 most abundant) to identify the environmental conditions that influence the abundance of the given diatom taxon. We learn two multi-target regression trees: one for modelling the complete community and the other for the top 10 most abundant diatoms. The multi-target regression trees approach is able to detect the conditions that affect the structure of a diatom community (as compared to other approaches that can model only a single target variable). We interpret and compare the obtained models. The models present knowledge about the influence of metallic ions and nutrients on the structure of the diatom community, which is consistent with, but further extends existing expert knowledge.     

Copepod recruitment and food composition: do diatoms affect hatching success?

Laboratory experiments were conducted to differentiate between factors controlling the hatching success of copepod eggs. Factors that could affect viability of eggs; viz food quality, female condition and external factors were investigated. In a series of experiments the copepod Acartia tonsa Dana was fed several different diets while egg production and hatching success were monitored. The diet was analysed for fatty acid content as an indicator of food quality. Both egg production and hatching were found to be affected by the nutritional quality of the food. Hatching was also highly dependent on female fertility. External effects were tested by exposing eggs to diatom extracts. Negative effects were only evident at high extract concentrations, but disappeared when aeration was supplied to the solution. Oxygen measurements showed that failure to hatch was due to hypoxia in the extracts. No inhibitory or toxic effects of diatom cell components on hatching could be found.     

Silicon and the ecology of marine plankton diatoms. I. Thalassiosira pseudonana (Cyclotella nana) grown in a chemostat with silicate as limiting nutrient

The small marine plankton diatom Thalassiosira pseudonana Hasle and Heimdal (Guillard's clone 3H) was grown in chemostats with silicate as the limiting nutrient. The calculated maximum growth rates were comparable to those previously reported for this species. The silica content of the diatom shells varied with the growth rate. As the growth rate approached zero, there were still measurable quantities of residual reactive silicate in the medium. In one of the two chemostats used, silicate assimilation by the cells was inefficient due to some unknown internal or external factor. In the other chemostat, statistically calculated half-saturation constants of growth were in the range of 0.5 to 0.8 g—at Si/l, depending on which kind of correction was made for residual silicate. Half-saturation constants of steady-state mean silicate uptake per cell and hour, calculated in a similar fashion, were in the range of 1.4 to 2.6 g—at Si/l. These results indicate that the silicate concentrations causing a reduced silicate uptake by this species in nature do not necessarily result in a correspondingly reduced growth rate. Growth in coastal waters is likely to become seriously limited by a shortage of silicate only when most of the silicate originally present has been removed in the course of a diatom bloom.