Biology subjects

Biard, T.

Publications and source records attributed to Biard, T..

7 recordsLinked to original sources

bioRxiv2025

Sinking particle export within and beneath the euphotic zone in the eastern Indian Ocean

The eastern Indian Ocean is substantially under sampled with respect to the biological carbon pump - the suite of processes that transport the carbon fixed by phytoplankton into the deeper ocean. Using sediment traps and other ecosystem measurements, we quantified sinking organic matter flux and investigated the characteristics of sinking particles in waters overlying the Argo Abyssal Plain directly downstream of the Indonesian Throughflow off northwest Australia. Carbon export from the euphotic zone averaged 7.0 mmol C m-2 d-1, which equated to an average export efficiency (export / net primary production) of 0.17. Sinking particle flux within the euphotic zone (beneath the mixed layer, but above the deep chlorophyll maximum) averaged slightly higher than flux at the base of the euphotic zone, suggesting that the deep euphotic zone was a depth stratum of net particle remineralization. Carbon flux attenuation continued into the twilight zone with a transfer efficiency (export at euphotic depth + 100m / export at euphotic depth) of 0.62 and an average Martins b-value of 1.1. Within the euphotic zone, fresh phytoplankton (chlorophyll associated with sinking particles, possibly contained within appendicularian houses) were an important component of sinking particles, but beneath the euphotic zone the fecal pellets of herbivorous zooplankton (phaeopigments) were more important. Changes in carbon and nitrogen isotopic composition with depth further reflected remineralization processes occurring as particles sank. We show similarities with biological carbon pump functioning in a similar semi-enclosed oligotrophic marginal sea, the Gulf of Mexico, including net remineralization across the deep chlorophyll maximum. Submitted to: Deep-sea Research II HighlightsDespite low productivity, export efficiency was 17% of primary production Flux attenuation beneath the euphotic zone (EZ) was low for a tropical region Sinking particle flux from the upper to lower EZ exceeded export from lower EZ The deep EZ was a stratum of net particle remineralization (and net heterotrophy)

ecology↗

bioRxiv2025

High throughput in situ imaging reveals complex ecological behaviour of giant marine mixotrophic protists

Mixotrophic organisms have the ability to combine photosynthetic and heterotrophic nutrition, and are ubiquitous in marine ecosystems. Here, we focus on Rhizaria, a diverse group of planktonic protists that included both mixotrophic and non-mixotrophic taxa, and that are particularly delicate and poorly sampled by standard plankton nets. Using high-frequency in situ imaging, we study the ecology of these organisms directly in their undisturbed environment. We examined the fine-scale distribution and orientation of ~230,000 organisms belonging to three groups of Rhizaria, including the mixotrophic taxa Acantharia and Collodaria, and the non-mixotrophic Phaeodaria. Overall, our results suggest that mixotrophic protists have better ability to control their position within the water column (depth, orientation) than non-mixotrophic ones. Our observations also point to several steps in the obscure life cycle of the mixotrophic Collodaria, during which de novo symbiont acquisition appears to involve active fine-scale buoyancy control. Taken together, these unprecedented results demonstrate that complex ecological behaviour can be achieved by "simple" organisms.

ecology↗

bioRxiv2024

Gelatinous matrix, an original convergent evolution to cope with oligotrophy in Nassellaria (Radiolaria)

Radiolaria are heterotrophic protists abundant in the worlds oceans playing important roles in biogeochemical cycles. Some species host photosynthetic algae also contributing to primary production. Such mixotrophic behaviour is believed to explain their ecological success in oligotrophic waters, notably Collodaria, exclusively mixotrophic radiolarians within a gelatinous matrix. Yet, our understanding of Radiolaria ecology is limited to direct observations, as they have so far withstood reproduction in culture and their genomes are unexplored. Sampling oligotrophic California Current communities revealed an abundant, rarely observed population of Nassellaria of the genus Phlebarachnium, characterized to live within a gelatinous matrix along with other Radiolaria. Phylogenetic reconstruction of the ribosomal DNA suggests that these distantly related lineages within Nassellaria independently developed the ability to produce a gelatinous matrix [~]150 million years ago. By matching physical samples with their genetic signature, we identified these rarely observed organisms in global metabarcoding datasets, revealing strong biogeographic affinity to oligotrophic water masses. Global ocean co-occurrence networks showed that Radiolaria with a gelatinous matrix have a distinct biogeography compared to those without the matrix. Results suggest that the gelatinous matrix is an adaptation to oligotrophic waters, but further research is needed to evaluate similarities between the gelatinous matrices across different Radiolaria groups. This strategy could increase the effective volume to weight ratio favoring prey capture and create a favorable microenvironment for symbionts, enhancing ecological success in nutrient-depleted waters. This study advances our understanding of eukaryotic diversity evolution, emphasizing specific advantages of certain adaptations, specifically when evolution occurs independently across lineages.

evolutionary biology↗

bioRxiv2023

Complete zooplankton size spectra re-constructed from in situ imaging and Multinet data in the global ocean

Plankton size spectra are important indicators of the ecosystem state, as they illustrate the quantity of organisms available for higher marine food web and reflect multiple size-dependent processes. Yet, such measurements are typically biased by the available sampling methods, either disrupting fragile organisms or lacking good resolution (in size and/or time and space). In this study, we combined two of the most common approaches to measure zooplankton Normalized Biomass/Biovolume Size Spectra (NBSS) to calculate a complete zooplankton distribution for organisms larger than 1 mm. The reconstructed NBSS slopes appeared steeper and closer to those measured by the UVP5 (+7.6%) and flatter than those of the Multinet (- 20%) particularly in tropics and temperate latitudes. The overall gain in polar biomass was relatively small for reconstructed biomass compared to bulk estimates from Multinet (+0.24 mgC/m3 or +4.25%) and high from the UVP5 (+2.0 mgC/m3 or +53%). In contrast, in the tropical and temperate ecosystems, the gain in biomass was small for UVP5 (+0.67 mgC/m3 or +30.44% and +0.74 mgC/m3 or +19.59% respectively) and high for Multinet (+1.66 mgC/m3 or +136% and +3.4 mgC/m3 or +309% respectively). Given these differences, we suggest here to combine in situ imaging sensors and net data in any comprehensive study exploring key living players in the ocean ecosystem and their contributions to the biological pump.

ecology↗

bioRxiv2020

Morpho-molecular diversity and evolutionary analyses suggest hidden life styles in Spumellaria (Radiolaria)

Spumellaria (Radiolaria, Rhizaria) are holoplanktonic amoeboid protists, ubiquitous and abundant in the global ocean. Their silicified skeleton preserves very well in sediments displaying an excellent fossil record, from the early middle Cambrian (ca. 509-521 Ma), extremely valuable for paleo-environmental reconstruction studies. Spumellaria are tedious to maintain in laboratory conditions preventing an accurate perception of their extant diversity and ecology in today’s oceans, most of which being inferred from sediment records. This study represents an integrated and comprehensive classification of Spumellaria based on the combination of ribosomal taxonomic marker genes (rDNA) and morphological characteristics. In contrast to established taxonomic knowledge, we demonstrate that symmetry of the skeleton takes more importance than internal structures at high taxonomic rank classification. Such reconsideration allows gathering different morphologies with concentric structure and a spherical or radial symmetry believed to belong to other Radiolaria orders from the fossil record, as for some Entactinaria families. Results obtained in this study suggest the existence of new Spumellaria diversity at early diverging positions, in which a non-bearing skeleton organism lives within shelled ones. Using fossil calibrated molecular clock we estimated the origin of Spumellaria in the middle Cambrian (ca. 515 Ma), in agreement with the appearance of the first radiolarian representatives in the fossil record. The morpho-molecular and evolutionary framework established herein allows a direct connection between living specimens and fossil morphologies from the Cambrian, bringing both a standpoint for future molecular environmental surveys and a better understanding for paleo-environmental reconstruction studies.Competing Interest StatementThe authors have declared no competing interest.View Full Text

molecular biology↗

bioRxiv2019

Vertical niche definition of test-bearing protists (Rhizaria) into the twilight zone revealed by in situ imaging

The Rhizaria is a super-group of ameoboid protists with ubiquitous distributions, from the euphotic zone to the twilight zone and beyond. While rhizarians have been recently described as important contributors to both silica and carbon fluxes, we lack the most basic information about their ecological preferences. Here, using the in situ imaging (Underwater Vision Profiler 5), we characterize the vertical ecological niches of different test-bearing rhizarian taxa in the southern California Current Ecosystem. We define three vertical layers between 0-500 m occupied, respectively, by 1) surface dwelling and mostly symbiont-bearing rhizarians (Acantharia and Collodaria), 2) flux-feeding phaeodarians in the lower epipelagic (100-200 m), and 3) Foraminifera and Phaeodaria populations adjacent to the Oxygen Minimum Zone. We then use Generalized Additive Models to analyze the response of each rhizarian category to a suite of environmental variables. The models explain between 13 and 93% of the total variance observed for the different groups. While temperature and the depth of the deep chlorophyll maximum, appear as the main factors influencing populations in the upper 200 m, silicic acid concentration is the most important variable related to the abundance of mesopelagic phaeodarians. The relative importance of biotic interactions (e.g., predation, parasitism) is still to be considered, in order to fully incorporate the dynamics of test-bearing pelagic rhizarians in ecological and biogeochemical models.

ecology↗

bioRxiv2018

Time calibrated morpho-molecular classification of Nassellaria (Radiolaria)

Nassellaria are marine radiolarian protists belonging to the Rhizaria lineage. Their skeleton, made of opaline silica, exhibit an excellent fossil record, extremely valuable in micro-paleontological studies for paleo-environmental reconstruction. Yet, to date very little is known about the extant diversity and ecology of Nassellaria in contemporary oceans, and most of it is inferred from their fossil record. Here we present an integrative classification of Nassellaria based on taxonomical marker genes (18S and 28S ribosomal DNA) and morphological characteristics obtained by optical and scanning electron microscopy imaging. Our phylogenetic analyses distinguished 11 main morpho-molecular clades relying essentially on the overall morphology of the skeleton and not on internal structures as previously considered. Using fossil calibrated molecular clock we estimated the origin of Nassellaria among radiolarians primitive forms in the Devonian (ca. 420 Ma), that gave rise to living nassellarian groups in the Triassic (ca. 250 Ma), during the biggest diversification event over their evolutionary history. This morpho-molecular framework provides both a new morphological classification easier to identify under light microscopy and the basis for future molecular ecology surveys. Altogether, it brings a new standpoint to improve our scarce understanding of the ecology and worldwide distribution of extant nassellarians.

molecular biology↗