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Picoplankton and Nanoplankton Aggregation by Appendicularians Picoplankton and nanoplankton aggregation by appendicularians: Fecal pellet contents of Megalocercus huxleyi in the equatorial Pacific G Gorsky, Mj Chretiennot-Dinet, J Blanchot, I Palazzoli To cite this version: G Gorsky, Mj Chretiennot-Dinet, J Blanchot, I Palazzoli. Picoplankton and nanoplankton aggregation by appendicularians: Fecal pellet contents of Megalocercus huxleyi in the equatorial Pacific. Journal of Geophysical Research. Oceans, Wiley-Blackwell, 1999, 104 (C2), pp.3381-3390. 10.1029/98JC01850. hal-03284913 HAL Id: hal-03284913 https://hal.archives-ouvertes.fr/hal-03284913 Submitted on 14 Jul 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Copyright JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. C2, PAGES 3381-3390, FEBRUARY 15, 1999 Picoplanktonand nanoplanktonaggregation by appendicularians- Fecal pellet contentsof Megalocercushuxleyi in the equatorial Pacific G. Gorsky,• M. J.Chr•tiennot-Dinet, 2J. Blanchot, 3and I. Palazzoli• Abstract. The contentof fecal pelletsof the freshlycollected warm water appendicularian Megalocercushuxleyi was studiedby light and electronmicroscopy and by flow cytometryin the superficial100 m of the water columnat 2øN, 165øE,in September1994, duringthe Flux dans l'Ouestdu ?acifiqueEquatorial (Joint Global OceanFlux Study-France)oceanographic cruise. Microscopicobservations showed that the fecal pellet contentsof M. huxleyireflected the natural compositionof the nanophytoplanktonand small microphytoplankton(<50 [tm). Larger cells were excludedfrom enteringthe filtering systemby the inlet filters. Coccolithophoridsappeared as the main componentfound in the feces.Evidence for ingestionof"naked" cellsby this appendicularianis given. Analysisof picoplanktonin fecal pelletsby flow cytometerconfirmed that appendiculariansefficiently collectsmall particles. Cyanobacteria, -1 gm in diameter,were found in large quantitiesand showedhigh fluorescencein the fecal pellets.Most of these cyanobacteriain the pelletsappeared to be intact,and thusmay be good indicatorsof the appendicularianingestion rate. The situationwas differentfor the prochlorophyteProchlorococcus abundantin the seawaterand for picoeucaryotes(<2 gm). Thesewere foundat very low quantities in the larvaceanfecal pellets.The calculationsshowed that with an averageconcentration of 5 3//. huxleyim -3, >7% of thesmall particulate matter will bedaily removed from the water. Some of thismatter will be assimilated,some trapped in the houses,and the restaggregated into rapidly sinkingfecal pellets.Ingestion of largequantities of coccolithophoridsindicates that appendiculariansare importantnot only in the cycleof organiccarbon but alsoof inorganic carbon.Moreover, if appendicularianssuccessfully aggregate and assimilateœrochlorococcus and picoeucaryotes,then their grazingactivity can representa major pathwayof carbontransformation in the tropical ecosystem 1. Introduction develop mechanisms for efficient adaptation to nutritionally dilute or to "extreme" environmental conditions. Pelagic The pelagic filter feeding tunicates, appendicularia(= tunicatesare consideredto be nonselectivegrazers, collecting larvacea),are an importantcomponent of the neritic food web. particlesby sieving and also by direct interceptiononto filter Their importanceis consideredinsignificant in openocean or in fibers [.4curiaet al. 1996]. Appendicularianspump water oligotrophic environments. Reports made from manned throughexternal and internal mucous nets in extrudedgelatinous submersiblesin the 1960s [see Barbara, 1979] and recent houses.Colloids >0.1 [tm and otherlarger particles are retained discoveriesof larvaecanpopulations auttptcu................. to ttpnouc oceanic and ingested[Fenaux, i986; Flood et ai.,,j992]. The retention environmentswith low food supplycontradict this view [Fenaux efficiencyis determinedby thepore size'•of the internalnet. It and Youngblurb, 1990; Gors192et al. 1991; Hamnet and generally exceeds90% for particles>3 [tin [Deibe! and Lee, Robison,1992]. Flood 1978, Flood et al. [1992], Deibel and Lee 1992]. The gut passagetime of larvaceansis rapid, and fecal [1992], and Bedo et al. [1993] documentedthe capabilityof pellet productionis high [7'aguchi,1982]. Fecal pellets are appendiculariansto shortcut the microbialfood web anddirectly compactand ellipsoid (Figure l a) and are numerousin sediment feed on submicronicparticles. Hopcroft and Roff [1995] traps [Urrare and Knauer, 1981; Buck and Newton, 1995]. describedthe rapid populationgrowth of larvaceansin warm Megalocercushuxleyi's maximumtrunk size can reach 5 mm, waters. These characteristics indicate that larvaceans can andthe housediameter of adultspecimens can reach more than 3 cm. Appendiculariansfeed continuously.The housesecretion is •ObservatoireOc6anologique, Universit6 Pierre et Marie also a continuousprocess (the daily houseproduction varies Curie/CentreNational de la RechercheScientifique/Institut National des from I to >10 dependingon speciesand temperature [see Flood Sciencesde l'Univers, Villefranche sur ruer, France. and Deibe/, 1998]). Thus, becauseof the rapid population 2ObservatoireOc•anologique de Banyuls,Laboratoire Arago. Laboratoired'Oc•anographie Biologique, Universit• Pierre et Marie growth basedon the short life cycle (<2 days in 29øC for Curie/CentreNational de la RechercheScientifique/Institut National des Oikopleuradioica [seeHoptcrofi and Roll, 1995]) and because Sciencesde l'Univers, Banyulssur mer, France. of three mechanismsrelated to their feedingbiology, (1) 3L'Institut Fran•ais de RechercheScientifique pour le retentionand ingestionof picoparticles,(2) continuoushouse Daveloppementen Cooparationde Noum•a, Noum•a, New Caledonia, France. secretion.and (3) high fecal pellet productionrate, the role of larvaceansin theprocesses of aggregationcan be importanteven Copyright1999 by the AmericanGeophysical Union. in weakly productiveenvironments. Studies on the content of larvaceanfecal pellets and its Paper number 98JC01850. comparisonwith natural sestonare few. The first observations 0148-0227/99/98JC-0185059.00 of the ingestedmatter were made by Lohmann [1909], who 3381 3382 GORSKY ET AL.: PICOPLANKTON AGGREGATION BY APPENDICULARIANS describeda new size classof plankton,the nanoplankton,while from the rectdm and intestines of freshly caught adult studyingthe gut contentof appendicularians.Alldredge [1977] individuals. showedin the Gulf of Californiathat largephytoplankton cells were oftenexcluded by the inlet filtersfrom enteringthe house. 2.2. Light Microscopy The internalfeeding filters contained diatoms and dinoflagellates in much lower proportionsthan in sea water. Accordingto A subsampleof fecal pelletswas immediatelyobserved with Alldredge [1977], 75%-96% of the materialin the gut and in a Leitz dialux 20 microscope,equipped with a HBO 50 light fecal pellets was composedof naked flagellatesand small sourceand a Ploemopak2.4 vertical illuminatorcontaining a particles.Coccolithophorids were also abundant in the gut,often 450-490 nm bandpath filter and excitingblock (Figure lb). For in higherproportion than in seawater.Deibel and Turner[1985] the studyof naturalphytoplankton, 250 mL seawatersamples studiedthe sizeof food in fecalpellets and compared it to the werefiltered through 0.8 gm nucleporemembranes. A few drops pore size of the inlet filters. They also comparedthe size and of 37% formaldehyde(Sigma) were addedduring the filtration type of particlesin fecal pellets with those in the environment. for cell fixation.The membranewas then placed upside down on Urban et al. [1992] describedthe seasonaldifferences in the a slide that had been coated with a poly-L-lysine(Sigma contentand composition of Oikopleuravanhoeffeni fecal pellets Diagnostics), an adhesive solution allowing collection of in coastal Newfoundland waters. materialon the slide.After removingthe excesswater the filter The surface waters of tropical oceanic provincesare waspeeled off, anda dropof liquidgelatin was deposited on the dominatedby phytoplanktonof extremelysmall size. In the filteredmaterial. A coverslipwas placed on the preparationand equatorialPacific, 50%-60% of the chlorophyllbiomass is wassealed when the gelatinsolid;fled. Slides were kept at room containedin the <1 [tm size fraction [Chavez, 1989; Le temperaturein the dark and viewed onboardwith the dialux or Bouteilleret al. 1992]. Prochlorococcus[Chisholm et al. 1988, backat the Laboratoryin Banyulssur mer, France,with a Zeiss 1992] seemsto contributethe majorpart of the chlorophyllof microscope,usually at x400 magnification.The contentof the thisfraction even in the equatorialupwelling zone [Landry et al. sampleswas displayed on a colorSony video system fitted to the 1996]. In the westerntropical Pacific, Blanchotand Rodlet microscopeand printed on a Sonyvideo graphic printer UP-850. [1996] foundthe highestProchlorococcus concentrations in the This transfermethod allowed only a semiquantitativestudy of superficial,oligotrophic, nitrate-depleted layer. M. huxleyiwas thenatural populations and was used to comparethe taxa
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