AME 17:311-321 (1999)  -  doi:10.3354/ame017311

Filter-type and sample handling affect determination of organic substrate uptake by bacterioplankton

Ronald P. Kiene*, Laura J. Linn

Department of Marine Sciences, University of South Alabama, Mobile, Alabama 36688, USA
Dauphin Island Sea Lab, Dauphin Island, Alabama 36528, USA

ABSTRACT: Natural assemblages of bacterioplankton (coastal seawater and seawater filtrates), treated with radiolabeled organic substrates, were used to test the effectiveness of various types of 0.2 μm pore size filters in capturing labeled cell materials. The highest retention of cell material was observed with filters made of either polyethersulfone (PES), mixed cellulose esters (MCE) or Nylon. PES filters retained up to 22% less than MCE or Nylon filters in some comparisons, but in most cases these filters gave similar, apparently high retention of cell materials. In contrast, retention of labeled cell material by track-etched polycarbonate (PC) or polyester (PE) filters was 10 to 80% less than for PES, MCE or Nylon filters, even at low filtration vacuum (5 cm Hg). Radiolabel lost from PC filters was recovered as dissolved compounds in the filtrates, and tests showed that cell breakage/leakage occurred, not during filtration per se, but rather at the end of filtration when filters ran dry. Higher filtration vacuum over the range 5 to 38 cm Hg caused greater losses with PC filters whereas retention of radiolabel by PES filters was nearly independent of vacuum. Losses on PC filters were greatest during early phases of incubations, when label in the cells was found mostly as untransformed parent substrate, and losses were greater for substrates known to function as intracellular osmolytes (e.g. glycine betaine and dimethylsulfoniopropionate) than for substrates (e.g. leucine and glucose) that were rapidly incorporated into macromolecules. Treatment of PES-filtered cells by a cold osmotic shock procedure resulted in nearly identical losses to those which occurred with non-shocked cells on PC filters. Likewise, addition of formalin (2% final concentration) to a seawater filtrate culture which had already taken up 14C-glycine betaine caused rapid losses of 14C from particulate materials, resulting in the same amount of loss as occurred when parallel live samples were filtered onto PC filters. The results suggest that a major fraction of the soluble components of bacterioplankton can be selectively lost during filtration through polycarbonate-type membranes, and by procedures which subject cells to osmotic shock and poisoning. Furthermore, our results indicate that PES, MCE and nylon filters function similarly well at retaining most cell materials.

KEY WORDS: Filtration · Artifact · Osmolyte · DOC · Filter comparison · Betaine

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