Intense hydrolytic enzyme activity on marine aggregates and implications for rapid particle dissolution

Abstract

LARGE, rapidly sinking organic aggregates are an important component of the carbon flux from the ocean's surface to its depths. Marine snow, the main type of large (<0.5 mm) aggregate, is heavily colonized by bacteria in surface waters¹, yet the carbon demand of the attached bacteria is so small that months to years are required to consume the aggregates' carbon^2–5. This has led to the conclusion that marine aggregates are resistant to degradation by attached bacteria, and thus act as refractory carriers of carbon to the deep ocean. Here we report that aggregates play host to intense activities of hydrolytic enzymes (presumably due to cell surface bound and released enzymes of the attached bacteria), which render the aggregates soluble. Particulate amino acids were hydrolysed rapidly (turnover time 0.2–2.1 days), with very little of the hydrolysate being taken up by the attached bacteria. Our results support the hypothesis^6,7 that such 'uncoupled' hydrolysis is a biochemical mechanism for large-scale transfer of organic matter from sinking particles to the dissolved phase, and may supply the slowly degradable dissolved organic matter for downward export postulated by recent models^8–10.