Abstract:

Proteinaceous materials are major components of marine organic matter, and can account for up to 80% of the organic nitrogen in marine organisms. In this study, we discussed the possible mechanisms how proteinaceous materials are preserved and transported in marine environments.

First, the accumulation of refractory prokaryotic cell membranes has been suggested as one of the possible sources of the proteinaceous materials observed in the deep ocean. Surface layer protein (S-layer) is a specific membrane glycoprotein that covers the outermost cell surface in a regularly ordered planar crystalline structure. With special attention to changes in S-layer protein and peptidoglycan, we studied the degradation of two species of marine cyanobacterium, Synechococcus sp.The changes in degradation biomarkers, elemental composition and cell morphology were followed, suggesting that S-layer protein functions as the skeleton to support the cell structures and removal of S-layer accelerates the degradation of cyanobacterial cells in marine environments.

Second, we provide evidence that Si is deposited on the extracellular polymeric substances (EPS) produced by cyanobacteria, particularly when they begin to decompose. We also found that Si associated with organic micro-blebs collected from the deep ocean. Both nano-particle imaging analysis and bulk organic geochemical analysis show a surprisingly similar appearance between EPS-associated Si in cyanobacteria and micro-blebs in the open ocean.  Accordingly, EPS-associated Si might be a precursor of the Si-enriched organic micro-blebs observed in the ocean. The previously unexplored source of particulate silicate minerals may play an important role in the oceanic silicate cycling as well as organic matter export from the surface waters.