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In the bathypelagic realm of the ocean, the role of marine snow as a carbon and energy source for the deep-sea biota and as a potential hotspot of microbial diversity and activity has not received adequate attention. Here, we collected bathypelagic marine snow by gentle gravity filtration of sea water onto 30 μm filters from~1000 to 3900 m to investigate the relative distribution of eukaryotic microbes. Compared with sediment traps that select for fast-sinking particles, this method collects particles unbiased by settling velocity. While prokaryotes numerically exceeded eukaryotes on marine snow, eukaryotic microbes belonging to two very distant branches of the eukaryote tree, the fungi and the labyrinthulomycetes, dominated overall biomass. Being tolerant to cold temperature and high hydrostatic pressure, these saprotrophic organisms have the potential to significantly contribute to the degradation of organic matter in the deep sea. Our results demonstrate that the community composition on bathypelagic marine snow differs greatly from that in the ambient water leading to wide ecological niche separation between the two environments.

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Macroscopic particles (>500 μm), including marine snow, large migrating zooplankton, and their fast-sinking fecal pellets, represent primary vehicles of organic carbon flux from the surface to the deep sea. In contrast, freely suspended microscopic particles such as bacteria and protists do not sink, and they contribute the largest portion of metabolism in the upper ocean. In bathy-and abyssopelagic layers of the ocean (2,000-6,000 m), however, microscopic particles may not dominate oxygen consumption. In a section across the tropical Atlantic, we show that macroscopic particle peaks occurred frequently in the deep sea, whereas microscopic particles were barely detectable. In 10 of 17 deep-sea profiles (>2,000 m depth), macroscopic particle abundances were more strongly crosscorrelated with oxygen deficits than microscopic particles, suggesting that biomass bound to large particles dominates overall deepsea metabolism.ecology | video analysis | marine snow | Atlantic ocean P articles have to be of a size visible to the unaided eye before they appreciably contribute to the vertical flux in the ocean (1-3). These particles-dubbed marine snow because they resemble snowflakes in the backscatter of dive lights (4, 5)-are best enumerated by photographic means. However, microscopic and colloidal particles, a large portion of which are freely suspended bacteria and protists (6), are better quantified using optical backscatter (7). Using both techniques in tandem, we can contrast the relative distribution of microscopic and macroscopic particles in the ocean. Most video profiles in the past have been taken to a maximum depth of 1,500 m (8-10), and very few profiles exist to 4,000 m (11, 12). Thus, the data presented here, with a maximum deployment depth of 6,000 m, represent the most comprehensive basin-scale video analyses of deep-sea particles to date. To what extent and at which depth these imaged particles contribute to vertical flux remains unknown; however, a link to oxygen deficits may reflect their significance as hotspots of microbial metabolic activity. Results and DiscussionThe Archimedes III research expedition on the research vessel Pelagia was conducted from December 17, 2007 to January 16, 2008; it followed a cruise track from Fortaleza, Brazil, along the equator to the Sierra Leone Basin and then, headed northwest toward the Cape Verde Islands. The video profiles reported here were collected over a transect of ∼4,000 km (Fig. 1). The Romanche Fracture Zone, the location of the greatest depth in the Mid-Atlantic Ridge, was sampled at a higher resolution than the western and eastern basins of the tropical Atlantic (Fig. 1). Microscopic particle abundance as assessed by optical backscatter generally decreased with depth, except for a pronounced peak in the oxygen minimum zone (250-500 m). Macroscopic particle numbers (>500 μm) as detected by video analysis usually decreased below the oxygen minimum zone to 2,000 m, but then, they frequently increased, forming peaks of high particle numbers ( Figs. 1 and 2). ...

In the Northern Adriatic Sea, extracellular enzymatic activity was measured during a Lagrangian study following a drifting buoy for 40 h. Dissolved free enzymatic activity represented 20 to 70% of total activity depending on the type of enzyme. a-and P-glucosidases exhibited a significantly higher free activity than proteolytic enzymes. In subsequent laboratory experiments we investigated the effect of zooplankton on the free enzyme pool. The 4-step approach included: ( l ) determinatlon of the enzymatic activities in copepods (mainly Acartia clausi); (2) enzymat~c activity in fecal pellets; (3) short-and long-term grazing experiments; and (4) degradab~lity of free glucosidase in seawater. a-and P-glucosidases, leu-aminopeptidase, lipase and chitinase were examined. Experiments in which zooplankton were selectively enriched revealed a significant increase in both particle-bound (due to the increase of bacterial density) and dissolved free enzymatic activity. Incubating water enriched in free enzymes released by zooplankton with natural bacterial consortia, we found that 70% of the original a-and P-glucosidase activity remained after 22 h. The presence of microorganisms did not enhance the degradation of these enzymes as compared to autoclaved controls. We found that a considerable amount of free dissolved enzymes is lost by 0.2 pm filtration using Nuclepore filters, thereby leading to an underestimation of dissolved enzymes by -30% in our experiments. Based on our results we conclude that mesozooplankton contribute to the free enzymatic activity in natural waters especially during periods of high grazing activity.