Proliferation of Purple Sulphur Bacteria at the Sediment Surface Affects Intertidal Mat Diversity and Functionality

There is a relative absence of studies dealing with mats of purple sulphur bacteria in the intertidal zone. These bacteria display an array of metabolic pathways that allow them to disperse and develop under a wide variety of conditions, making these mats important in terms of ecosystem processes and functions. Mass blooms of purple sulphur bacteria develop during summer on sediments in the intertidal zone especially on macroalgal deposits. The microbial composition of different types of mats differentially affected by the development of purple sulphur bacteria was examined, at low tide, using a set of biochemical markers (fatty acids, pigments) and composition was assessed against their influence on ecosystem functions (sediment cohesiveness, CO₂ fixation). We demonstrated that proliferation of purple sulphur bacteria has a major impact on intertidal mats diversity and functions. Indeed, assemblages dominated by purple sulphur bacteria (Chromatiaceae) were efficient exopolymer producers and their biostabilisation potential was significant. In addition, the massive growth of purple sulphur bacteria resulted in a net CO₂ degassing whereas diatom dominated biofilms represented a net CO₂ sink.     

Pre-zygotic factors best explain reproductive isolation between the hybridizing species of brittle-stars <Emphasis Type="Italic">Acrocnida brachiata</Emphasis> and <Emphasis Type="Italic">A. spatulispina</Emphasis> (Echinodermata: Ophiuroidea)

The two brittle-stars Acrocnida brachiata (Montagu 1804) and A. spatulispina (Stöhr and Muths in J Mar Biol Assoc, 2009) exhibit strong spatial segregation along the coast of Brittany (France), the first being subtidally distributed relative to the other intertidal species. Despite a very high degree of mitochondrial DNA divergence, previous preliminary results hinted at the potential for hybridization to occur. Therefore, we specifically aim to determine local levels of hybridization between these two species and to investigate the relative roles of pre- and post- zygotic isolation processes acting to decrease local hybridization patterns. Mitochondrial DNA, allozymes and the Internal Transcribed Spacer 2 region of the ribosomal DNA were all used on 529 brittle-stars sampled locally in June and September 2005, among six stations in Douarnenez Bay, a site situated at the tip of Brittany. Only 2.6% of all samples analyzed were identified as potential hybrids. However, these were twice more frequent in June, just after the reproductive period, than in September after selective mortality acted to reduce the proportions of hybrids. In addition to the abrupt bathymetric segregation between the two species, spawning asynchrony also clearly restricts hybridization to low levels, which shows the importance of pre-zygotic mechanisms in maintaining reproductive isolation. Moreover, both limited hybridization events and adult mortalities following reproduction tend to generate local genetic differentiation at the intra-species level. On the contrary, the genetic structure is homogenized by migration of juveniles or adults and hybrids mortalities over the summer period.     

Seasonal variations of community production,respiration and biomass of different primary producers in an intertidal <Emphasis Type="Italic">Zostera noltii</Emphasis> bed (Western English Channel,France)

Gross primary production (GPP) and respiration (R) in an intertidal Zostera noltii (Hornemann, 1832) community were estimated each month from in situ CO₂ flux measurements during the emersion period in the Bay of Morlaix (France). Simultaneously, the Chlorophyll a biomass of Z. noltii leaves, associated epiphytes and sediment within the benthic chambers were also monitored. The significant positive exponential relationship established between R and the maximum daily temperature emphasized that temperature was a good predictor of seasonal variation in the community R in Z. noltii bed. In contrast, the amplitude of the fitted curve on GPP was low and represented <25% of the annual mean value (90.9 ± 15.2 mgC m⁻² h⁻¹). The Chlorophyll a biomass of sediment was high (133.70 ± 22.96 mg m⁻²) but it was likely constituted by detritus which did not contribute much to the community GPP. The Chlorophyll a biomass of associated epiphytes was higher in winter and varied over the year inversely with Z. noltii. Therefore, the asynchronous dynamic of primary producers could support a constant rate of production at community scale over the year.     

Combined effects of global climate change and nutrient enrichment on the physiology of three temperate maerl species

Made up of calcareous coralline algae, maerl beds play a major role as ecosystem engineers in coastal areas throughout the world. They undergo strong anthropogenic pressures, which may threaten their survival. The aim of this study was to gain insight into the future of maerl beds in the context of global and local changes. We examined the effects of rising temperatures (+3°C) and ocean acidification (?0.3 pH units) according to temperature and pH projections (i.e., the RCP 8.5 scenario), and nutrient (N and P) availability on three temperate maerl species (Lithothamnion corallioides, Phymatolithon calcareum, and Lithophyllum incrustans) in the laboratory in winter and summer conditions. Physiological rates of primary production, respiration, and calcification were measured on all three species in each treatment and season. The physiological response of maerl to global climate change was species‐specific and influenced by seawater nutrient concentrations. Future temperature–pH scenario enhanced maximal gross primary production rates in P. calcareum in winter and in L. corallioides in both seasons. Nevertheless, both species suffered an impairment of light harvesting and photoprotective mechanisms in winter. Calcification rates at ambient light intensity were negatively affected by the future temperature–pH scenario in winter, with net dissolution observed in the dark in L. corallioides and P. calcareum under low nutrient concentrations. Nutrient enrichment avoided dissolution under future scenarios in winter and had a positive effect on L. incrustans calcification rate in the dark in summer. In winter conditions, maximal calcification rates were enhanced by the future temperature–pH scenario on the three species, but P. calcareum suffered inhibition at high irradiances. In summer conditions, the maximal calcification rate dropped in L. corallioides under the future global climate change scenario, with a potential negative impact on CaCO₃ budget for maerl beds in the Bay of Brest where this species is dominant. Our results highlight how local changes in nutrient availability or irradiance levels impact the response of maerl species to global climate change and thus point out how it is important to consider other abiotic parameters in order to develop management policies capable to increase the resilience of maerl beds under the future global climate change scenario.     

Accumulation of detached kelp biomass in a subtidal temperate coastal ecosystem induces succession of epiphytic and sediment bacterial communities

Kelps are dominant primary producers in temperate coastal ecosystems. Large amounts of kelp biomass can be exported to the seafloor during the algal growth cycle or following storms, creating new ecological niches for the associated microbiota. Here, we investigated the bacterial community associated with the kelp Laminaria hyperborea during its accumulation and degradation on the seafloor. Kelp tissue, seawater and sediment were sampled during a 6-month in situ experiment simulating kelp detritus accumulation. Evaluation of the epiphytic bacterial community abundance, structure, taxonomic composition and predicted functional profiles evidenced a biphasic succession. Initially, dominant genera (Hellea, Litorimonas, Granulosicoccus) showed a rapid and drastic decrease in sequence abundance, probably outcompeted by algal polysaccharide-degraders such as Bacteroidia members which responded within 4 weeks. Acidimicrobiia, especially members of the Sva0996 marine group, colonized the degrading kelp biomass after 11 weeks. These secondary colonizers could act as opportunistic scavenger bacteria assimilating substrates exposed by early degraders. In parallel, kelp accumulation modified bacterial communities in the underlying sediment, notably favouring anaerobic taxa potentially involved in the sulfur and nitrogen cycles. Overall, this study provides insights into the bacterial degradation of algal biomass in situ, an important link in coastal trophic chains.     

Degradation dynamics and processes associated with the accumulation of Laminaria hyperborea (Phaeophyceae) kelp fragments: an in situ experimental approach

A high proportion of the kelp Laminaria hyperborea production is exported from kelp forests following seasonal storms or natural annual old blade loss. Transport of drifting kelp fragments can lead to temporary accumulations in benthic subtidal habitats. We investigated the degradation processes of L. hyperborea in a low subtidal sandy bottom ecosystem by setting up a 6-month cage experiment to simulate accumulations of kelp fragments on the seafloor. We monitored temporal changes in biomass, nutritional quality (C:N ratio), respiration, quantum efficiency of photosystem II (F_v/F_m), bacterial colonization, and chemical defense concentrations. Biomass decomposition started after 2 weeks and followed a classic negative exponential pattern, leading to 50% degradation after 8 weeks. The degradation process seemed to reach a critical step after 11 weeks, with an increase in respiration rate and phlorotannin concentration in the tissues. These results likely reflect an increase in bacterial activity and a weakening of the kelp cell wall. After 25 weeks of degradation, only 16% of the initial biomass persisted, but the remaining large fragments looked intact. Furthermore, photosystems were still responding to light stimuli, indicating that photosynthesis persisted over time. Reproductive tissues appeared on some fragments after 20 weeks of degradation, showing a capacity to maintain the reproductive function. Our results indicate that L. hyperborea fragments degrade slowly. As they maintain major physiological functions (photosynthesis, reproduction, etc.) and accumulate on adjacent ecosystems, they may play a long-term ecological role in coastal ecosystem dynamics.     

Multi-scale heterogeneity in a macrobenthic epifauna community

A quantitative study, based on monthly samples taken by divers with 0.1m² quadrats, was conducted on a benthic community settling coarse sediments in an area exposed to strong water currents.The richness of the community was high both in terms of biomass (270±107 g m^–2 in ash-free dry weight) and diversity (ten zoologicals groups for a total of 98 species). Three suspension feeding species were dominant: Ophiothrix fragilis, Urticina felina and Alcyonium digitatum accounted for more than 95% of the total biomass at all sampling periods.Spatial heterogeneity analysis confirmed the role of these three dominant species. Heterogeneity revealed at a scale of 0.1 m² by dendrograms (qualitative similarity index) or by Factor Correspondence Analysis (FCA) (on biomasses) was due to the strong dispersion of rare species.The influence of the observation scale was visualised by building Rank-Frequency Diagrams using progressively aggregated replicates. The distribution of relative biomasses and the rank of the dominant species always stabilized at scale of 0.4 or 0.5 m².The three dominant species were overdispersed at a scale of 0.1 m² compared with a Poisson distribution. Spatial competition among the three species was suggested by their dispersion in the plane of the FCA. Calculation of a multiple linear regression (at the scale of 0.8 m²) showed negative relationships among these dominant species.Temporal homogeneity was indicated (at a scale of 0.8 m²) by dendrograms of species similarities among samples and by FCA performed on total biomasses.     

Physiological reactions to a change in light regime in cultured Skeletonema costatum (Bacillariophyta): implications for estimation of phytoplankton biomass

The marine planktonic diatom Skeletonema costatum (Cleve) was grown in batch culture under a 12 h light: 12 h dark (LD) regime for several generations before the experiment. At Time 0, half of the culture was transferred to continuous light (CL). Particulate organic carbon (POC), nitrogen (PON), and photosynthetic pigments (measured by spectrophotometry and high performance liquid chromatography) were monitored for 7 to 9 days in the two cultures. Under CL, POC and PON production were always lower. In addition, we measured a much higher proportion of chlorophyll degradation products (as chlorophyllid a, phaeophytin a and phaeophorbid a (PrD)). These indicate different physiological conditions between the two cultures, as reflected also be the lower POC/PON and chlorophyll a/POC ratios. Under CL illumination cells appear stressed, probably due to the total quantity of light to which cells are exposed. In fact, a higher proportion of the two xanthophylls diadinoxanthin and diatoxanthin in the CL culture indicates a photoprotective reaction of the cells. In contrast with the LD culture, parameters measured were not significantly inter-correlated in the CL culture. The lack of correlation between chlorophyll a and POC in CL does not encourage the use of the chlorophyll a/POC ratio as a biomass estimator. On the opposite, the ratio of total pigment content to POC did not vary much as a function of the light condition of the culture, remaining near 0.040 in both light regimes. Although further observations are needed, this index seems to be a reliable indicator of phytoplankton carbon biomass.     

Secondary production,calcification and CO<Subscript>2</Subscript> fluxes in the cirripedes <Emphasis Type="Italic">Chthamalus montagui</Emphasis> and <Emphasis Type="Italic">Elminius modestus</Emphasis>

Calcification, a process common to numerous marine taxa, has traditionally been considered to be a significant source of CO₂ in tropical waters only. A number of relatively recent studies, however, have shown that significant amounts of CO₂ are also produced in temperate waters, although none of these studies was carried out on rocky shores, which are considered to be very productive systems. We compared the CO₂ fluxes due to respiration and calcification in two temperate species, the cirripedes Chthamalus montagui and Elminius modestus. The population dynamics of both species were estimated at two sites during a 1-year experimental period in order to establish mean organic (ash-free dry weight) and CaCO₃ (dry shell weight) production. Based on these parameters, we estimated the CO₂ fluxes due to respiration and calcification. CaCO₃ production was estimated to be 481.0 and at each site, representing 3.4 and respectively, of released CO₂. These fluxes represent each 47% of the CO₂ released as a result of respiration and calcification. The production of CaCO₃ at the high-density site was: (1) among the highest values obtained for temperate organisms, and (2) comparable to the estimated CO₂ fluxes for coral reefs. As calcifying organisms are well represented in temperate ecosystems in terms of both density and biomass, our results provide clear evidence that calcification of temperate organisms should not be underestimated. Additional studies on other rocky shore taxa are needed before the relative importance of calcification in rocky intertidal carbon budgets can be generalized.     

BENTHIC METABOLISM ON A SHELTERED ROCKY SHORE: ROLE OF THE CANOPY IN THE CARBON BUDGET1

While the importance of canopy‐forming algae in structuring ecosystems is recognized, their role in the carbon budget is still not well understood. To our knowledge, no measurements of rocky shores primary production and respiration under emersion periods have been carried out in situ. A benthic chamber coupled to a CO₂‐infrared gas analyzer was used to measure gross primary production and respiration on the Ascophyllum nodosum (L.) Le Jol. zone of a sheltered rocky shore in Brittany, France. Over a year of monthly measurements on the zone with and without the A. nodosum canopy showed fairly high production and respiration values for the global community as well as carbon fluxes due to the canopy that largely dominated the benthic metabolism of the zone. The strong canopy respiration relative to the primary production also suggested a high metabolic activity by microscopic heterotrophs on the surface of the alga. Both the canopy and the understory annual primary production and respiration were under the control of light and temperature seasonal variations. Finally, the range of the amount of carbon produced on the A. nodosum zone during diurnal emersions was estimated. Additional measures accounting for the day–night cycles and seasonal light variations over an entire tidal cycle are, however, necessary to establish an annual carbon budget. Such measures using the benthic chamber together with complementary techniques would allow a better understanding of the functioning of sheltered rocky shores.