Spatiotemporal Changes in the Structure and Composition of a Less-Abundant Bacterial Phylum (Planctomycetes) in Two Perialpine Lakes

International audienceWe used fingerprinting and cloning-sequencing to study the spatiotemporal dynamics and diversity of Planctomycetes in two perialpine lakes with contrasting environmental conditions. Planctomycetes, which are less-abundant bacteria in freshwater ecosystems, appeared to be structured in the same way as the entire bacterial community in these ecosystems. They were more diversified and displayed fewer temporal variations in the hypolimnia than in the epilimnia. Like the more-abundant bacterial groups in aquatic systems, Planctomycetes communities seem to be composed of a very small number of abundant and widespread operational taxonomic units (OTUs) and a large number of OTUs that are present at low abundance. This indicates that the concept of "abundant or core" and "rare" bacterial phylotypes could also be applied to less-abundant freshwater bacterial phyla. The richness and diversity of Planctomycetes were mainly driven by pH and were similar in both of the lakes studied, whereas the composition of the Planctomycetes community seemed to be determined by a combination of factors including temperature, pH, and nutrients. The relative abundances of the dominant OTUs varied over time and were differently associated with abiotic factors. Our findings demonstrate that less-abundant bacterial phyla, such as Planctomycetes, can display strong spatial and seasonal variations linked to environmental conditions and suggest that their functional role in the lakes studied might be attributable mainly to a small number of phylotypes and vary over space and time in the water column

Maximum in the Middle: Nonlinear Response of Microbial Plankton to Ultraviolet Radiation and Phosphorus

The responses of heterotrophic microbial food webs (HMFW) to the joint action of abiotic stressors related to global change have been studied in an oligotrophic high-mountain lake. A 2×5 factorial design field experiment performed with large mesocosms for >2 months was used to quantify the dynamics of the entire HMFW (bacteria, heterotrophic nanoflagellates, ciliates, and viruses) after an experimental P-enrichment gradient which approximated or surpassed current atmospheric P pulses in the presence vs. absence of ultraviolet radiation. HMFW underwent a mid-term (<20 days) acute development following a noticeable unimodal response to P enrichment, which peaked at intermediate P-enrichment levels and, unexpectedly, was more accentuated under ultraviolet radiation. However, after depletion of dissolved inorganic P, the HMFW collapsed and was outcompeted by a low-diversity autotrophic compartment, which constrained the development of HMFW and caused a significant loss of functional biodiversity. The dynamics and relationships among variables, and the response patterns found, suggest the importance of biotic interactions (predation/parasitism and competition) in restricting HMFW development, in contrast to the role of abiotic factors as main drivers of autotrophic compartment. The response of HMFW may contribute to ecosystem resilience by favoring the maintenance of the peculiar paths of energy and nutrient-mobilization in these pristine ecosystems, which are vulnerable to threats by the joint action of abiotic stressors related to global change.This research was supported by Junta de Andalucía (Excelencia P07-CVI-02598 to PC, and P09-RNM-5376 to JMMS), the Spanish Ministries of Medio Ambiente, Rural y Marino (PN2009/067 to PC) and Ciencia e Innovación (GLC2008-01127/BOS and CGL2011-23681 to PC), the ERC Advanced Grant project number 250254 “MINOS” (to GB), and two Spanish government grants (to JADM and FJB)

Resource Supply Overrides Temperature as a Controlling Factor of Marine Phytoplankton Growth

The universal temperature dependence of metabolic rates has been used to predict how ocean biology will respond to ocean warming. Determining the temperature sensitivity of phytoplankton metabolism and growth is of special importance because this group of organisms is responsible for nearly half of global primary production, sustains most marine food webs, and contributes to regulate the exchange of CO2 between the ocean and the atmosphere. Phytoplankton growth rates increase with temperature under optimal growth conditions in the laboratory, but it is unclear whether the same degree of temperature dependence exists in nature, where resources are often limiting. Here we use concurrent measurements of phytoplankton biomass and carbon fixation rates in polar, temperate and tropical regions to determine the role of temperature and resource supply in controlling the large-scale variability of in situ metabolic rates. We identify a biogeographic pattern in phytoplankton metabolic rates, which increase from the oligotrophic subtropical gyres to temperate regions and then coastal waters. Variability in phytoplankton growth is driven by changes in resource supply and appears to be independent of seawater temperature. The lack of temperature sensitivity of realized phytoplankton growth is consistent with the limited applicability of Arrhenius enzymatic kinetics when substrate concentrations are low. Our results suggest that, due to widespread resource limitation in the ocean, the direct effect of sea surface warming upon phytoplankton growth and productivity may be smaller than anticipated

Rates of growth and microbial grazing mortality of phytoplankton in a recent artificial lake

International audienceDilution experiments were conducted from May to September 1998 in the epilimnion of a recently flooded reservoir (Sep Reservoir, Puy-de-Dôme, France: 46° 2'N, 3° 1' E), to estimate growth and microzooplankton (20 to 200 μm) grazing mortality of 2 size classes of the phytoplankton community. This community was dominated by 25 to 200 μm size fraction. Micrograzers were largely dominated by ciliated protozoa (86 to 96% of total abundance), and also comprised rotifers and copepod nauplii. Experiments with and without added nutrients (N and P) indicated, together with changes in particulate protein, RNA and DNA, that phosphorus is a limiting element in the Sep Reservoir. Grazing activity of microzooplankton was significant on nanoalgae, averaging 0.38 ± 0.19 d–1 (range 0.16 to 0.66 d–1). The grazing activity balanced the daily production of the target algal community by 71 ± 11% (range 54 to 88%). Microzooplankton herbivory and the growth rates of nanoalgae were strongly correlated, suggesting the existence of an operating homeostatic interaction between the prey and the predators. The high mortality:growth ratio indicated that a substantial fraction of phytoplankton carbon is recycled in surface waters through microbial grazing. We conclude that microzooplankton herbivory provides an effective and substantial link to higher trophic levels in the Sep Reservoir, and might contribute to fueling planktonic communities with the limiting nutrient through regeneration

Spatiotemporal Changes in the Structure and Composition of a Less-Abundant Bacterial Phylum (Planctomycetes) in Two Perialpine Lakes

International audienceWe used fingerprinting and cloning-sequencing to study the spatiotemporal dynamics and diversity of Planctomycetes in two perialpine lakes with contrasting environmental conditions. Planctomycetes, which are less-abundant bacteria in freshwater ecosystems, appeared to be structured in the same way as the entire bacterial community in these ecosystems. They were more diversified and displayed fewer temporal variations in the hypolimnia than in the epilimnia. Like the more-abundant bacterial groups in aquatic systems, Planctomycetes communities seem to be composed of a very small number of abundant and widespread operational taxonomic units (OTUs) and a large number of OTUs that are present at low abundance. This indicates that the concept of "abundant or core" and "rare" bacterial phylotypes could also be applied to less-abundant freshwater bacterial phyla. The richness and diversity of Planctomycetes were mainly driven by pH and were similar in both of the lakes studied, whereas the composition of the Planctomycetes community seemed to be determined by a combination of factors including temperature, pH, and nutrients. The relative abundances of the dominant OTUs varied over time and were differently associated with abiotic factors. Our findings demonstrate that less-abundant bacterial phyla, such as Planctomycetes, can display strong spatial and seasonal variations linked to environmental conditions and suggest that their functional role in the lakes studied might be attributable mainly to a small number of phylotypes and vary over space and time in the water column

The microbial food web in the recently flooded Sep reservoir : Diel fluctuations in bacterial biomass and metabolic activity in relation to phytoplankton and flagellate grazers

International audienceThe spatial distribution of the bacterial biomass and production and of potential heterotrophic activity (PHA) were measured every 4 h between 23 July (10:00 h) and 25 July (10:00 h) 1997 in a recently flooded oligo-mesotrophic reservoir (the Sep Reservoir, Puy-de-Dôme, France), in relation to temperature, the phytoplankton biomass and production, and the abundance of heterotrophic flagellates. The temperature varied slightly with time during the study, but the well-established thermal stratification agreed well with vertical distribution of the biological variables that were measured. Only the bacterial production and the PHA showed significant diel changes (t-test, p < 0.05), with maxima at 18:00 h and minima at 02:00 h. A significant positive relation was found between bacterial abundance and that of heterotrophic flagellates, which, rather than being an association related to the thermal stratification of the water column, was considered to reflect a trophic relation between these two communities. A carbon balance analysis suggested that at least 30% of the C from primary production measured during the sampling period was used by bacteria, and that 42% of this secondary production, or 6% of the primary production, would be used for the development of the heterotrophic flagellates present. We conclude that the bacterioplankton forms, at least occasionally, an important source of carbon for higher trophic levels, and reject the hypothesis that bacterial production in the Sep Reservoir depends exclusively on organic matter of allochthonous origin

Primary productivity in the recently flooded 'Sep Reservoir' (Puy-de-Dôme, France)

International audienceSeasonal variations in phytoplankton biomass and production were studied in 1996 and 1997 in the newly flooded (1995) Sep Reservoir (France) in relation to various physico-chemical environmental factors. No trophic upsurge occurred in the reservoir as has been found in most of the newly flooded reservoirs. Phytoplankton composition changed drastically from 1996 to 1997, the latter year being characterized by a rarity of diatoms. Areal euphotic chlorophyll a concentration and phytoplankton carbon biomass were typical of mesotrophic lakes in 1996 and of somewhat oligotrophic environments in 1997. In contrast, the primary production (PP) rates were similar in both years, indicating a change in the factors controlling phytoplankton populations from 1996 to 1997. The low PP (compared with the algal biomass present) in 1996 seemed to be related to the instability of the water column and the Zmix:Zeu ratio, whose negative effects on the phytoplankton community were apparently more pronounced in 1996 than in 1997. Orthophosphate concentrations, and their contribution to total phosphorus, remained low in 1996 but increased with time in 1997. The temporal losses of orthophosphates from June to September 1996 accounted for only 42% of the measured PP, indicating that PP in 1996 was also strongly affected by P availability and probably took place due to other P sources such as regenerated nutrients

Short-term vertical distribution of phytoplankton populations in a shallow tropical lake (Lake Municipal, Yaoundé, Cameroon)

International audienceThe circadian (i.e. 24-hours) vertical distribution of phytoplankton community was examined for the first time in a small, shallow and nutrient-rich tropical lake (Lake Municipal, Yaoundé, Cameroon), in relation to environmental factors. Sampling was done during the late dry season and the water column was very stable. Phytoplankton community was diverse and was dominated by cyanobacterial species which averaged (± SD) 97 ± 1% and 62 ± 14% of total phytoplankton abundance and biovolume, respectively. The abundances of these species consistently peaked in the surface layer. Their dominance was related to their positive buoyancy in stable environments, and the resulting shading of surface waters that possibly outcompeted eukaryotic algae. Although most of the extracted chlorophyll was apparently not from the dominant cyanobacterial species, chlorophyll-a concentrations (25.3 to 234 μg/l), as well as phytoplankton total abundance (1.42 to 11.73 x 108 cells/l) and biovolume (18.24 to 135.78 x 109 μm3/l), were substantial and characteristic of productive waters. The vertical distribution of the dominant phytoplankton species exhibited three different patterns, according to the depths where their maximum abundances were recorded: (1) surface water populations represented by the cyanobacteria Oscillatoria limosa and Synechocystis pevalekii, (2) mid-layer species, the euglenoid flagellate Lepocinclis fusiformis, and (3) populations that actively avoided the surface water during the nighttime, the euglenoid Trachelomonas hispida and the cryptomonad Cryptomonas ovata. We conclude that these patterns were related to several factors, including light conditions, temperature, cell buoyancy, the potential of motile flagellates to avoid adverse conditions such as grazing pressure, and accorded to the autoecologies of algal species involved

Short-term variations in the abundance and cell volume of bacterioplankton in an artificial tropical lake

International audienceCircadian and spatial fluctuations in bacterioplankton abundance and cell volume were examined, for the first time, in the Municipal Lake located in the down town area of Yaoundé (Capital of Cameroon, Central Africa, ca 3 520 N, 11 310 E). Bacterial cell volumes (range, 0.05 to 0.2 m3) were consistent with those reported for other aquatic systems while bacterial densities (0.8 to 2 108 cells m

Bacterial population dynamics, production, and heterotrophic activity in a recently formed reservoir

International audienceLes variations spatiales et saisonnières de l'abondance, la production de biomasse et l'activité hétérotrophe potentielle (mesurée par incorporation du 14C-glucose) du bactérioplancton, sont examinées dans un lac-réservoir vieux de seulement 1 an (le réservoir de la Sep, Puy-de-Dôme, France), en relation avec la température, les teneurs en chlorophylle a et la production primaire (PP). Les variables biologiques retenues présentent des valeurs qui nous ont amené à considérer le réservoir de la Sep comme étant un réservoir de type oligo-mésotrophe. Les fluctuations spatiotemporelles des variables bactériennes se sont revélées comme étant la conséquence du couplage entre, d'une part, l'évolution saisonnière et notamment thermique du réservoir et, d'autre part, la disponibilité en ressources. En effet, un total de 40% de la variance observée pour la densité et l'activité bactériennes s'expliquerait par ce couplage, dont 14% par la température et 26% par la chlorophylle a ou par la production primaire. L'établissement d'un budget en carbone a indiqué que entre 4 et 126% (moyenne = 20%) de la production primaire ambiante transite par la boucle microbienne, via la production de biomasse bactérienne qui peut ainsi représenter, occasionnellement, une source importante de carbone pour les maillons trophiques supérieurs