Colony growth and evidence for colony multiplication in Phaeocystis pouchetii (Prymnesiophyceae) isolated from mesocosm blooms

Using well plates of Phaeocystis pouchetii colonies isolatedfrom experimental mesocosms in western Norway, increases incolony size and division were documented. Median longest lineardimensions increased 0–7 µm h^–1; literaturePhaeocystis globosa values are 0.9–4.7 µm h^–1.Ten to twelve percent of colonies divided at rates of 0.21–0.28divisions day^–1. Daughter colonies were 100 µm smallerthan mother colonies. Colonies delayed 3.5–4.9 days tofirst division, compared with literature values of 4–5days for P. globosa. This study provides the first experimentalevidence for colony division of wild P. pouchetii.     

Grazing and colony size development in Phaeocystis globosa (Prymnesiophyceae): the role of a chemical signal

The bloom-forming prymnesiophyte Phaeocystis globosa forms hollow,spherical, mucilaginous colonies that vary from micrometresto millimetres in size. A recent paper gave the first empiricalevidence that colony size increase in P. globosa is a defensiveresponse against grazers, and knowing the signalling mechanism(s)behind this response will thus be a key to understanding thetrophodynamics in systems dominated by this species. I conductedexperiments with specially designed diffusion incubators, eachof which consists of a non-grazing chamber (with P. globosaonly) and a grazing chamber (grazers + phytoplankton) connectedby 2 µm polycarbonate membrane filters. The results showedthat physical contact with grazers was not required to initiatethe defensive response; instead, P. globosa colony size increasewas found to be stimulated by dissolved chemicals generatedby ambient grazing activities. This signal was non-species specific,such that various combinations of three species of grazers andfour species of phytoplankton in the grazing chambers all resultedin significant, but different extents of colony enlargementin P. globosa in the non-grazing chambers (30–300% largerthan the ‘grazer-free’ control). High concentrationsof ambient solitary P. globosa cells and other phytoplanktonseemed to suppress colony enlargement in P. globosa, and grazerswould help reduce this inhibition by removing the ambient solitaryP. globosa cells and other phytoplankton. These non-species-specificmechanisms would allow P. globosa to regulate colony size developmentand defend itself in diverse planktonic systems, which may helpto explain the global success of this species.     

The vitamin B requirement of Phaeocystis globosa (Prymnesiophyceae)

In batch cultures of flagellates and non-flagellate cells ofPhaeocystis globosa, the biomass yield was significantly enhancedby the addition of a mixture of the vitamins thiamine (B1),cyanocobalamin (B12) and biotin (H). A bioassay with B1 andB12 using the non-flagellate cells of P.globosa showed thatthis prymnesiophyte is a B1 auxotroph. The bioassay also indicateda significant difference in growth rate between culture mediumwith 10 nmol l^–1 B1 (µ = 0.80 day^–1) and culturemedium with 10 nmol l^–1 B12 (µ = 0.52 day^–1).These findings are discussed in relation to the hypothesis thatcentric diatoms, through vitamin B1 excretion or B12 depletion,initiate Phaeocystis blooms. It is concluded, however, thatan alternative hypothesis, that diatoms provide a solid substratefor colony initiation, has more experimental support.     

Growth and mortality of flagellates and non-flagellate cells of Phaeocystis globosa (Prymnesiophyceae)

Two cell types of the same clone of Phaeocystis globosa, solitarynon-flagellate cells and flagellates, were grown in batch culturesunder identical conditions. The non-flagellate cells had a shorterlag phase (1.4 versus 2.8 days) and a higher growth rate (0.72versus 0.65 day^–1) than flagellate cells. The flagellateshad a longer stationary phase (15.6 versus 9.5 days) and a lowerdeath rate (0.07 versus 0.52 day^–1) than non-flagellatecells. All differences were statistically significant. Biomassyield did not differ between the two cell types. The short lagphase and high growth rate of nonflagellate cells correspondsto field observations of rapidly developing non-flagellate Phaeocystisblooms that are typically observed in nutrient-rich environmentssuch as temperate seas in spring. The flagellate cell type,with its longer stationary phase and lower death rate than non-flagellatecells, is better equipped for survival in oligotrophic environments.This explains why the flagellates of Phaeocystis are abundantafter the spring phytoplankton bloom in temperate seas and inother nutrient-poor environments such as the open ocean.     

Survival and recovery of Phaeocystis antarctica (Prymnesiophyceae) from prolonged darkness and freezing

The colony-forming haptophyte Phaeocystis antarctica is an important primary producer in the Ross Sea, and must survive long periods of darkness and freezing temperature in this extreme environment. We conducted experiments on the responses of P. antarctica-dominated phytoplankton assemblages to prolonged periods of darkness and freezing. Chlorophyll and photosynthetic capacity of the alga declined nonlinearly and independently of each other in the dark, and darkness alone would potentially reduce photosynthetic capacity by only 60 per cent over 150 days (approximately the length of the Antarctic winter in the southern Ross Sea). The estimated reduction of colonial mucous carbon is higher than that of colonial cell carbon, suggesting metabolism of the colonial matrix in the dark. The alga quickly resumed growth upon return to light. Phaeocystis antarctica also survived freezing, although longer freezing durations lengthened the lag before growth resumption. Particulate dimethylsulfoniopropionate relative to chlorophyll increased upon freezing and decreased upon darkness. Taken together, the abilities of P. antarctica to survive freezing and initiate growth quickly after darkness may provide it with the capability to survive in both the ice and the water column, and help explain its repeated dominance in austral spring blooms in the Ross Sea and elsewhere in the Southern Ocean.     

Isolation and Phylogenetic Analysis of Novel Viruses Infecting the Phytoplankton Phaeocystis globosa (Prymnesiophyceae)

Viruses infecting the harmful bloom-causing alga Phaeocystis globosa (Prymnesiophyceae) were readily isolated from Dutch coastal waters (southern North Sea) in 2000 and 2001. Our data show a large increase in the abundance of putative P. globosa viruses during blooms of P. globosa, suggesting that viruses are an important source of mortality for this alga. In order to examine genetic relatedness among viruses infecting P. globosa and other phytoplankton, DNA polymerase gene (pol) fragments were amplified and the inferred amino acid sequences were phylogenetically analyzed. The results demonstrated that viruses infecting P. globosa formed a closely related monophyletic group within the family Phycodnaviridae, with at least 96.9% similarity to each other. The sequences grouped most closely with others from viruses that infect the prymnesiophyte algae Chrysochromulina brevifilum and Chrysochromulina strobilus. Whether the P. globosa viruses belong to the genus Prymnesiovirus or form a separate group needs further study. Our data suggest that, like their phytoplankton hosts, the Chrysochromulina and Phaeocystis viruses share a common ancestor and that these prymnesioviruses and their algal host have coevolved.     

Isolation and phylogenetic analysis of novel viruses infecting the phytoplankton Phaeocystis globosa (Prymnesiophyceae)

Viruses infecting the harmful bloom-causing alga Phaeocystis globosa (Prymnesiophyceae) were readily isolated from Dutch coastal waters (southern North Sea) in 2000 and 2001. Our data show a large increase in the abundance of putative P. globosa viruses during blooms of P. globosa, suggesting that viruses are an important source of mortality for this alga. In order to examine genetic relatedness among viruses infecting P. globosa and other phytoplankton, DNA polymerase gene (pol) fragments were amplified and the inferred amino acid sequences were phylogenetically analyzed. The results demonstrated that viruses infecting P. globosa formed a closely related monophyletic group within the family Phycodnaviridae, with at least 96.9% similarity to each other. The sequences grouped most closely with others from viruses that infect the prymnesiophyte algae Chrysochromulina brevifilum and Chrysochromulina strobilus. Whether the P. globosa viruses belong to the genus Prymnesiovirus or form a separate group needs further study. Our data suggest that, like their phytoplankton hosts, the Chrysochromulina and Phaeocystis viruses share a common ancestor and that these prymnesioviruses and their algal host have coevolved.     

Molecular genetic delineation of Phaeocystis species (Prymnesiophyceae) using coding and non-coding regions of nuclear and plastid genomes

Sequence variation among 22 isolates representing a global distribution of the prymnesiophyte genus Phaeocystis has been compared using nuclear-encoded 18S rRNA genes and two non-coding regions: the ribosomal DNA internal transcribed spacer 1 (ITS1) separating the 18S rRNA and 5.8S rRNA genes and the plastid ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) spacer flanked by short stretches of the adjacent large and small subunits (rbcL and rbcS). 18S rRNA can only resolve major species complexes. The analysis suggests that an undescribed unicellular Phaeocystis sp. (isolate PLY 559) is a sister taxon to the Mediterranean unicellular Phaeocystis jahnii; this clade branched prior to the divergence of all other Phaeocystis species, including the colonial ones. Little divergence was seen among the multiple isolates sequenced from each colonial species complex. RUBISCO spacer regions are even more highly conserved among closely related colonial Phaeocystis species and are identical in Phaeocystis antarctica, Phaeocystis pouchetii and two warm-temperate strains of Phaeocystis globosa, with a single base substitution in two cold-temperate strains of P. globosa. The RUBISCO spacer sequences from two predominantly unicellular Phaeocystis isolates from the Mediterranean Sea and PLY 559 were clearly different from other Phaeocystis strains. In contrast, ITS1 exhibited substantial inter- and intraspecific sequence divergence and showed more resolution among the taxa. Distinctly different copies of the ITS1 region were found in P. globosa, even among cloned DNA from a single strain, suggesting that it is a species complex and making this region unsuitable for phylogenetic analysis in this species. However, among nine P. antarctica strains, four ITS1 haplotypes could be separated. Using the branching order in the ITS1 tree we have attempted to trace the biogeographic history of the dispersal of strains in Antarctic coastal waters.     

Carotenoids of Prymnesiophyceae (Haptophyceae)

A quantitative study, including mass spectrometric identification, of the carotenoids isolated from some selected prymnesiophytes harvested in exponential growth phase has been carried out. Isochrysis galbana, Hymenomonas carterae, Prymnesium parvum, Pavlova (Monochrysis) lutheri and a Pavlova sp. all produce β,β-carotene, diatoxanthin, diadinoxanthin and fucoxanthin (major carotenoid) in various proportions, in addition to several minor carotenoids were found characteristic of each alga. 19'-Hexanoyloxyfucoxanthin, previously shown to be the major carotenoid in the prymnesiophyte Emeliana (Coccolithus) huxleyi, was not encountered in the Prymnesiophyceae studied here, and we conclude that this carotenoid should be critically looked for in other members of the Gephyrocapsaceae to which E. huxleyi belongs. We further conclude that the carotenoid complement of the Chrysophyceae (in the narrow sense) should be compared with that of the Prymnesiophyceae.     

Observations of flagellates in colonies of Phaeocystis globosa (Prymnesiophyceae); a hypothesis for their position in the life cycle

Flagellates of Phaeocystis globosa were observed inside coloniesin North Sea samples in 1992. Field data suggest that low phosphateconcentrations (     

Evidence for high iron requirements of colonial Phaeocystis antarctica at low irradiance

We have carried out field and laboratory experiments to examine the iron requirements of colonial Phaeocystis antarctica in the Ross Sea. In December 2003, we performed an iron/light-manipulation bioassay experiment in the Ross Sea polynya, using an algal assemblage dominated by colonial Phaeocystis antarctica, collected from surface waters with an ambient dissolved Fe concentration of ∼0.4 nM. Results from this experiment suggest that P. antarctica growth rates were enhanced at high irradiance (∼50% of incident surface irradiance) but were unaffected by iron addition, and that elevated irradiance mediated a significant decrease in cellular chlorophyll a content. We also conducted a laboratory iron dose–response bioassay experiment using a unialgal, non-axenic strain of colonial P. antarctica and low-iron (<0.2 nM) filtered seawater, both collected from the Ross Sea polynya in December 2003. By using rigorous trace-metal clean techniques, we performed this dose–response iron-addition experiment at ∼0°C without using organic chelating reagents to control dissolved iron levels. At the relatively low irradiance of this experiment (∼20 μE m⁻² s⁻¹), estimated nitrate-specific growth rate as a function of dissolved iron concentration can be described by a Monod relationship, yielding a half-saturation constant with respect to growth of 0.45 nM dissolved iron. This value is relatively high compared to reported estimates for other Antarctic phytoplankton. Our results suggest that seasonal changes in the availability of both iron and light play critical roles in limiting the growth and biomass of colonial Phaeocystis antarctica in the Ross Sea polynya.     

Serum lipoproteins inhibitors of granulocyte-macrophage (GM) colony formation

Normal and chloroform-extracted human sera, fractionated by Sephadex column chromatography, were tested for inhibitory activity on granulocyte-macrophage (GM) colony formation. This activity was found to be connected with lipoproteins with a molecular weight of about 200,000. Serum native fractions of lipoproteins were isolated and mainly high density lipoproteins (HDL) and very low density lipoproteins (VLDL) were shown to have an unspecific inhibitory activity directed on colony stimulating factor (CSF) action.     

Toxicity of coastal coccolithophores (Prymnesiophyceae, Haptophyta)

Over the last decade, certain coccolithophores have been thesubject of extensive multidisciplinary research. Several speciesof coccolithophore, belonging mainly to the families Pleurochrysidaceaeand Hymenomonadaceae, inhabit inshore coastal waters where theymay occasionally bloom and hence impact aquaculture resources.The toxicity to Artemia salina larvae of 11 species of coccolithophore(nine coastal and two oceanic members of the order Coccolithales)was tested. For the nine coastal species, tests were conductedwith rapidly growing and stationary phase cultures at a rangeof cellular concentrations and for two different exposure times(24 and 48 h). Five of the coastal species (four in the genusPleurochrysis as well as Jomonlithus littoralis) were foundto be toxic to A. salina nauplii. Allelopathic effects of acell-free filtrate of a culture of a toxic coccolithophore werealso tested on three flagellate microalgal species: Scrippsiellatrochoidea, Tetraselmis sp. and Isochrysis galbana. Negativeeffects of the filtrate on growth rates and motility of S. trochoideaand Tetraselmis sp. were recorded, suggesting that the toxinof the coccolithophore tested could be an exotoxin similar tothat produced by other non-calcifying members of the Prymnesiophyceae.The fact that certain coccolithophores were found to be toxicto invertebrates and were shown to exhibit allelopathic activitycould imply negative effects at different trophic levels incoastal areas.     

Trypanosoma cruzi: colony formation and clonal growth in agar

Trypanosoma cruzi exhibited colonial growth when incorporated into 0.5–0.6% agar. Colonies were established from a single organism and clones readily derived. The plating efficiencies were variable depending on the original inoculum but were consistently over 50% when 10⁴ to 10⁵ parasites were added. The use of this technique for evaluation of an antitrypanosomal agent, nifurtimox, was demonstrated, making possible large-scale testing of potential antitrypanosomal agents and assessment of microbicidal and microbio-static drug levels.     

Photophysiology in two major southern ocean phytoplankton taxa: photosynthesis and growth of Phaeocystis antarctica and Fragilariopsis cylindrus under different irradiance levels

The Ross Sea, Antarctica, supports two distinct populations of phytoplankton, one that grows well in sea ice and blooms in the shallow mixed layers of the Western marginal ice zone and the other that can be found in sea ice but thrives in the deeply mixed layers of the Ross Sea. Dominated by diatoms (e.g. Fragilariopsis cylindrus) and the prymnesiophyte Phaeocystis antarctica, respectively, the processes leading to the development of these different phytoplankton assemblages are not well known. The goal of this article was to gain a better understanding of the photophysiological characteristics that allow each taxon to dominate its specific habitat. Cultures of F. cylindrus and P. antarctica were each grown semi-continuously at four different constant irradiances (5, 25, 65, and 125 μmol quanta/m2/s). Fragilariopsis cylindrus produced far less photosynthetic pigment per cell than did P. antarctica but much more photoprotective pigment. Fragilariopsis cylindrus also exhibited substantially lower rates of photosynthesis and growth but also was far less susceptible to photoinhibition of cell growth. Excess photosynthetic capacity, a measure of the ability of phytoplankton to exploit variable light environments, was significantly higher in both strains of P. antarctica than in F. cylindrus. The combination of these characteristics suggests that F. cylindrus has a competitive advantage under conditions where mixed layers are shallow and light levels are relatively constant and high. In contrast, P. antarctica should dominate waters where mixed layers are deep and light levels are variable. These results are consistent with distributions of phytoplankton in the Ross Sea and suggest that light is the primary factor determining composition of phytoplankton communities.     

Plasticity in relative growth rate and its components following a change in irradiance

A total of 244 plants from two species, Lythrum salicaria and Epilobium glandulosum, were grown individually in hydroponic sand culture from seed for 36 d. Until day 27 all plants experienced an irradiance of 550 μmol m^?2 s^?1 PFD and on day 27 half of the plants were subjected to a neutral shade treatment in which irradiance was reduced to 100 μmol m^?2 s^?1 photon fluy density (PFD). Measures of relative growth rate, net assimilation rate, specific leaf area, biomass partitioning to leaves, roots, structural tissues (i.e. stems, petioles and inflorescences) and tissue density were obtained from intensive harvests three or four times per day. The shade treatment caused an immediate decrease in relative growth rate and net assimilation rate. Within hours the specific leaf area of the shaded plants increased and leaf tissue density decreased, thus partially offsetting the decrease in relative growth rate. Biomass partitioning was not affected.     

The ultrastructure of mitosis inIsochrysis galbana parke (Prymnesiophyceae)

Summary Mitosis and cytokinesis have been studied in the flagellate algaIsochrysis galbana Parke (Prymnesiophyceae). Nuclear division is preceded by replication of the flagella and haptonema, the Golgi body and the chloroplast; fission in the chloroplast occurs in the region of the pyrenoid. During prophase, spindle microtubules radiating from two ill-defined poles are formed. The nuclear envelope breaks down and the chromatin condenses. At metaphase the spindle is fully developed, some pole-to-pole microtubules passing through the well-defined chromatin plate, others terminating at it. No kinetochores or individual chromosomes were observed. By late metaphase, many Golgi-derived vesicles may be seen against the two poleward faces of the metaphase plate. During anaphase, the two daughter masses of chromatin move towards the poles. In early telophase, the nuclear envelope of each daughter nucleus is complete only on the side towards the adjacent chloroplast, remaining open on the interzonal side. However, during telophase each nucleus becomes reorientated so that it lies lateral to the long axis of the spindle and with its open side towards the chloroplasts. By late telophase, each new nuclear envelope is complete and confluence with the adjacent chloroplast ER established.Cytokinesis and subsequent segregation of the daughter cells are effected by the dilation of Golgi- and ER-derived vesicles in the interzonal region. No microtubular structures are involved. Comparisons with the results from other studies of mitosis in members of thePrymnesiophyceae show that they all have a number of features in common, but that there are differences in detail between species.