Abundant protein-containing particles in the sea

Abstract

The interaction of bactena with particulate organic matter has implications for organic matter cycling and bacterial ecology in the ocean. Until recently, the focus has been on 'classical' particles visible by unaided eye (marine snow) or light rnicroscopy. Recent discovenes of several new types of abundant particles, from sub-micrometer to sub-manne snow, are changing our ideas of the physical and chermcal nature of the particle field with which pelagic bactena interact. Previous workers have discovered polysacchande-contain~ng (Alcian Blue stainable) transparent exopolymer particles (TEP) ranging from 3 to 100s of micrometers. Looklng for additional components of the submanne snow particle field, we took into considerat~on that since protein is a major component of biogenic organic matter, protelnaceous particles might also be abundant and important in bacteriaparticle interactions. We stalned seawater with Coomassie Brilliant Blue G-250 (CBB), a protein stain, to reveal light to dark blue stained particles similar in shapes and size range to TEP. In samples filtered on Nuclepore f~lters, Coomassie Stained Particles (CSP) appeared globular, sheet-or string-like, whlle stalnlng unf~ltered water revealed 3-dimens~onal cloud-like shapes as well. Whether CSP are in fact TEP which also contain protein was tested by staining parallel samples with Alcian Blue and CBB (double stainlng a single sample was not possible since both dyes stain blue). CSP were 3 to 13 times more numerous and had up to 2 orders of magnitude greater area than TEP. Thus, while TEP and CSP may overlap, most CSP were distinct from TEP. Treatment of samples with Pronase E decreased CSP abundance and area by 7 8 % and 96% respectively, confirming the proteinaceous nature of CSPs. The CSP abundance in coastal waters was 10' to 10"-l, and their area was 102 to 10" mm2 I-'; both generally decreased with depth. Small particles were 2 to 3 orders of magnitude more abundant than large particles. Double staining with CBB and a fluorescent nucleic acid stain, DAPI, revealed that 20 to 40 % of CSP were colonized by bacteria. Since they contain protein, CSP may serve as a N source for bacteria and other organisms, and t h e ~r production and utilization, which we did not study, may influence the flux and cycling of nitrogen in pelagic ecosystems.