Rising nutrient-pulse frequency and high UVR strengthen microbial interactions

Solar radiation and nutrient pulses regulate the ecosystem’s functioning. However, little is known about how a greater frequency of pulsed nutrients under high ultraviolet radiation (UVR) levels, as expected in the near future, could alter the responses and interaction between primary producers and decomposers. In this report, we demonstrate through a mesocosm study in lake La Caldera (Spain) that a repeated (press) compared to a one-time (pulse) schedule under UVR prompted higher increases in primary (PP) than in bacterial production (BP) coupled with a replacement of photoautotrophs by mixotrophic nanoflagellates (MNFs). The mechanism underlying these amplified phytoplanktonic responses was a dual control by MNFs on bacteria through the excretion of organic carbon and an increased top-down control by bacterivory. We also show across a 6-year whole-lake study that the changes from photoautotrophs to MNFs were related mainly to the frequency of pulsed nutrients (e.g. desert dust inputs). Our results underscore how an improved understanding of the interaction between chronic and stochastic environmental factors is critical for predicting ongoing changes in ecosystem functioning and its responses to climatically driven changes.This study was supported by the Ministerio de Economía y Competitividad and Fondo Europeo de Desarrollo Regional (FEDER) (CGL2011-23681 and CGL2015-67682-R to PC), Ministerio de Medio Ambiente, Rural, y Marino (PN2009/067 to PC) and Junta de Andalucía (Excelencia projects P09-RNM-5376 and P12-RNM-327 to PC and JMMS, respectively). M.J.C. was supported by the Spanish Government “Formación de Profesorado Universitario” PhD grant (FPU12/01243) and I.D.-G. by the Junta de Andalucía “Personal Investigador en Formación” PhD grant (FPI RNM-5376). This work is in partial fulfillment of the Ph. D. thesis of M.J.C

Routine quantification of phytoplankton groups - microscopy or pigment analyses?

International audiencePhytoplankton pigments in samples taken from nutrient-enriched and non-enriched 3 m(3) seawater enclosures were separated and quantified using high-performance liquid chromatography (HPLC). The enclosures were with and without inorganic (N, P, Si) and organic (glucose, C) nutrient enrichments, resulting in a variation of phytoplankton groups in time and space. The relative contribution of the major phytoplankton groups to the total chlorophyll a (i.e. chlorophyll a plus chlorophyllide a) was estimated by the CHEMTAX program. The results were compared to phytoplankton groups identified and quantified by light and epifluorescence microscopy. For the pigmented flagellate groups the results obtained by microscopy and pigment analyses using the CHEMTAX program showed similar trends. The picocyanobacteria were readily quantified by microscopy and the results were similar to those obtained by flow-cytometry, while the CHEMTAX program for the cyanobacteria revealed different trends. Microscopy and pigment analyses provided similar trends in diatom population development. Estimated diatom contributions to total phytoplankton biomass, however, were considerably higher when based on microscopy than when based on the CHEMTAX program, especially in Si-amended enclosures. Total chlorophyll a:carbon ratios for diatoms were at the lower end of a previously reported range between 1:27 and 1:67. For the pigmented flagellate groups the total chlorophyll a:carbon ratios were above that range. In routine monitoring of phytoplankton we recommend the use of the CHEMTAX program based on HPLC pigment analyses accompanied by a screening for the dominating species by microscopy, and by flow-cytometry for quantification of picocyanobacteria

Silicate and labile DOC interfere in structuring the microbial food web via algal-bacterial competition for mineral nutrients: Results of a mesocosm experiment

International audienceThe effects of organic and inorganic nutrient enrichments on algal-bacterial competition were investigated using mesocosms. Interactions were followed over 10 d in 12, 3-m(3) seawater mesocosms in the Isefjord, Denmark. Two sets of four mesocosms were given the same daily addition of ``phytoplankton nutrients'' (phosphate and nitrate) but received different amounts of glucose, and one set was kept in excess with respect to silicate. Four additional mesocosms served as controls and received either no additions, silicate alone, or glucose alone. In the mesocosm set where no silicate was added, enrichment with phytoplankton nutrients and glucose led to a replacement of diatoms, not by other algae, but by heterotrophic bacteria, mainly bacteria > 2 mum. In the mesocosm set where silicate was kept replete, diatoms competed successfully with bacteria for the uptake of mineral nutrients. Even in mesocosms enriched with high amounts of glucose, primary production increased throughout the experimental period, while bacterial production, after an initial increase, leveled off. In addition, turnover time of glucose increased in the silicate-replete mesocosm set, consistent with the idea that bacterial consumption was hampered by diatoms competing successfully for phosphate and nitrate. The size and shape of different algal and bacterial groups in relation to nutrient uptake and grazer avoidance are discussed. Both accumulation and consumption of dissolved organic carbon could depend on the. structure of the microbial food web

Mixotrophic haptophytes are key bacterial grazers in oligotrophic coastal waters

13 pages, 6 figures, 1 tableGrazing rate estimates indicate that approximately half of the bacterivory in oligotrophic oceans is due to mixotrophic flagellates (MFs). However, most estimations have considered algae as a single group. Here we aimed at opening the black-box of the phytoflagellates (PFs) <20 μm. Haptophytes, chlorophytes, cryptophytes and pigmented dinoflagellates were identified using fluorescent in situ hybridization or by standard 4′,6-diamidino-2- phenylindole staining. Their fluctuations in abundance, cell size, biomass and bacterivory rates were measured through an annual cycle in an oligotrophic coastal system. On average, we were able to assign to these groups: 37% of the total pico-PFs and 65% of the nano-PFs composition. Chlorophytes were mostly picoplanktonic and they never ingested fluorescently labeled bacteria. About 50% of the PF <20 μm biomass was represented by mixotrophic algae. Pigmented dinoflagellates were the least abundant group with little impact on bacterioplankton. Cryptophytes were quantitatively important during the coldest periods and explained about 4% of total bacterivory. Haptophytes were the most important mixotrophic group: (i) they were mostly represented by cells 3-5 μm in size present year-round; (ii) cell-specific grazing rates were comparable to those of other bacterivorous non-photosynthetic organisms, regardless of the in situ nutrient availability conditions; (iii) these organisms could acquire a significant portion of their carbon by ingesting bacteria; and (iv) haptophytes explained on average 40% of the bacterivory exerted by MFs and were responsible for 9-27% of total bacterivory at this site. Our results, when considered alongside the widespread distribution of haptophytes in the ocean, indicate that they have a key role as bacterivores in marine ecosystems. © 2014 International Society for Microbial Ecology All rights reservedThis study was supported by EU project BASICS (EVK3-CT-2002-00078) and a post-doctoral fellowship of the former MECD (SB2001-0166). It was also partially funded by MEC projects ESTRAMAR (CTM2004-12631/MAR), GENmMAR (CTM2004-02586/MAR) and FLAME (CGL2010-16304, MICINN), and by the Spanish-Argentina project PROBA (2007AR0018, CSIC). FN was supported by the Marie-Curie fellowship ESUMAST (MEIF-CT-2005-025000)Peer Reviewe