Expression of secondary metabolites by the Mediterranean sponges Aplysina aerophoba and Aplysina cavernicola

Aplysina aerophoba and Aplysina cavernicola are two sibling sponge species common in Mediterranean sea. External and internal morphology is almost identical in the two species. So, according to a lack of traditional taxonomic character (spicule comparison), taxonomists have longly debated if the two taxa, showing different ecological requirements but identical morphology, should be regarded as ecotypes or distinct species. Now, molecular phylogeny has brought response to this question: A. aerophoba and A. cavernicola are lately distinct species. The two species, also called bacteriosponges because of their dense community of symbiotic bacteria that can amount up to 40 % of their dry weight, can contain high amounts of secondary metabolites: brominated alkaloids derived from tyrosine (up to 13% of dry weight). As part of a program in chemical ecology where the main goal is to better understand the processes controlling the chemical diversity and its variation in marine invertebrates, (1) we finalized the methodology to quantify the major secondary metabolites produced by the two Mediterranean Aplysina species, (2) we identified chemotaxonomic markers in order to clearly discriminate the two sibling species, (3) we explored whether variation, in quality and quantity, of secondary metabolites expression occurs at different biogeographic scales, and (4) we investigated the role of microbial symbionts in bromotyrosine alkaloids biosynthesis

La méiofaune du biofilm épilithique de rivière : dynamique et interactions trophiques

La dynamique à long terme de la méiofaune a été examinée dans les biofilms épilithiques de la Garonne, en considérant les contraintes environnementales. En complément, les contenus pigmentaires intestinaux des nématodes ont été analysés par HPLC, et une multi-approche isotopique incluant une mesure des signatures naturelles d13C et d15N et une expérience de marquage au 13C a été réalisée in situ, afin d'étudier les interactions trophiques impliquant la méiofaune habitant ces biofilms. Les résultats montrent que les scénarios hydrologiques (durée des étiages et fréquence des crues) et l'état du biofilm (dynamique de sa biomasse et de sa composition microphytique) ont façonné la structure de la communauté méiobenthique. De possibles interférences avec les macro-invertébrés brouteurs ont aussi été soupçonnées. Les nématodes Chromadorina spp. consommaient non-selectivement les diatomées épilithiques, tandis que l'essentiel de leur régime alimentaire était basé sur l'utilisation de substances polymériques extra-cellulaires sécrétées par les microphytes. Le carbone fraîchement photosynthétisé était rapidement incorporé par la méiofaune, bien que quantitativement, les larves macrobenthiques de Chironomidae broutaient davantage.The long-term dynamics of meiofauna with regards to environmental constraints was monitored in epilithic biofilms of the Garonne River, France. In addition, HPLC-analysis of nematode gut pigment content and a stable isotope multi-approach-including measurement of d13C and d15N natural signatures and an in situ 13C-labelling experiment-were performed to disentangle trophic interactions involving biofilm-dwelling meiofauna. Hydrological scenarios and fluctuation of microphyte availability primarily shaped the structure of the meiobenthic community. Also, possible interferences with macro-invertebrate grazers were suspected during July. Chromadorina spp. nematodes grazed non-selectively on epilithic diatom contents, while most of their diet likely derived from extra-cellular polymeric substances produced by microphytes. Freshly photosynthesized carbon was rapidly incorporated by meiofauna, although quantitatively, macrofaunal Chironomidae were the most important grazers

Meiofauna in river epilithic biofilm: Dynamics and trophic relationships

The long-term dynamics of meiofauna with regards to environmental constraints was monitored in epilithic biofilms of the Garonne River, France. In addition, HPLC-analysis of nematode gut pigment content and a stable isotope multi-approach—including measurement of δ13C and δ15N natural signatures and an in situ 13C-labelling experiment—were performed to disentangle trophic interactions involving biofilm-dwelling meiofauna. Hydrological scenarios and fluctuation of microphyte availability primarily shaped the structure of the meiobenthic community. Also, possible interferences with macro-invertebrate grazers were suspected during July. Chromadorina spp. nematodes grazed non-selectively on epilithic diatom contents, while most of their diet likely derived from extra-cellular polymeric substances produced by microphytes. Freshly photosynthesized carbon was rapidly incorporated by meiofauna, although quantitatively, macrofaunal Chironomidae were the most important grazers

Quantification of sediment reworking by the Asiatic clam Corbicula fluminea Müller, 1774

Active organisms modify the substratum in which they dwell. This process, called “bioturbation”, affects the way that biogeochemical fluxes are mediated at the substratum–water interface. In the frame of this work, the bioturbation potential of the Asiatic clam Corbicula fluminea was characterized and quantified. We measured the displacement of fluorescent particles by C. fluminea burying in a size-based experimental design in order to explore the effects of body-size on sediment reworking. Our results stress that C. fluminea belongs to the functional group of biodiffusors, and that C. fluminea can be considered as an intermediate sediment reworker. We suggest that bioturbation was mainly induced by the pedal-feeding activity of the clams. Results also showed that, though large clams induced displacement of particles deeper into the sediment, small clams showed the highest net sediment reworking activity. This result was in contrast to the initial hypothesis of biovolume as the main driver for particle displacement by bioturbating organisms. Life-history traits and specific features of pedal-feeding could explain the observed pattern

The early catfish catches the worm: predation of Corydoras aeneus (Siluriformes, Callichthyidae) on freshwater nematodes

Majdi N, Weber S, Traunspurger W. The early catfish catches the worm: predation of Corydoras aeneus (Siluriformes, Callichthyidae) on freshwater nematodes. ANNALES DE LIMNOLOGIE-INTERNATIONAL JOURNAL OF LIMNOLOGY. 2018;54: UNSP 29.In this study, we measured the daily consumption of four different nematode species by a small freshwater catfish species, Corydoras aeneus (Gill, 1858). Consumption of nematodes by fishes was significant with a single C. aeneus individual being able to consume in 24 h between 40 581 and 75 849 adult nematodes depending on the nematode species offered. This represented the ingestion of up to 238 mg wet weight when considering the largest nematode species: Panagrellus redivivus. Our results strengthen the growing evidence of a significant trophic channel existing between meiobenthic invertebrates like nematodes and small bottom-feeding fishes like C. aeneus. We also discuss the relevance of using P redivivus as live food for rearing C. aeneus which is a popular ornamental fish

Contribution of epilithic diatoms to benthic−pelagic coupling in a temperate river

Water residence time in the middle course of rivers is often too short to allow substantial phytoplankton development, and primary production is essentially provided by benthic phototrophic biofilms. However, cells occurring in the water column might derive from biofilm microalgae, and, reciprocally, sedimenting microalgae could represent a continuous source of colonizers for benthic biofilms. A comparative study of biofilm and pelagic microphytic communities (with special focus on diatoms) was carried out over 15 mo in the Garonne River, France. Diatoms dominated both biofilm and pelagic microphytic communities. Typically benthic diatoms were found in high abundance in the water column, and their biomass in the water was correlated with their biomass in the biofilm, indicating the benthic origin of these cells. Variations in river discharge and temperature drove the temporal distribution of benthic and pelagic communities: under high flow mixing (winter) communities showed the greatest similarity, and during low flow (summer)they differed the most. Even during low flow, typical benthic species were observed in the water column, indicating that benthic−pelagic exchanges were not exclusively due to high water flow. Moreover, during low flow periods, planktonic diatoms typically settled within biofilms, presumably because of higher water residence times, and/or upstream reservoir flushing

Nematodes down under: State of knowledge and future trends

Nematodes inhabit most environments that address their need of water, carbon, and energy. They also play an important role in epigean food webs by occupying different trophic levels, while providing essential ecosystem services such as regulating decomposition and nutrient mineralization. Conversely, little is known about the role that nematodes play in subterranean ecosystems. For this reason, an in-depth review was undertaken in order to concatenate data from existing literature and provide a baseline for future studies. A total of 41 surveyed scientific works, published over a time period of 138 years, reported 295 unique taxa from 78 different cave systems. Although an increase in scientific outputs was recorded from the late 1990s, there is a considerable lack of information on cave-dwelling nematodes from Asia, South America, and North America. Also, only seven true cave-dwelling (troglobitic) species have been discovered, which may indicate a lack of comprehensive taxonomic efforts. From an ecological perspective, very few works have reported on the importance of nematode assemblages associated with cave ecosystems. This has resulted in the causal factors leading to population resilience processes, as well as trophic interactions, remaining mostly speculative. Nonetheless, the isolated cave-dwelling nematode assemblage associated with Movile Cave, including the true cave-dwelling species Chronogaster troglodytes, fulfills an important role by feeding on microbial mats and in turn serving as a food source for predators. This example of a cave nematode assemblage providing ecosystem services has inspired the undertaking of the Gcwihaba Caves Research Project, which aims at studying artificially opened caves in Ngamiland, Botswana. Compared to other caves in the area with natural openings, the artificially opened systems present different food source pathways, atmospheric conditions, and community assemblage structures. This provides a unique opportunity to study isolated cave communities, including nematode assemblages, as well as their ecological importance

Effects of a broad range of experimental temperatures on the population growth and body-size of five species of free-living nematodes

Majdi N, Traunspurger W, Füser H, Gansfort B, Laffaille P, Maire A. Effects of a broad range of experimental temperatures on the population growth and body-size of five species of free-living nematodes. Journal of Thermal Biology. 2019;80:21-36

Duration and frequency of non‐flow periods affect the abundance and diversity of stream meiofauna

Majdi N, Colls M, Weiss L, Acuña V, Sabater S, Traunspurger W. Duration and frequency of non‐flow periods affect the abundance and diversity of stream meiofauna. Freshwater Biology. 2020;65(11):1906-1922.Abstract Intermittent streams (IS) comprise a large proportion of the drainage network in many parts of the world. The non‐flow period of IS are known to impact stream biota because aquatic habitats dry out. However, less well understood are the relative effects of the temporal component of these drying events including their duration and frequency. Here, we characterised effects of temporal component of drying events on abundant and species‐rich meiofauna. The effects were assessed in 22 streams in the north‐eastern Iberian Peninsula. The duration and frequency of non‐flow events was characterized over a period of 250 days prior to sampling the sediment‐dwelling meiofauna in riffle zones that completely dried out. Overall, meiofauna abundances were amongst the highest ever reported for streambeds. Most meiofaunal taxa correlated positively with the frequency of drying events and correlated positively with the length of dry periods recorded shortly before sampling, suggesting that the community was able to recover quickly. Tardigrades were the only group to correlate positively with the longest dry periods, suggesting that they had the best resilience capabilities in streams that had experienced the longest droughts. On average, nematodes made up half of the meiofauna. We identified a total of 113 different nematode species. The nematode community was more taxonomically diverse in IS, with a smaller proportion of bacterivores and a higher proportion of fungivore species such as Filenchus vulgaris. Thereby resembling the trophic structure commonly observed in soil ecosystems. Our results show that most meiofauna were positively influenced by drying disturbance, that is being able to quickly recover after them. This suggests outstanding resilience capabilities, and points out meiofaunal organisms as key players for kick‐starting stream food webs and functions once flow returns

The relationship between epilithic biofilm stability and its associated meiofauna under two patterns of flood disturbance

Habitat stability is an important driver of ecological community composition and development. River epilithic biofilms are particularly unstable habitats for the establishment of benthic communities because they are regularly disturbed by floods. Our aim was to determine the influence of habitat instability on meiobenthic organisms. We hypothesized that hydrologic variables are the most important predictors of meiofauna distribution. We monitored epilithic communities (meiofauna and microalgae) with a high sampling frequency during 2 sampling periods with contrasting hydrodynamic patterns in a temperate river (the Garonne, France). Nematodes and rotifers dominated meiofaunal assemblages. The critical flow velocity threshold for their maintenance in the biofilm was ,30 cm/s, a result suggesting that meiofauna can resist higher flow velocity within the biofilm than within sediments. Nematode distribution was primarily influenced by the duration of undisturbed periods, whereas rotifer distribution was also correlated with the thickness of the biofilm. During the periods after floods, rotifers were faster colonizers than nematodes. Collectively, our results show that flow regime was an essential driver for biofilm community development