Mikel Latasa

A Single Species, Micromonas pusilla (Prasinophyceae), Dominates the Eukaryotic Picoplankton in the Western English Channel

ABSTRACT The class Prasinophyceae (Chlorophyta) contains several photosynthetic picoeukaryotic species described from cultured isolates. The ecology of these organisms and their contributions to the picoeukaryotic community in aquatic ecosystems have received little consideration. We have designed and tested eight new 18S ribosomal DNA oligonucleotide probes specific for different Prasinophyceae clades, genera, and species. Using fluorescent in situ hybridization associated with tyramide signal amplification, these probes, along with more general probes, have been applied to samples from a marine coastal site off Roscoff (France) collected every 2 weeks between July 2000 and September 2001. The abundance of eukaryotic picoplankton remained high (>103 cells ml−1) during the sampling period, with maxima in summer (up to 2 × 104 cells ml−1), and a single green algal species, Micromonas pusilla (Prasinophyceae), dominated the community all year round. Members of the order Prasinococcales and the species Bathycoccus prasinos (Mamiellales) displayed sporadic occurrences, while the abundances of all other Prasinophyceae groups targeted remained negligible.

Climate influence on deep sea populations.

Dynamics of biological processes on the deep-sea floor are traditionally thought to be controlled by vertical sinking of particles from the euphotic zone at a seasonal scale. However, little is known about the influence of lateral particle transport from continental margins to deep-sea ecosystems. To address this question, we report here how the formation of dense shelf waters and their subsequent downslope cascade, a climate induced phenomenon, affects the population of the deep-sea shrimp Aristeus antennatus. We found evidence that strong currents associated with intense cascading events correlates with the disappearance of this species from its fishing grounds, producing a temporary fishery collapse. Despite this initial negative effect, landings increase between 3 and 5 years after these major events, preceded by an increase of juveniles. The transport of particulate organic matter associated with cascading appears to enhance the recruitment of this deep-sea living resource, apparently mitigating the general trend of overexploitation. Because cascade of dense water from continental shelves is a global phenomenon, we anticipate that its influence on deep-sea ecosystems and fisheries worldwide should be larger than previously thought.

A comparison of phytoplankton populations of the Arabian Sea during the Spring Intermonsoon and Southwest Monsoon of 1995 as described by HPLC-analyzed pigments

We used diagnostic pigments to estimate the relative abundances of different algal groups in the Arabian Sea during the Spring Intermonsoon (cruise TN045) and late Southwest Monsoon (cruise TN050) of 1995. Northern and southern transect lines were occupied during each cruise. These transects started near the coast of the Arabian Peninsula at 22.38°N and 18.50°N, respectively, and extended >1200 km offshore. The pigment concentrations at the offshore stations (>1000 km from the coast) were low during both cruises, and the composition of the phytoplankton resembled that found in oligotrophic, open-ocean waters. The pigment compositions at the inshore stations (<1000 km from the coast) were markedly different during both cruises. During TN045, the inshore waters of the northern transect were dominated by a mixture of diatoms and haptophytes. Large particles (>18 μm) accounted for up to 23% of the total chlorophyll a-related pigments (TCHLA, chlorophyllide a+monovinyl chlorophyll a+divinyl chlorophyll a). Pigment biomass (=500 ng TCHLA L-1) was homogeneously distributed in the upper water column at these stations. By contrast, pigment biomass at the inshore stations of the southern transect was concentrated in a pronounced deep chlorophyll maximum. The phytoplankton were clearly partitioned in the water column, with photosynthetic prokaryotes and eukaryotes dominating in the upper mixed layer and below, respectively. The high levels of divinyl chlorophyll a observed (up to 300 ng l-1) represent the highest concentrations ever recorded for this prochlorophyte marker. The size-fractionated pigment analyses revealed that 85% of the TCHLA passed through 2 μm filters, indicating dominance by the picophytoplankton. During TN050, the inshore portion of the northern transect displayed a clear example of a diatom-to-Phaeocystis successional event. Diatoms dominated at coastal upwelling stations and were progressively replaced by Phaeocystis at offshore stations. At the inshore stations of the southern transect, diatoms dominated the phytoplankton community, and a large fraction of the TCHLA (21–48%) was retained by 18 μm filters. A filament with high diatom biomass was observed 700 km offshore, documenting that horizontal advection is an important mechanism for removing phytoplankton from coastal waters in this region. However, lower than expected TCHLA concentrations were measured during this period relative to those observed in earlier years in the CZCS imagery. Several hypotheses are discussed to explain this enigma, including sampling biases, losses by grazing and sinking, phytoplankton dilution via upwelling, and Si limitation of diatom growth.

Iron‐stimulated changes in 13C fractionation and export by equatorial Pacific phytoplankton: Toward a paleogrowth rate proxy

We present δ13C values for phytol, an algal biomarker, which document up to 7 per mil isotopic enrichment during the IronEx II iron fertilization experiment. We evaluate these data using a laboratory-derived 13C fractionation model and show this variability is largely the result of elevated growth rates. Isotopic enrichment and stimulation of growth rate were accompanied by a sevenfold increase in the export of particulate organic carbon as estimated from 234Th activities. This is the first direct evidence that enhanced productivity following iron enrichment can lead to both increased export of organic matter and an associated isotopic signal in an algal biomarker. On the basis of these results, we propose biomarker isotopic data be used in conjunction with paleo-CO2 records to reconstruct paleogrowth rates. This approach provides a means to test for iron-stimulated changes in algal growth in sedimentary records.

PIGMENT SUITES AND TAXONOMIC GROUPS IN PRASINOPHYCEAE

Pigment analysis performed on 30 Prasinophyceae strains revealed two main groups: the prasinoxanthin‐containing and prasinoxanthin‐less Prasinophyceae. Prasinoxanthin‐containing Prasinophyceae comprised the orders Mamiellales, Pseudoscourfieldiales (Pycnococcaceae), and Prasinococcales. For this group, classification with pigment composition showed a good agreement with molecular phylogeny. Mamiellales, except Crustomastix stigmatica, accumulated uriolide, micromonal, dihydrolutein, and the pigment Unidentified M1 as characteristic pigments. Prasinococcales and Pseudoscourfieldiales (Pycnococcaceae) lacked micromonal and Unidentified M1. In addition, Pseudoscourfieldiales (Pycnococcaceae) lacked uriolide. A chl c3‐like pigment was present in prasinoxanthin‐containing strains isolated from the deep sea. Common green algae pigments, a loroxanthin derivative, and siphonaxanthin plus derivatives were found in the prasinoxanthin‐less Prasinophyceae, which included strains from Pyramimonadales, Pseudoscourfieldiales (Nephroselmidiaceae), Chlorodendrales, and a new order. Although some associations could be observed, the correspondence between pigments and molecular taxonomy was less clear for this group.

Factors controlling seasonal variability of benthic ammonium release and oxygen uptake in Alfacs Bay (Ebro Delta, NW Mediterranean)

The seasonal variability of sediment–water ammonium flux andoxygen uptake was studied in an estuarine bay (Alfacs Bay, Ebro Delta, NWMediterranean) influenced by temporal freshwater discharges. Three stationswith different organic loading were sampled. The relationships of benthicfluxes to bottom water (temperature, dissolved oxygen, ammonium, nitrateplus nitrite) and to sediment (porosity, chlorophyll a derivative pigments,organic carbon and nitrogen) variables were examined. Oxygen uptake rangedfrom 0.3 to 2.5 mmol m−2 h−1 and ammoniumrelease ranged from 6 to 230 µmol m−2 h−1.The lowest value was recorded at the station furthest from the freshwaterinputs, and the highest was at the littoral station nearest the freshwaterdischarge channels (for oxygen uptake) and at the deep station at the saltwedge front (for ammonium flux). Water temperature and the concentration ofchlorophyll a derivative pigments on the surface sediment were revealed asthe main variables to be taken into account to explain the variabilityfound. Changes in fluxes reflecting temperature changes were found at thestation furthest from the freshwater inputs, while at the other, fluxvariability was found to be related to the cycle of functioning offreshwater discharge channels. The different patterns of variability arediscussed in relation to the dynamics of the estuary and to the mainfeatures of benthic nitrogen cycling.

Factors controlling spatial variability in ammonium release within an estuarine bay (Alfacs Bay, Ebro Delta, NW Mediterranean)

Sediment-water ammonium fluxes, oxygen uptake and sediment characteristics were studied in an estuarine bay influenced by temporal freshwater discharges. Sediment at seven stations representing a gradient imposed by freshwater inputs was sampled for sediment-water ammonium and oxygen fluxes, chlorophyll a derivative pigments, organic content, porosity and elemental composition (Fe, Mn, Si, Al). Oxygen uptake decreased along the gradient and correlated with total chlorophyll a derivatives indicating the close coupling between aerobic metabolism and short-time sedimentation events. Ammonium release showed a discontinuous pattern of decrease along the gradient and only correlated with the Fe:Mn atomic ratio. Correlation between the structural properties of the sediment (Si:Al atomic ratio, porosity and organic content) and ammonium release was also found (excluding data from the station with the highest ammonium flux). The extent of the influence of metabolism and sediment structure on ammonium release is discussed.

Variations in biochemical parameters of Heterocapsa sp. and Olisthodiscus luteus grown in 12:12 light:dark cycles I. Cell cycle and nucleic acid composition

The division cycle of two phytoplankton species, Olisthodiscus luteus and Heterocapsa sp. was studied in relation to a 12:12 light:dark cycle. Batch cultures in exponential phase were sampled every three hours during 48 hours. Cell number, cellular volume and DNA and RNA concentrations were measured. Microscopic observations of the nuclei of Heterocapsa sp. were also performed. In both species, cell division took place in the dark. In Heterocapsa sp., DNA and RNA showed a similar diel variability pattern, with synthesis starting at the end of the light period, previously to mitosis and cytokinesis. In O. luteus. Major RNA synthesis occurred during darkness, and DNA was produced almost continuously. Both species presented different values and diel rhythmicity on the RNA/DNA ratios.

Variations in biochemical parameters of Heterocapsa sp. and Olisthodiscus luteus grown in 12:12 light:dark cycles II. Changes in pigment composition

Photosynthetic pigment composition was studied in batch cultures of Heterocapsa sp. and Olisthodiscus luteus growing exponentially in a 12:12 light:dark cycle. Both species divided in the dark. The synthesis of pigments was continuous for both species. However for chlorophyll c and peridinin, in Heterocapsa sp., and chlorophyll c and fucoxanthin, in O. luteus, (pigments belonging to light harvesting complexes) the synthesis was significantly higher during the light period. Concentrations per total cell volume (TCV) of chlorophyll a, chlorophyll c, peridinin and diadinoxanthin in Heterocapsa sp., and chlorophyll a, chlorophyll c, fucoxanthin and violaxanthin in O. luteus, showed a maximum at the onset of light and decreased during the light period. The values of the chlorophyll a:chlorophyll c, chlorophyll a:peridinin and chlorophyll a:fucoxanthin ratios are compared with data reported in the literature.

Variations in biochemical parameters of Heterocapsa sp. and Olisthodiscus luteus grown in 12:12 light:dark cycles

Photosynthetic pigment composition was studied in batch cultures of Heterocapsa sp. and Olisthodiscus luteus growing exponentially in a 12:12 light:dark cycle. Both species divided in the dark. The synthesis of pigments was continuous for both species. However for chlorophyll c and peridinin, in Heterocapsa sp., and chlorophyll c and fucoxanthin, in O. luteus, (pigments belonging to light harvesting complexes) the synthesis was significantly higher during the light period. Concentrations per total cell volume (TCV) of chlorophyll a, chlorophyll c, peridinin and diadinoxanthin in Heterocapsa sp., and chlorophyll a, chlorophyll c, fucoxanthin and violaxanthin in O. luteus, showed a maximum at the onset of light and decreased during the light period. The values of the chlorophyll a:chlorophyll c, chlorophyll a:peridinin and chlorophyll a:fucoxanthin ratios are compared with data reported in the literature.