Picoplanktonic Algae Research Articles

Planktonic protist diversity across contrasting Subtropical and Subantarctic waters of the southwest Pacific

Planktonic protists are an essential component of marine pelagic ecosystems where they mediate important trophic and biogeochemical functions. Although these functions are largely influenced by their taxonomic affiliation, the composition and spatial variability of planktonic protist communities remain poorly characterized in vast areas of the ocean. Here, we investigated the diversity of these communities in contrasting oceanographic conditions of the southwest Pacific (33–58 °S) using DNA metabarcoding of the 18S rRNA gene. Seawater samples collected during twelve cruises (n = 482, 0–3100 m) conducted east of New Zealand were used to characterize protist communities in Subtropical (STW) and Subantarctic (SAW) surface water masses and the Subtropical Front (STF) that separates them. Diversity decreased with increasing latitude and increasing temperature but tended to be lowest in the STF. Sample ordination resulting from the abundance of amplicon single variants (ASVs) corresponded to the different water masses. Overall, Dinoflagellata (Syndiniales, 27%; Dinophyceae, 24% of standardized number of reads) dominated the euphotic zone followed by Chlorophyta (20%), but their relative abundance and composition at class and lower taxonomic levels varied consistently between water masses. Among Chlorophyta, several picoplanktonic algae species of the Mamiellophyceae class including Ostreococcus lucimarinus dominated in STW, while the Chloropicophyceae species Chloroparvula pacifica was most abundant in SAW. Bacillariophyta (5%), Prymnesiophyceae (5%), and Pelagophyceae (2%) classes were less abundant but showed analogous water mass specificity at class and finer taxonomic levels. Protist community composition in the STF had mixed characteristics and showed regional differences with the southern STF (50 °S) having more resemblance with subantarctic communities than the STF over the Chatham Rise region (42–44 °S). Below the euphotic zone, Syndiniales sequences (40%) dominated the dataset followed by Radiolaria (31%), Dinophyceae (14%) and other heterotrophic groups like Marine Stramenopiles and ciliates (1–1.5%). Among Radiolaria, several unidentified ASVs assigned to Spumellaria were most abundant, but showed significantly different distributions between STW and SAW highlighting the need to further investigate the taxonomy and ecology of this group. The present study represents a significant step forward towards characterizing protistan communities composition in relation to major physical oceanographic features in the southwest Pacific providing new insights about the biogeography and ecological preferences of different planktonic protist taxa from class to species and genotypic level.

Capturing open ocean biodiversity: Comparing environmental DNA metabarcoding to the continuous plankton recorder.

Environmental DNA (eDNA) metabarcoding is emerging as a novel, objective tool for monitoring marine metazoan biodiversity. Zooplankton biodiversity in the vast open ocean is currently monitored through continuous plankton recorder (CPR) surveys, using ship-based bulk plankton sampling and morphological identification. We assessed whether eDNA metabarcoding (2L filtered seawater) could capture similar Southern Ocean zooplankton biodiversity as conventional CPR bulk sampling (~1,500L filtered seawater per CPR sample). We directly compared eDNA metabarcoding with (a) conventional morphological CPR sampling and (b) bulk DNA metabarcoding of CPR collected plankton (two transects for each comparison, 40 and 44 paired samples, respectively). A metazoan-targeted cytochrome c oxidase I (COI) marker was used to characterize species-level diversity. In the 2L seawater eDNA samples, this marker amplified large amounts of non-metazoan picoplanktonic algae, but eDNA metabarcoding still detected up to 1.6 times more zooplankton species than morphologically analysed bulk CPR samples. COI metabarcoding of bulk DNA samples mostly avoided nonmetazoan amplifications and recovered more zooplankton species than eDNA metabarcoding. However, eDNA metabarcoding detected roughly two thirds of metazoan species and identified similar taxa contributing to community differentiation across the subtropical front separating transects. We observed a diurnal pattern in eDNA data for copepods which perform diel vertical migrations, indicating a surprisingly short temporal eDNA signal. Compared to COI, a eukaryote-targeted 18S ribosomal RNA marker detected a higher proportion, but lower diversity, of metazoans in eDNA. With refinement and standardization of methodology, eDNA metabarcoding could become an efficient tool for monitoring open ocean biodiversity.

Characterization of a high lipid-producing thermotolerant marine photosynthetic pico alga from genus Picochlorum (Trebouxiophyceae)

ABSTRACT A new marine strain of picoplanktonic algae, PMPFPPE4, was isolated from a mixed net-phytoplankton sample taken from the upper euphotic layer of the southeastern Adriatic Sea. Evaluation of the new strain included morphological investigation (by light and electron microscopy), phylogenetic analysis (utilizing plastid 16S rRNA and nuclear 18S rRNA genes), and physiological characterization (screening of pigment/lipid composition and capturing photosynthesis measurements). The new strain was proven to belong to the genus Picochlorum and the lipid composition revealed an unexpected accumulation of triacylglycerols, indicating an evolutionary adaptation for growth under unfavourable conditions. In addition, lipid remodelling in the exponential to stationary growth phase was characterized by an increased share of membrane-forming digalactosyldiacylglycerols and phosphatidylcholines. Maximum photosynthetic activity measured was at 30°C, but the most rapid increase of photosynthetic activity was at lower temperatures (15–20°C). Moreover, the thermotolerant strain did not exhibit photoinhibition below 40°C and survived a one-month cultivation period in complete darkness. The strain’s survival in low light and dark conditions suggests a potential shift from autotrophy to mixotrophy under unfavourable growth conditions. Thus, the unique physiological attributes represented by a high growth rate, thermotolerance, phototolerance and high triacylglycerol synthesis may render the strain highly attractive for biofuel production and growth in large outdoor systems.

Chloropicophyceae, a new class of picophytoplanktonic prasinophytes

Prasinophytes are a paraphyletic group of nine lineages of green microalgae that are currently classified either at the class or order level or as clades without formal taxonomic description. Prasinophyte clade VII comprises picoplanktonic algae that are important components of marine phytoplankton communities, particularly in moderately oligotrophic waters. Despite first being cultured in the 1960s, this clade has yet to be formally described. Previous phylogenetic analyses using the 18S rRNA gene divided prasinophyte clade VII into three lineages, termed A, B and C, the latter formed by a single species, Picocystis salinarum, that to date has only been found in saline lakes. Strains from lineages A and B cannot be distinguished by light microscopy and have very similar photosynthetic pigment profiles corresponding to the prasino-2A pigment group. We obtained phenotypic and genetic data on a large set of prasinophyte clade VII culture strains that allowed us to clarify the taxonomy of this important marine group. We describe two novel classes, the Picocystophyceae and the Chloropicophyceae, the latter containing two novel genera, Chloropicon and Chloroparvula, and eight new species of marine picoplanktonic green algae.

The carbonic anhydrase CAH1 is an essential component of the carbon-concentrating mechanism in Nannochloropsis oceanica

Aquatic photosynthetic organisms cope with low environmental CO2 concentrations through the action of carbon-concentrating mechanisms (CCMs). Known eukaryotic CCMs consist of inorganic carbon transporters and carbonic anhydrases (and other supporting components) that culminate in elevated [CO2] inside a chloroplastic Rubisco-containing structure called a pyrenoid. We set out to determine the molecular mechanisms underlying the CCM in the emerging model photosynthetic stramenopile, Nannochloropsis oceanica, a unicellular picoplanktonic alga that lacks a pyrenoid. We characterized CARBONIC ANHYDRASE 1 (CAH1) as an essential component of the CCM in N. oceanica CCMP1779. We generated insertions in this gene by directed homologous recombination and found that the cah1 mutant has severe defects in growth and photosynthesis at ambient CO2 We identified CAH1 as an α-type carbonic anhydrase, providing a biochemical role in CCM function. CAH1 was found to localize to the lumen of the epiplastid endoplasmic reticulum, with its expression regulated by the external inorganic carbon concentration at both the transcript and protein levels. Taken together, these findings show that CAH1 is an indispensable component of what may be a simple but effective and dynamic CCM in N. oceanica.

Chloroparva pannonica gen. et sp. nov. (Trebouxiophyceae, Chlorophyta) – a new picoplanktonic green alga from a turbid, shallow soda pan

Somogyi B., Felföldi T., Solymosi K., Makk J., Homonnay Z.G., Horváth G., Turcsi E., Böddi B., Márialigeti K. and Vörös L. 2011. Chloroparva pannonica gen. et sp. nov. (Trebouxiophyceae, Chlorophyta) – a new picoplanktonic green alga from a turbid, shallow soda pan. Phycologia 50: 1–10. DOI: 10.2216/10-08.1We describe Chloroparva pannonica Somogyi, Felföldi & Vörös gen. et sp. nov., a new trebouxiophycean picoplanktonic alga isolated from a turbid, shallow soda pan in Hungary. The cells are spherical to oval, less than 2 µm in diameter, with simple ultrastructure typical to small green algae. Cells divide by autosporulation, forming two daughter cells per autosporangium. Cell wall structure consists of an outer trilaminar layer, an inner microfibrillar layer and an electron-transparent layer covering the plasma membrane. The trilaminar layer of the mother cell wall often persists around the autospores. Typical chlorophyte pigments have been found, including chlorophyll a and b and lutein as the dominant carotenoid. The main fatty acid was oleic acid. The phylogenetic position of the new chlorophyte confirms the proposal of a new genus within the Trebouxiophyceae. Based on its 18S rRNA gene sequence, this isolate is distantly related to Nannochloris eucaryotum UTEX 2502, Chlorella minutissima C-1.1.9 and C. minutissima SAG 1.80 (≤ 97.6% 18S rRNA gene pairwise similarities).

Computational Analysis of the Evolution of 1‐Deoxy‐D‐xylulose‐5‐phosphate Reductoisomerase, an Important Enzyme in Plant Terpene Biosynthesis

Isoprenoids are a highly diverse and important group of natural compounds. The enzyme 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) catalyzes a key regulatory step in the non-mevalonate isoprenoid biosynthetic pathway in eubacteria and in plant plastids. For example, in Artemisia annua DXR participates in regulation of the biosynthesis of artemisinin, an important antimalarial drug. We performed phylogenetic analysis using DXR protein sequences from a model prokaryote, Escherichia coli, a picoplanktonic alga, Ostreococcus lucimarinus, and higher plants. The functional domain of DXR was conserved, allowing molecular evolutionary comparisons of both prokaryotic and eukaryotic sequences of DXR. Despite this conservation, for some plant species such as Campthoteca acuminata and Arabidopsis thaliana, phylogenetic relationships of their lineages were consistently violated. Our analysis revealed that plant DXR has an N-terminal transit domain that is likely bipartite, consisting of a chloroplast transit peptide (cTP) and a lumen transit peptide (lTP). Several features observed in the lTP suggest that, while DXR is targeted to the chloroplast, it is localized to the thylakoid lumen. These features include a twin arginine motif, a hydrophobic region, and a proline-rich region. The transit peptide also showed putative motifs for a 14-3-3 binding site with a chaperone phosphorylation site at Thr.

Microbial control of phosphate in the nutrient‐depleted North Atlantic subtropical gyre

Little is known about the dynamics of dissolved phosphate in oligotrophic areas of the world's oceans, where concentrations are typically in the nanomolar range. Here, we have budgeted phosphate uptake by the dominant microbial groups in order to assess the effect of the microbial control of this depleted nutrient in the North Atlantic gyre. Low concentrations (2.2 +/- 1.2 nM) and rapid microbial uptake (2.1 +/- 2.4 nM day(-1)) of bioavailable phosphate were repeatedly determined in surface waters of the North Atlantic oligotrophic gyre during spring and autumn research cruises, using a radiotracer dilution bioassay technique. Upper estimates of the concentration of bioavailable phosphate were 7-55% of the dissolved mineral phosphate suggesting that a considerable part of the chemically measured nanomolar phosphate was in a form unavailable for direct microbial uptake. A 1:1 relationship (r(2) = 0.96, P < 0.0001) was observed between the bioavailable total phosphate uptake and the phosphate uptake of all the flow sorted bacterioplankton cells, demonstrating that bacterioplankton were the main consumers of phosphate. Within the bacterioplankton a group of heterotrophic bacteria and Prochlorococcus phototrophic cyanobacteria, were the two major competing groups for bioavailable phosphate. These heterotrophic bacteria had low nucleic acid content and 60% of them comprised of SAR11 clade cells based on the results of fluorescence in situ hybridization. Each of the two competing bacterial groups was responsible for an average of 45% of the phosphate uptake, while Synechococcus cyanobacteria (7%) and picoplanktonic algae (0.3%) played minor roles in direct phosphate uptake. We have demonstrated that phosphate uptake in the oligotrophic gyre is rapid and dominated by two bacterial groups rather than by algae.

The trophic position of planktonic ciliate populations in the food webs of some East African lakes

The trophic role of planktonic ciliates was investigated in the food webs of 17 East African lakes of different salinity and trophy. Experiments with 1μm latex beads showed that Cyclidium, Halteria, Dysteria, Aspidisca and Vorticella are bacterivores. Examination of food vacuole contents showed that the ciliate communities also included herbivores, carnivores, omnivores and mixotrophs. While some herbivorous ciliates grazed picoplanktonic algae or flagellates, large omnivores such as Frontonia and Linostoma ingested cyanobacterial filaments or preyed on other ciliates. Most predaceous ciliates were haptorids, and consumed other ciliates or metazoa. Growth rate estimates for planktonic ciliate populations in East African lakes ranged from 0.18 to 6.56d−1. From these growth rates and biomass, the estimated production of ciliate populations ranged from 5.9 to 2 335μg carbon l−1 d−1 and increased with lake trophic status and water electrical conductivity. Bacterivores, herbivores, carnivores and mixotrophs accounted for 36%, 39%, 21% and 4% of total production, respectively, over all the lakes. Zooplankton were the probable consumers of much of this ciliate production, although some taxa were consumed mostly within the microzooplankton community, presumably by other ciliates.

MYCOSPORINE‐LIKE AMINO ACIDS AND PHYLOGENIES IN GREEN ALGAE: PRASIOLA AND ITS RELATIVES FROM THE TREBOUXIOPHYCEAE (CHLOROPHYTA)1

A UV‐absorbing mycosporine‐like amino acid (324 nm‐MAA), so far only known from the green macroalgal genus Prasiola (Trebouxiophyceae), was also identified in other morphologically diverse green algae closely related to Prasiola spp. in 18S rDNA phylogenies. Using HPLC, a second UV‐absorbing compound was found only in Myrmecia incisa Reisigal among all studied strains. This substance showed an absorption maximum at 322 nm and hence was designated as putative 322 nm‐MAA. Preliminary UV‐exposure experiments indicated that all species containing one or the other MAA showed a strong accumulation of the respective compound, thus supporting their function as putative UV sunscreen. Both UV‐absorbing substances were only identified in the studied members of the Trebouxiophyceae but were absent in members of the Ulvophyceae and Chlorophyceae. When mapped on an 18S rDNA phylogeny, the distribution of 324 nm‐MAA was found to be scattered within the Trebouxiophyceae but was consistent with a distribution that follows phylogenetic patterns rather than ecological adaptations. The 324 nm‐MAA was also detected in two phylogenetically related species from freshwater as well as from subaerial habitats, Watanabea reniformis Hanagata et al. and isolate UR7/5, which were phylogenetically independent of Prasiola and its closer allies. MAAs were absent in another Trebouxiophyceae clade comprising lichen photobionts (Coccomyxa pringsheimii Jaag) as well as freshwater picoplanktonic algae (Choricystis minor (Skuja) Fott). The data presented suggest a chemotaxonomic value of the 324 nm‐MAA in green algal taxonomy. To address the paraphyly of the genus Myrmecia Printz as presently circumscribed, we propose the new combination Lobosphaera incisa.

Autotrophic picoplankton in Lake Baikal: Abundance, dynamics, and distribution

The autotrophic picoplankton of Lake Baikal was discovered more than 30 years ago; the productivity of the endemic member of the group, Synechocystis limnetica, is of particular interest. In the course of long-term investigations, the abundance of this species was established using the sedimentation technique. Maximum abundance was reached at 104 cells per ml in summer. In addition to estimating the distribution of this species throughout a defined lake area during the spring-summer period, the character of interrelations with large phytoplankton was revealed. Baikalian autotrophic picoplankton was studied from 1997 to 2000 using epifluorecsence and scanning electron microscopy. The study of seasonal and annual dynamics of picoplankton abundance was conducted monthly at the monitoring station in southern Baikal. The spatial distribution of autotrophic picoplankton was observed in the period when Lake Baikal was free of ice; the lake area was defined by 50 stations. Microscopic examination of planktonic samples revealed that the phycoerythrin-rich cyanobacteria of genus Synechococcus generated the largest biomass of autotrophic picoplankton in Lake Baikal. Morphotype diversity of the representatives of genus Synechecoccus was revealed in the study: Synechococcus spp. of coccoid and ellipsoid forms were dominant by abundance. The coccoid and ellipsoid forms had diameters of 0.9 and 0.7 to 1.3 μm, respectively. The portion of picoplanktonic algae in the Baikalian picoplankton community is not high and does not exceed 10% in the summer time when growth is highest. The autotrophic picoplankton abundance throughout the year in the 0–50 m layer varies by more than one hundred times. Maximal concentration of picocyanobacteria occurs in late July–August at 10 to 25 m depths where it reached 106 cells ml−1. The minimal concentration (103 cells ml−1) was observed in December at the same depths. A small peak of the picoplankton growth occurred in April of each year when the total numbers of picocyanobacteria increased 2 to 4 times after winter. In 1999 the peak rose to 105 cells ml−1 . During the investigation period the highest indices of autotrophic picoplankton numbers were observed in 1999 and the lowest in 1997. The spatial distribution of this group in the whole Baikal aquatorium was characteristically heterogeneous; its abundance varied by 2 to 10 times in different parts of the lake. The largest fluctuation occurred in August and the least in September. The maximum picoplankton abundance was observed in the epilimnion during the periods of thermal stratification. In the course of homothermy a comparably homogeneous distribution of picocyanobacteria was revealed up to 300 m depth, but the highest numbers were found in the upper 30 m layer.

Picocystis salinarum gen. et sp. nov. (Chlorophyta) – a new picoplanktonic green alga

We describe Picocystis salinarum R.A. Lewin, gen. et sp. nov., a hitherto unrecognized kind of green picoplanktonic alga from a saline pond, now isolated in pure culture. The cells are normally spherical or oval, 2–3 μm in diameter, but tend towards a trilobed shape under conditions of nutrient depletion. The ultrastructure is typical for green algae. The pigment complement comprises chlorophylls a and b and the carotenoids neoxanthin, violaxanthin, alloxanthin, monadoxanthin, diatoxanthin, lutein and zeaxanthin. The major component of the cell wall is polyarabinose. Such a unique combination of features suggests that this alga might be assigned to a new lineage within the Chlorophyta.

Consumption of picoplankton by the bivalve larvae of Japanese pearl oyster Pinctada fucata martensii

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 192:195-202 (2000) - doi:10.3354/meps192195 Consumption of picoplankton by the bivalve larvae of Japanese pearl oyster Pinctada fucata martensii Yuji Tomaru1,*, Zen'ichiro Kawabata2, Shin-ichi Nakano3 1Department of Environmental Conservation Sciences, The United Graduate School of Agricultural Sciences, Ehime University, Tarumi 3-5-7, Matsuyama 790-8566, Ehime, Japan 2Center for Ecological Research, Kyoto University, Otsuka 509-3, Kamitanakami-Hirano-cho, Otsu 520-2113, Shiga, Japan 3Department of Environmental Conservation, Faculty of Agriculture, Ehime University, Tarumi 3-5-7, Matsuyama 790-8566, Ehime, Japan *E-mail: tomaruy@agr.ehime-u.ac.jp ABSTRACT: We examined grazing on bacteria and algal picoplankton (APP) by the larvae of Pinctada fucata martensii, using fluorescently labeled bacteria (FLB) and algae (FLA). Ingestion of both FLB and FLA by the larvae indicated that bacteria and APP serve as food sources for the larvae. With natural assemblages of bacteria and APP, the clearance rates of bacteria by the larvae (70 to 200 µm) ranged between 0.08 and 0.12 µl larva-1 h-1, and those of APP between 0.12 and 1.5 µl larva-1 h-1. Grazing pressure on natural populations of bacteria and APP by the larvae was estimated as <\ 0.01% d-1 in Uchiumi Bay, suggesting that these larvae play a minor role in the consumption of picoplankton. KEY WORDS: Pearl oyster larvae · Clearance rate · Picoplankton Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 192. Publication date: January 31, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Ultrastructure, cell wall composition, and phylogenetic position of Pseudodictyosphaerium jurisii (Chlorococcales, Chlorophyta) including a comparison with other picoplanktonic green algae

A pure culture of a common colonial freshwater picoplanktonic chlorophyte, Pseudodictyosphaerium jurisii, was isolated from Lake Stechlin (Brandenburg, Germany). This strain was compared with solitary picochlorophytes with respect to morphology, ultrastructure, and the biochemical composition of the cell walls. Its phylogenetic position and relationships were determined by analysis of 18S rDNA sequence data. It is similar in cell morphology and ultrastructure to Choricystis minor, Mychonastes homosphaera, and the marine picoplanktonic alga Nanochlorum eucaryotum. In their ultrastructure, biochemistry of the cell wall, and 18S rDNA, these picoplanktonic algae are clearly different from Chlorella species, and these features indicate a close relationship between Pseudodictyosphaerium and Mychonastes. In contrast to Nannochloris, which propagates by binary fission, Pseudodictyosphaerium, Choricystis, and Mychonastes multiply by autosporulation. This study provides further evidence that picoplanktonic chlorophytes evolved independently in several lineages among the Chlorophyceae and Trebouxiophyceae.

Nitrogen, Silicate and Zooplankton Control of the Planktonic Food-web in Spring

The control of silicate, nitrogen and zooplankton was studied with a special emphasis on the planktonic food web on spring bloom on the coastal area of the Baltic Sea in a laboratory experiment. Samples of 150l of surface seawater were manipulated in transparent plastic laboratory tanks for 3 weeks atin situtemperature (+2°C) with additions of nitrate and silicate (NO3–N, SiO4–Si), as well as removal of mesozooplankton by filtration through a 100μm mesh-sized plankton net. A diatom bloom developed in the experimental tanks within 7 days together with maxima in both picoplanktonic eukaryote and heterotrophic nanoflagellate abundances, during which time inorganic nitrogen and phosphorus were exhausted. Despite the clear response of phytoplankton to the silicate and nitrogen additions, the remainder of the food web was not stimulated by the nutrients. The production and abundance of picoplanktonic algae and bacteria were low and probably controlled by low temperature and bacterial substrates. Maximum growth rates of the organisms varied between 0·1 and 1·2 day−1, being the highest among diatoms and the lowest amongAcartiacopepods. The removal of mesozooplankton through prefiltration enhanced the growth of ciliates with a weak cascading effect through the microbial food web. Most of the biomass and production in the community was concentrated in the net phytoplankton, clearly indicating that in a diatom-dominated spring bloom community in the Baltic sea, only a minor fraction of carbon flow is channelled through the heterotrophic food web. Moreover, increased inputs of silicate and nitrogen to the coastal area before the spring bloom will be channelled to the production of net phytoplankton, therefore not stimulating the heterotrophic food web.

Seasonal Changes in Horizontal Distribution of Algal Picoplankton in Lake Biwa with Special Reference to Water Temperature, Nutrient Leveles and Heterotrophic Flagellates.

Seasonal changes in the horizontal distributions of algal picoplankton (APP) were examined in relation to those of heterotrophic flagellates and some environmental factors at 8 sampling stations covering both the northern and southern basins of Lake Biwa from June 1992 to March 1993. Water temperature and nutrient levels differed little among these stations. Horizontal distribution of APP was variable during the summer and fall. There was a consistent trend in the changing pattern of APP density at each station: cell density of APP at all stations was highest in August and low during the winter. Horizontal distributions of both flagellates and APP were changeable during the summer and fall. However, we could not find a seasonal trend in the changing pattern of flagellate density at each station.

A new marine picoeucaryote: Ostreococcus tauri gen. et sp. nov. (Chlorophyta, Prasinophyceae)

A new picoplanktonic alga, Ostreococcus tauri Courties et Chrétiennot-Dinet, gen. et sp. nov. (Chlorophyta, Prasinophyceae) is described from the Thau Lagoon on the Mediterranean coast, France. Almost undetectable by light or fluorescence microscopy in field studies, the cells were discovered by their flow cytometric signature and appeared numerically as the main component of the phytoplankton. Their ultrastructure is described, with additional information on cell size, pigment and DNA content. Each cell contains a nucleus, a chloroplast, one mitochondrion, one Golgi body and a very reduced cytoplasmic compartment. A starch granule and Chi a and b demonstrate its affinity to the Chlorophyta and the presence of a Chi c-like pigment, Mg 3,8 DVP (=Mg 2,4 DVP) argues for placing it in the Prasinophyceae. It differs from other coccoid taxa in ultrastructural details, mode of division and detailed pigment composition. The size and DNA content make Ostreococcus tauri the smallest eucaryote known.

Photosynthesis and growth response of the oceanic picoplankter Pycnococcus provasolii Guillard (clone Ω48-23) (Chlorophyta) to variations in irradiance, photoperiod and temperature

The growth, photosynthesis and respiration rates of the green picoplanktonic algae Pycnococcus provasolii Guillard were measured as a function of irradiance, temperature and photoperiod. The algae showed positive photoadaptation to low irradiance and from an analysis of the photosynthesis versus irradiance curves, it is suggested that this is achieved mainly by increasing the size of the photosynthetic units. In accordance with this conclusion, chlorophyll b to a ratios increased with decreasing photon flux density. The algae further compensated for low light energy supply by reducing the rates of respiration. Values of the initial slope of the growth versus irradiance curve were higher than average (0.0016–0.0022 h −1 (μ Em −2 · s −1) −1 at 20°C). It is thus concluded that P. provasolii Guillard is a well suited organism to grow at sites of low irradiance and this may explain its success in colonizing the pycnocline area of stratified oceanic waters. This capacity, however, was not accompanied by a reduced ability to photosynthesize at high irradiances. A 24 h light regime did not seem harmful to P. provasolii Guillard, however, light energy was utilized less efficiently under 24 h than under 12:12 h photoperiod. Values of Q 10 for P max and μ max were in the region of 2.

Picoplankton Associations in an Ultra-Oligotrophic Lake on Vancouver Island, British Columbia

Epiphytic associations or "microaggregates" of picoplankton with larger phytoplankton in Sproat Lake, British Columbia, occurred in the epilimnion throughout the 1-yr study, but were most frequently seen at times of severe nutrient limitation in summer and autumn. Both bacterial and algal picoplankton epiphytes were attached to the cell surface of algal hosts by either fine protrusions or by their fibrillar extracellular matrix (glycocalyx). This surface coat increased in thickness as ambient nutrient concentrations became limiting, and harboured a range of heterogeneous organic and inorganic particles and phytoplankters. The phytoplanktonic host cells for these picoplankton associations included filamentous cyanobacteria, dinoflagellates, chlorophytes, coccolithophores, and diatoms. Under controlled nutrient-depleted conditions, the filamentous nitrogen-fixing cyanobacterium Anabaena flos-aquae became a common host of chroococcoid cyanobacterium. Some picoplankters were endophytic within the cytoplasm and vacuoles of protozoa and microflagellates, serving either as prey or as symbionts. These associations are thought to facilitate rapid carbon and nutrient recycling in pelagic ecosystems.

Survey of algal picoplankton from lakes in five continents

Survey of algal picoplankton from lakes in five continents