Marine Phytoplankton Species Research Articles

Utilization of phosphate diester by the marine diatom Chaetoceros ceratosporus

Utilization of phosphate diester (PDE) and phosphodiesterase (PDEase) production by 5 marine phytoplankton s pecies were examined in the laboratory to evaluate the contribution of PDE to the growth of phytoplankton. Among the 5 marine phytoplankton species tested, only Chaetoceros ceratosporus was able to use the PDE compound (bis(p-nitrophenyl) phosphate) effectively as a sole phosphorus source. In addition, C. ceratosporus simultaneously produced both PDEase and alkaline phosphatase (APase) at almost equal activity-levels under the phosphate-deficient condition. These results suggest that PDE compounds presumably play an important role as a phosphorus source for PDEase-producing phytoplankton in coastal environments. Many studies have generally focused on dissolved inorganic phosphate (orthophosphate: PO4 3- ) as the major phosphorus source for the growth of phytoplankton to evaluate the factors controlling occurrences of algal blooms (Nalewajko and Lean, 1980). However, the

Characterization of Fe/Mn−Superoxide Dismutase from DiatomThallassiosira weissflogii: Cloning, Expression, and Property

A cDNA clone of 1114 bp encoding a putative Mn-superoxide dismutase (Mn-SOD) from diatom Thallassiosira weissflogii was cloned by the PCR technique. Nucleotide sequence analysis of this cDNA clone revealed that it was translated into 201 amino acid residues. When the sequence was compared with Mn-SODs from Vibrio mimicus and Escherichia coli, as well as two Fe-SODs from E. coli and Photobacterium leiognathi, this SOD showed higher homology to Mn-SOD. The amino acid residues required to coordinate the single manganese ion were conserved in all reported Mn-SOD sequences. This cDNA was introduced in an expression vector, pET-20b(+), and transformed into E. coli BL21(DE3)pLysS. The expressed SOD protein was then purified by a His-tag column. The recombinant enzyme was heated at 55 degrees C with a time-dependent assay; the time interval for 50% inactivation was 23 min, and its thermal inactivation rate constant K(d) was 3.03 x 10(-)(2) min(-)(1). The enzyme was inactivated either in acidic pH (below 4.0) or in the presence of imidazole (above 1.6 M) and had only a moderate effect under SDS (above 4%), whereas it was not affected under an alkaline pH (above 9.0). The atomic absorption spectrometric assay showed that 0.6 atom of iron/manganese (3:1) was present in each subunit of SOD. Reconstitution study was suggested that diatom SOD was cambialistic (Fe/Mn)-SOD. The finding of this SOD cDNA could be used for a reference in comparing the differences among marine phytoplankton species and as a probe to detect the transcription level of this enzyme, which can be applied in cosmetics for skin protection or defending unesthetic effects caused by oxygen-containing free radicals.

Modelling allelopathy among marine algae

Allelopathy among microalgae has been modelled in order to fit the dynamics of mixed cultures of Chrysocromulina polylepis and Heterocapsa triquetra, carried out with three different initial concentrations of the allelopathic species C. polylepis. The experimental data have been analysed with a simple Lotka–Volterra type model including an allelopathic term. Based on the population dynamics of the two species in unialgal and mixed cultures, the model has been simplified and solved analytically. The best fit of the allelopathy parameter ( γ ≈ 1 0 − 5 to 1 0 − 6 ) provides a good agreement between the theoretical curve and the experimental data for the two highest initial concentrations of C. polylepis but, interestingly, shows a less accurate fit for low initial concentrations of the toxic alga. Following the same procedures, a modified model, in which the allelopathic effect is dependent on the square of the concentration of the toxic alga, has been adjusted to the experimental data. This modified model presents a good fit for all the range of initial concentrations of the toxic alga. These results allow a quantification of the strength of the allelopathic interaction between two marine phytoplankton species. The growth curve of the non-toxic alga is significantly affected by the allelopathy term only after the toxic alga has reached relatively high concentrations. This supports field observations suggesting that, at least in the case of C. polylepis, allelopathy may be important once a bloom is well developed, but is not likely to be a key factor in initial phases of the proliferation.

INFLUENCE OF LOW LIGHT AND A LIGHT: DARK CYCLE ON NO3– UPTAKE, INTRACELLULAR NO3–, AND NITROGEN ISOTOPE FRACTIONATION BY MARINE PHYTOPLANKTON1

The nitrogen isotope enrichment factor (ɛ) of four species of marine phytoplankton grown in batch cultures was determined during growth in continuous saturating light, continuous low light, and a 12:12‐h light:dark cycle, with nitrate as a nitrogen source. The low growth rate that resulted from low irradiance caused an increased accumulation of the intracellular nitrate pool and/or a reduction in cell volume and was correlated to a species‐specific increase in the measured ɛ value, compared with the saturating light conditions. The largest response was in the diatom Thalassiosira weissflogii (Grun.) Fryxell et Hasle, which showed a nearly 3‐fold increase between high and low light conditions (6.2–15.2‰). The smallest response was in T. pseudonana (Hustedt) Hasle et Heimdal, which showed no change in the ɛ value of approximately 5‰ in both high and low light conditions. There was significant but smaller increases in the ɛ value for the diatom T. rotula Meunier (2.7–5.6‰) and the prymnesiophyte Emiliania huxleyi (Lohm.) Hay et Mohler (4.5–9.4‰) between high and low light levels. In the light:dark experiments, all three diatoms but not the prymnesiophyte exhibited an increase in ɛ. This increase was linked to the ability of diatoms to assimilate nitrate at night. The results of the these experiments suggest that the light regime influences the relative uptake, assimilation, and efflux rates of nitrate and results in differences in the expression of the isotope effect by the enzyme nitrate reductase. Therefore, variations in nitrate isotope fractionation in nature can be more accurately interpreted when the light regime and species composition are taken into consideration.

Preferential PCR amplification of parasitic protistan small subunit rDNA from metazoan tissues.

A "universal non-metazoan" polymerase chain reaction (UNonMet-PCR) that selectively amplifies a segment of nonmetazoan Small Subunit (SSU) rDNA gene was validated. The primers used were: 18S-EUK581-F (5'-GTGCCAGCAGCCGCG-3') and 18S-EUK1134-R (5'-TTTAAGTTTCAGCCTTGCG-3') with specificity provided by the 19-base reverse primer. Its target site is highly conserved across the Archaea, Bacteria, and eukaryotes (including fungi), but not most Metazoa (except Porifera, Ctenophora, and Myxozoa) which have mismatches at bases 14 and 19 resulting in poor or failed amplification. During validation, UNonMet-PCR amplified SSU rDNA gene fragments from all assayed protists (n = 16 from 7 higher taxa, including two species of marine phytoplankton) and Fungi (n = 3) but amplified very poorly or not at all most assayed Metazoa (n = 13 from 8 higher taxa). When a nonmetazoan parasite was present in a metazoan host, the parasite DNA was preferentially amplified. For example, DNA from the parasite Trypanosoma danilewskyi was preferentially amplified in mixtures containing up to 1,000 x more goldfish Carassius auratus (host) DNA. Also, the weak amplification of uninfected host (Chionoecetes tanneri) SSU rDNA did not occur in the presence of a natural infection with a parasite (Hematodinium sp.). Only Hematodinium sp. SSU rDNA was amplified in samples from infected C. tanneri. This UNonMet-PCR is a powerful tool for amplifying SSU rDNA from non-metazoan pathogens or symbionts that have not been isolated from metazoan hosts.

The evolutionary inheritance of elemental stoichiometry in marine phytoplankton.

Phytoplankton is a nineteenth century ecological construct for a biologically diverse group of pelagic photoautotrophs that share common metabolic functions but not evolutionary histories. In contrast to terrestrial plants, a major schism occurred in the evolution of the eukaryotic phytoplankton that gave rise to two major plastid superfamilies. The green superfamily appropriated chlorophyll b, whereas the red superfamily uses chlorophyll c as an accessory photosynthetic pigment. Fossil evidence suggests that the green superfamily dominated Palaeozoic oceans. However, after the end-Permian extinction, members of the red superfamily rose to ecological prominence. The processes responsible for this shift are obscure. Here we present an analysis of major nutrients and trace elements in 15 species of marine phytoplankton from the two superfamilies. Our results indicate that there are systematic phylogenetic differences in the two plastid types where macronutrient (carbon:nitrogen:phosphorus) stoichiometries primarily reflect ancestral pre-symbiotic host cell phenotypes, but trace element composition reflects differences in the acquired plastids. The compositional differences between the two plastid superfamilies suggest that changes in ocean redox state strongly influenced the evolution and selection of eukaryotic phytoplankton since the Proterozoic era.

Individual and Mixture Toxicity of Three Pesticides; Atrazine, Chlorpyrifos, and Chlorothalonil to the Marine Phytoplankton Species Dunaliella tertiolecta

This study analyzed the toxicity of three pesticides (the herbicide atrazine, the insecticide chlorpyrifos and the fungicide chlorothalonil) individually, and in two mixtures (atrazine and chlorpyrifos; atrazine and chlorothalonil) to the marine phytoplankton species Dunaliella tertiolecta (Chlorophyta). A standard 96 h static algal bioassay was used to determine pesticide effects on the population growth rate of D. tertiolecta. Mixture toxicity was assessed using the additive index approach. Atrazine and chlorothalonil concentrations ≥ 25 µg/L and 33.3 µg/L, respectively, caused significant decreases in D. tertiolecta population growth rate. At much higher concentrations (≥ 400 µg/L) chlorpyrifos also elicited a significant effect on D. tertiolecta population growth rate, but toxicity would not be expected at typical environmental concentrations. The population growth rate EC50 values determined for D. tertiolecta were 64 µg/L for chlorothalonil, 69 µg/L for atrazine, and 769 µg/L for chlorpyrifos. Atrazine and chlorpyrifos in mixture displayed additive toxicity, whereas atrazine and chlorothalonil in mixture had a synergistic effect. The toxicity of atrazine and chlorothalonil combined was approximately 2 times greater than that of the individual chemicals. Therefore, decreases in phytoplankton populations resulting from pesticide exposure could occur at lower than expected concentrations in aquatic systems where atrazine and chlorothalonil are present in mixture. Detrimental effects on phytoplankton population growth rate could impact nutrient cycling rates and food availability to higher trophic levels. Characterizing the toxicity of chemical mixtures likely to be encountered in the environment may benefit the pesticide registration and regulation process.

The decomposition of hydrogen peroxide by marine phytoplankton

All nine species of marine phytoplankton tested (a cyanobacterium: Synechococcus sp., three diatoms: Chaetoceros simplex, Thalassiosira oceanica and Skeletonema costatum, two prymnesiophytes: Pleurochrysis carterae and Isochrysis galbana, a prasinophyte: Tetraselmis sp., a green alga: Dunaliella tertiolecta, and a dinoflagellate: Amphidinium carterae) were able to decompose hydrogen peroxide in the dark. Since these phytoplankton species can be found widely in a variety of marine sub-environments, this indicates that the dark decomposition of hydrogen peroxide by phytoplankton is a general phenomenon in the oceans. The decomposition rates were first order with respect to the concentration of hydrogen peroxide and biomass. The second-order rate constants for these nine species of phytoplankton ranged between 2 × 10 –4 and 2.7 × 10 –2 μg Chl- a –1 1 h –1. Synechococcus sp. and S. costatum were the most efficient, while P. carterae and D. tertiolecta were the least efficient decomposers. While the magnitudes and patterns in the changes were species-dependent, in general, increasing salinity, temperature, the presence of light and the depletion of nutrients enhanced the decomposition of hydrogen peroxide. The effect of growth phase was small.

Isoprene production by Prochlorococcus, a marine cyanobacterium, and other phytoplankton

The oceans are a small source of light (C 2–C 6) non-methane hydrocarbons (NMHC), which influence the photo-oxidant chemistry of the remote marine atmosphere. Previous work has shown that water column sources of alkenes include photochemical processes, and that various phytoplankton species can produce isoprene. However, only a few phytoplankton species have been studied, and no assessment has been performed of the effects of other pelagic microorganisms on NMHC cycling. The dependence of phytoplanktonic isoprene production on light, temperature, and organism size has also not been investigated. In this work, laboratory cultures of five different marine phytoplankton species ( Prochlorococcus, Synechococcus, Micromonas pusilla, Pelagomonas calceolata, and Emiliania huxleyi) were examined for NMHC production capabilities. All species were found to produce isoprene at constant rates during the balanced exponential growth phase; rates ranged from 1×10 −21 to 4×10 −19 mol cell −1 day −1 over all cell species and growth conditions tested. No other NMHC was consistently produced or consumed by these cells. The presence of heterotrophic bacteria in phytoplankton cultures had no effect on isoprene production rates per phytoplankton cell. A positive allometric relationship was observed between isoprene production rate and cell volume; highest production rates were found for the largest cell, E. huxleyi, and lowest rates for Prochlorococcus, the smallest cell. Isoprene production was a function of light intensity and temperature in Prochlorococcus, with patterns that were similar to those between growth rate and these environmental variables. The maximum production with light intensity occurred in the photoinhibited regime, and the maximum with temperature was at the maximum of growth rate for this species, near 23 °C. Nanoflagellate grazing by Cafeteria roenbergensis on, and phage infection of, Prochlorococcus controlled total isoprene produced in the flask by controlling cell abundances. Phage infection also decreased the isoprene production rate per cell during latent period of infection as compared to healthy cells. With certain assumptions, combining the measured laboratory isoprene production rates with observed water column phytoplankton abundances resulted in a maximum estimated sea-to-air flux of isoprene that was on the same order of magnitude as previously reported values determined using in situ measured seawater and atmospheric measurements.

Nitrogen isotope fractionation in 12 species of marine phytoplankton during growth on nitrate

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 255:81-91 (2003) - doi:10.3354/meps255081 Nitrogen isotope fractionation in 12 species of marine phytoplankton during growth on nitrate Joseph A. Needoba1,*, N. A. Waser2, P. J. Harrison1,3, S. E. Calvert2 1Department of Botany, and 2Department of Earth and Ocean Sciences, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada 3Atmospheric and Marine Coastal Environmental Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR *Email: needoba@interchange.ubc.ca ABSTRACT: The nitrogen isotopic composition of 12 species of marine phytoplankton were determined by isotope ratio mass spectrometry in order to investigate isotope fractionation associated with growth on nitrate. The species, representing diatoms, coccolithophores, dinoflagellates, green algae, and cyanobacteria, were grown in batch cultures in artificial seawater under the same laboratory conditions of constant light and temperature. The species (with isotope fractionation values in parenthesis) were: Thalassiosira weissflogii (6.2 ± 0.4‰); Chaetoceros simplex (2.7 ± 0.3‰); Ditylum brightwellii (3.3 ± 0.4‰); Skeletonema costatum (2.7 ± 0.3‰); Phaeodactylum tricornutum (4.8 ± 0.3‰); Emiliania huxleyi (4.5 ± 0.2‰), Isochrysis galbana (3.2 ± 0.4‰); Pavlova lutheri, (3.6 ± 0.5‰); Amphidinium carterae (2.2 ± 0.3‰); Prorocentrum minimum (2.5 ± 0.3‰); Dunaliella tertiolecta (2.2 ± 0.2‰); and Synechococcus sp. (5.4 ± 0.6‰). There was no relationship between isotope fractionation and organism group, nor was there a direct effect of cell size or growth rate on the degree of isotope fractionation among all the groups. Overall, the results show that isotope fractionation during growth on nitrate is lower than values obtained from field samples (i.e. 4 to 9‰). These results indicate that there is no simple mechanism for describing differences in isotope fractionation between groups of phytoplankton, and that a physiological understanding of isotope fractionation during uptake and assimilation of nitrate is needed to properly understand the δ15N signal generated by phytoplankton in the ocean. KEY WORDS: Marine phytoplankton · Nitrogen isotope fractionation · Nitrate · 15N:14N · Stable isotopes · Batch culture Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 255. Online publication date: June 24, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Phycotoxines et produits de la mer: risques sanitaires associés et mesures de prévention

Among the known thousands of marine phytoplankton species, sixty of them have been recognised as toxin producers nowadays. These algal toxins — or phycotoxins- can contaminate the marine food chain and reach content levels, which are very hazardous for humans. Scientists have identified until now six food borne outbreaks generated by these phycotoxins, five due to shellfish consumption: diarrhetic shellfish poisoning, paralytic shellfish poisoning, amnesic shellfish poisoning, neurologic shellfish poisoning, and azapiracid shellfish poisoning; and one due to fish consumption: ciguateric fish poisoning. During the last two decades, both public authorities and scientific community have noticed an increasing number of toxic events, of endemic areas and of new toxin identification. Efficient preventive measures are limited regarding shellfish contamination except the surveillance of both production areas and shellfish meat on market place. Besides human health risks, occurrence of phycotoxins in shellfish can generate important economical disturbances since during toxic events, shellfish harvesting is banned, inducing benefit lost for marine farmers. In France, national monitoring programmes avoiding outbreaks, the main risk is related to imported shellfish and fish products. Above food sanitary objectives, the severe measures conducted by public authorities contributes strongly to keep the positive image of seafood for the consumers in favour of promoting aquaculture and fisheries.

DETERMINATION OF CELLULAR LEVELS OF NONPROTEIN THIOLS IN PHYTOPLANKTON AND THEIR CORRELATIONS WITH SUSCEPTIBILITY TO MERCURY1

We determined the intracellular contents and concentrations of cysteine and glutathione in five species of marine phytoplankton, Tetraselmis tetrathele (West) Butcher (Prasinophyceae), Porphyridium purpureum (Bory) Drew et Ross (Rhodophyceae), Pavlova sp. (Haptophyceae), Isochrysis sp. (Haptophyceae), and Pleurochrysis carterae (Braarud et Fagerl) Christensen (Haptophyceae), and examined relationships to mercury susceptibility. Intracellular contents (concentrations) of nonprotein thiols in the five species ranged from 119 to 1210 amol (0.66–12.0 mM) for cysteine, 78 to 719 amol (0.65–2.52 mM) for cystine, 31 to 677 amol (0.13–1.25 mM) for reduced glutathione (GSH), and 12 to 123 amol (0.15–0.26 mM) for oxidized glutathione (GSSG). The intracellular contents of the nonprotein thiols were not proportional to the intracellular concentrations because the cell sizes differed. Oxidation ratios of cysteine:cystine and GSH:GSSG were also wide ranging in the five species, and the higher the concentration of the reduced form of nonprotein thiols, the less they tended to be oxidized. Flow cytometric analyses with fluorescein diacetate were used to monitor the effect of HgCl2 on esterase, and the 50% effect concentrations (EC50) were compared in the five species. The EC50 after 3 h exposure to HgCl2 correlated well with the GSH concentrations but not with those of cysteine. These results indicate that the intracellular concentrations of the nonprotein thiols reflect antioxidant activity and susceptibility to heavy metals.

Effects of nutrient-limitation and irradiance on marine phytoplankton pigments

Effects of nutrient and light regimes on the pigment composition of 12 species of marine phytoplankton, representing eight algal classes, were examined in batch cultures. During exponential growth, significant differences in pigment ratios caused by light were found for several pigments in six species. Cellular contents of lutein increased with increasing irradiance in Pyramimonas disomata and Brachiomonas sp. indicating a photoprotective role of this pigment. Nutrient regimes had a significant effect only on fucoxanthin:Chl a in Ditylum brightwellii. During stationary growth phases, fewer pigment to Chl a ratios were significantly affected by irradiance, but significant effects from nutrientlimitation were found on the ratio of most marker pigments to Chl a. In four cryptophytes, the alloxanthin:Chl a ratio increased from 1.3-fold up to 9-fold from exponential growth phase to N-limited stationary phase, illustrating different stability of the marker pigment to Chl a ratio within the same phytoplankton group and even within the same genus. Pigment ratios obtained from exponential and stationary growth phases and from low and high light conditions were applied to pigment data from natural phytoplankton populations from two Danish fjords using the matrix factorization program CHEMTAX. Comparison of calculated phytoplankton compositions with direct phytoplankton counts showed the best agreement for exponential growth phase ratios from medium or low light intensities. Discrepancies between community structure described by microscopy and by using pigment analysis are discussed.

CELL CYCLE DEPENDENT VIRUS PRODUCTION IN MARINE PHYTOPLANKTON1

In this study we investigated virus production in two marine phytoplankton species and how it relates to the host's cell cycle. Phaeocystis pouchetii (Hariot) Lagerheim and Pyramimonas orientalis McFadden, Hill & Wetherby, growing synchronously in batch cultures, were infected with their respective viruses (PpV and PoV) at four different stages in the cell cycle and the production of free virus was then measured for 30 h. The virus production in P. orientalis infected with PoV depended on the time of infection, whereas no such relation was found for P. pouchetii infected with PpV. The P. orientalis cultures infected at the end of the dark period and at the beginning of the light period produced three times more virus than those infected in the middle of the light period and eight times more virus than those infected at the beginning of the dark period. The latent periods for PpV and PoV were 12–14 h and 18–20 h, respectively, and in both cases were independent of the host cell cycle. The differences in virus production may be attributed to light or cell cycle dependent regulation of host infection, metabolism, or burst size. Regardless of the mechanism, these differences may be related to differences in the ecological strategies of the hosts and their ability to form blooms.

Benzo[a]pyrene absorption and exposure pathways in the green mussel Perna viridis

The absorption efficiency (AE) of benzo[a]pyrene (BaP), a representative polycyclic aromatic hydrocarbon compound, in the green mussel Perna viridis was measured under different biological and food conditions (body size, food quantity, and food species). Uptake of BaP from the aqueous phase by different species of phytoplankton and green mussels was also experimentally quantified. A bioenergetic-based kinetic model was then used to separate the exposure pathways of BaP accumulation in the green mussels. Our results demonstrated that the AE of BaP in the green mussels ranged between 17 and 47%. Body size did not significantly affect the AE of BaP in the mussels. The AEs increased with a decrease in food availability, largely as a result of lengthening the gut passage of chemicals. The AEs differed by up to 2.3 times (17-39%) among the different food diets tested (four phytoplankton diets and one sediment diet). The BaP associated with the sediment was assimilated least efficiently. The difference in gut passage also appeared to account partially for the difference in AEs observed among different food types. No correlation between the AE and the distribution of BaP in the cytoplasm of phytoplankton cells was found. The concentration factor of BaP measured in four different marine phytoplankton species was in the range of 10(4)-5 x 10(4) (L/kg). The calculated uptake rate constant of BaP in the green mussels was 6.37 L/g/d, and the efficiency of absorption of BaP from the aqueous phase was 6.6 to 8.8%. The model calculation indicated that under most circumstances, >56% of BaP in mussels can indeed be derived from the uptake from the aqueous phase. However, the relative importance of aqueous and dietary exposure can be affected by the variation of AE, mussel's ingestion rate, and concentration of BaP in ingested food particles.

Photosensitizing hemolytic toxin in Heterocapsa circularisquama, a newly identified harmful red tide dinoflagellate

Red tides of Heterocapsa circularisquama ( H. circularisquama), recently identified as a novel species of dinoflagellate, have frequently caused mass mortality of several species of bivalves in Japan, while no harmful effects of this flagellate on fish have been reported so far. We found that the cell-free ethanol extract prepared from H. circularisquama caused hemolysis of rabbit erythrocytes and demonstrated cytotoxic effects in HeLa cells and on the microzooplankton rotifer ( B. plicatilis) in a dose- and time-dependent manner. Interestingly, the hemolytic activity and cytotoxic effects of the extract were completely dependent on the presence of light. When the experiments were conducted in the dark, no hemolysis was observed even at very high concentration of the extract. These results suggest that H. circularisquama has photosensitizing hemolytic toxin which can be easily extracted into ethanol. This may be the first report documenting the occurrence of photosensitizing hemolytic toxin in marine phytoplankton species.

Photoacclimation of four marine phytoplankton species to irradiance and nutrient availability

Photoacclimation to different light and nutrient regimes was studied in 4 common marine phytoplankton species from northern European coastal waters. Significant changes were observed for all species in their cellular nutrient content, pigment packaging and composition, and chl a-specific in vivo light absorption (a* chla (λ)). From exponential to stationary growth, the C:N and carotenoid:chl a ratios increased, while the cellular chl a content decreased. All phytoplankton species acclimated to increasing irradiance by reducing their cellular chl a content and increasing the carotenoid:chl a ratio, resulting in higher chl a-specific absorption coefficients (a* chla ). Similar results were observed for N-depleted cultures. Although acclimation to different irradiance and nutrient conditions introduced a substantial variability in pigment composition and intracellular pigment concentration, significant interspecies differences in a* chla were still apparent. The interspecies differences were primarily due to differences in cell size, which covaried with pigment packaging and composition. On this basis, we suggest that optical acclimation (i.e. changes in pigment composition and packaging) to the prevailing growth conditions (light, temperature and nutrient availability) is less important than differences in phytoplankton composition (dominant cell size and pigment composition) for the variation in a* chla (λ) of natural phytoplankton communities.

A modified toxicity testing method using tropical marine microalgae.

Toxicity testing of four heavy metals (Cd, Cu, Mn and As) using four species of tropical marine phytoplankton, Chaetoceros calcitrans, Isochrysis galbana, Tetraselmis tetrahele and Tetraselmis sp., was carried out in multiwell plates with test volumes of 2 mL and the results compared to those of standard, large volume, shake-flasks. IC50 values (concentrations of metals estimated to inhibit 50% growth relative to the control) were determined after 96 hours based on automated O.D. readings measured in Elisa microplates by a Multiskan spectrophotometer. Good agreement was achieved between O.D. readings and cell counts indicating that this new method is a simple, economical, practical and rapid technique for toxicity testing, and provides good reproducibility of IC50 values. Results of the toxicity tests indicate that Cu was the most toxic metal (average IC50 values ranging from 0.04 to 0.37 mg L(-1)), followed by Cd (0.06-5.7 mg L(-1)), Mn (7.2-21.4 mg L(-1)) and As (33.9-319.3 mg L(-1)). Test species had different degrees of sensitivity to the metals tested, with I. galbana and C. calcitrans the most sensitive to Cu, Cd and Mn. Based on these findings it is recommended that the existing Malaysian Interim Standards for Marine Water Quality for Cd and Cu be reviewed.

Effects of pH on coastal marine phytoplankton

Twenty-one studies on the effects of pH on marine phytoplankton were found and are herein reviewed. Under laboratory conditions, the optimum pH for growth is between pH 6.3 and 10. Some species can grow well at a wide range of pH, while others have growth rates that vary greatly over a 0.5 to 1 pH unit change. Different clones of the same species were found to have slightly to strikingly different relationships between pH and growth rate. The pH in typical coastal environ- ments may vary by 1 or more pH units, with over 10% of observations being more than 0.5 units above or below equilibrium pH. This range is great enough, relative to the observed pH effect on growth rate for many species, for seawater pH to affect the growth rate, and hence the timing and abundance of coastal marine phytoplankton species. Effects of pH are not limited to extreme pH con- ditions. The growth rates of some species are influenced significantly by changes in pH near the equi- librium pH of coastal seawater. Care must be taken in growth experiments with phytoplankton to avoid effects due to pH of the culture media. Eutrophication of coastal waters may amplify the range of pH found in coastal environments.

Support vector machines for identifying organisms — a comparison with strongly partitioned radial basis function networks

Biological identification poses many problems which are not commonly met in other areas where artificial neural networks (ANNs) are applied. One such problem is that the number of possible classes is often unbounded and it is necessary to be able to deal with novel categories (taxa) either by rejecting them or retraining to allow for them. Where it is necessary to incorporate them, retraining a neural network may be time consuming and or impossible. One way of negating this problem is to use single-category networks that can be combined and configured as required. This paper compares support vector machines (SVMs) with strongly partitioned traditional radial basis function (RBF) networks for the discrimination of single species of phytoplankton against a background of N other species. SVMs resulted in greater identification success than the unpartitioned, large single RBF networks (81 and 77%, respectively), though partitioned RBF ANNs performed relatively poorly (50% success) at discrimination of 61 marine phytoplankton species from flow cytometry data. Greatest success was achieved by combining the outputs of the individual networks by means of a ‘winner takes all’ strategy; with RBF ANNs identification success dramatically increased (from 16 to 50%), though there was only a modest increase with SVMs (77–81%). When SVMs trained on one data set were tested with data on cells grown under different light conditions, an overall successful identification was low (13%), but when SVMs were trained on a combined data set identification was high (>70%). Clearly, it is essential to cover the whole spectrum of biological variation in the training data set.