Association of the stramenopilan protists, the aplanochytrids, with zooplankton of the equatorial Indian Ocean

Abstract

Thraustochytrids and aplanochytrids, belonging to the Labyrinthulomycetes of the Kingdom Stramenopila, have been frequently reported to occur as parasites or symbionts in a number of coastal invertebrates. Given the fact that these protists are widespread in coastal and oceanic waters, it is possible that they are also associated with pelagic zooplankton. This study examines their occurrence in zooplankton from equatorial waters of the Indian Ocean. A total of 171 of 2100 individual specimens of zooplankton yielded Labyrinthulomycetes in culture, the colony morphology and/or life cycles of which corresponded to aplanochytrids. Small-subunit ribosomal DNA (SSU rDNA) sequence analysis of 8 of the isolates placed them in a distinct clade among aplanochytrids, but closest to Aplanochytrium yorkensis or A. kerguelensis. The 8 isolates were further segregated into 2 clusters, corresponding to isolates obtained from 2 different seasons. Biotinylated probes for 2 isolates of A. kerguelensis from the 2 different seasons were developed based on internal transcribed spacer (ITS) sequences. In situ hybridization (ISH) of zooplankton using the probes with streptavidin-alkaline phosphatase showed that aplanochytrid cells fed to copepods were grazed and could subsequently be detected in the animals. ISH on natural samples of zooplankton yielded a positive but diffuse reaction in copepods, while cells resembling aplanochytrids were detected within chaetognaths. ISH using streptavidin-peroxidase conjugate lent further support for their presence within chaetognaths. This study suggests the predominant occurrence of A. kerguelensis in association with mesozooplankton of the equatorial Indian Ocean, particularly with chaetognaths. Further studies are suggested to determine whether genetically distinct populations of A. kerguelensis are associated with chaetognaths in oceanic waters and if such an association is parasitic, mutualistic or commensalistic.KEY WORDS: Stramenopila · Labyrinthulomycetes · Aplanochytrids · Zooplankton · Internal transcribed spacer · In situ hybridization · Biotinylated probes Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 399: [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68] 2010 require these fatty acids for their growth and reproduction but possess a limited or no ability to synthesize them (Kanazawa et al. 1979a,b, Lubzens et al. 1985, Nichols & Nichols 2008, Labyrinthulomycetes present in the water column might play an important role in the marine food web by serving as sources of these fatty acids for the zooplankton (Fell & Newell 1998, Kimura et al. 1999, Raghukumar 2002.Thraustochytrids and aplanochytrids are known to be associated with invertebrates in a variety of ways. observed thraustochytrids in the faecal pellets of salps. They were found to be endolithic in mollusk shell fragments, especially mussel shells and clam shells (Porter & Lingle 1992), and were isolated from oyster tissue (Perkins 1973) and surface mucus...

Novel uncultivated labyrinthulomycetes revealed by 18S rDNA sequences from seawater and sediment samples

“…These results suggest that Aplanochytrium is a geographically widespread genus with restricted phylogenetic diversity. Damare & Raghukumar (2006) recently suggested that aplanochytrids might be more abundant than thraustochytrids in open-ocean waters. The group of environmental sequences named uLa8 may represent either a novel species of Aplanochytrium or a novel related genus.…”

“…Aplanochytrids have been isolated from a variety of plant, algal, and animal substrates as well as sediment and seawater samples collected in polar, temperate, and tropical habitats (e.g. Leander et al 2004, Damare & Raghukumar 2006. In addition to the 18S-F/LABY-Y amplicon sequences recovered from Peconic Bay, Port Jefferson Harbor, and Long Island Sound sediment samples and a Peconic Bay seawater sample, 18S rDNA sequences very similar to cultivated aplanochytrids have been recovered in universal 18S rDNA libraries constructed from a variety of sediment (including arctic and hydrothermal areas) and seawater samples (including normoxic Mediterranean surface seawater and the suboxic part of the Cariaco Basin water column) (Edgcomb et al 2002, Stoeck et al 2003, Massana et al 2004a,b, Zuendorf et al 2006.…”

“…Raghukumar (2002) concluded that members of the genera Labyrinthula and Aplanochytrium are often associated with living plants and algae, either as parasites or commensals, while thraustochytrids may be more sensitive to the antimicrobial compounds produced by living plant substrates and more commonly found on plant and algal detritus. Labyrinthula has never been reported from open waters, but Aplanochytrium is abundant in the Indian Ocean (Damare & Raghukumar 2006). In addition to normoxic habitats, labyrinthulomycetes have been observed in and cultivated from suboxic and sulfiderich water column and sediment habitats, and have even been cultivated anaerobically under a nitrogen atmosphere (Goldstein 1973, Riemann & Schrage 1983, Raghukumar et al 2001.…”

Increasing evidence for the important role of Labyrinthulomycetes in marine ecosystems

“…Likewise, none of the aplanochytrids from the zooplankton of oceanic waters produced chitinase, indicating that they may not significantly degrade zooplankton exoskeletons (Damare and Raghukumar, 2006). Members of Labyrinthulomycetes appear to be highly efficient in degrading protein.…”

“…Thraustochytrids secrete a wide variety of extracellular enzymes in vitro, such as proteases, lipases, cellulases, amylases, xylanases, gelatinase, urease, phosphatase, chitinase and α-Glucosidase , Bremer and Talbot 1995, Bongiorni et al 2005b, Damare and Raghukumar 2006, Taoka et al 2009, Nagano et al 2010). …”