Synechococcus is widely distributed in the world's ocean surfaces, and is often found in sediment traps. However, its distribution and ecological importance have not been well studied in meso-and bathypelagic waters. We measured Synechococcus abundance in the Suruga Bay (central Japan) and the subtropical NW Pacific. Synechococcus abundance at depths of 200 m and below varied from 2.4 to 190 cells ml -1 , but was in proportion to the surface abundance, suggesting transport of epipelagic populations to greater depths. Surprisingly, Synechococcus was evenly distributed from 200 m down to 1420 m (Suruga Bay) or to 2000 m (subtropical NW Pacific), regardless of season. The contribution of deep Synechococcus to the total population was highest in spring in the Suruga Bay (36 to 77%), and lowest in summer in the Suruga Bay (1 to 9%) and in the subtropical NW Pacific (4 and 10%). These results suggest effective transport of Synechococcus cells down the water column during productive seasons by attachment to large particles and limited transport under oligotrophic and stratified conditions. Deep Synechococcus abundance decreased from fall to winter in the Suruga Bay, though in filtered deep seawater it did not significantly decrease for 30 d in the dark, and it increased in a light/dark cycle. Our investigations show that the standing stock of Synechococcus has been significantly underestimated in previous studies of epipelagic waters conducted during productive seasons and that Synechococcus seems to be grazed and to contribute to biogeochemical cycles in the dark ocean.
KEY WORDS: Synechococcus · Dark ocean · Vertical distribution · Vertical exportResale or republication not permitted without written consent of the publisher ability to utilize organic substrates (Zubkov et al. 2003, Zubkov & Tarran 2005, suggesting that Synechococcus can survive in darkness (Cottrell & Kirchman 2009).These earlier studies led us to examine how Synechococcus is dispersed throughout the dark ocean. The purpose of the present study is to clarify the abundance and distribution of Synechococcus in the dark ocean and to make inferences about the ecological and biogeochemical roles of the organism. Surprisingly, at depths of 200 m and below, Synechococcus was evenly distributed regardless of the season or area studied. These deep populations accounted for up to 77% of total Synechococcus population in the entire water column.
MATERIALS AND METHODSSampling. Seawater was collected in Niskin bottles mounted on a CTD-carousel sampler at Station (Stn) 2 (34°51' N, 138°37' E, Suruga Bay) and Stn C2700 (27°00' N, 138°00' E, subtropical NW Pacific), during cruises on board the RV 'Suruga-maru' (Shizuoka Prefectural Research Institute of Fisheries) and RV 'Soyomaru' (National Research Institute of Fisheries Science, Fisheries Research Agency). The sampling location and hydrological conditions are shown in Table 1. Stn 2 is located near the Suruga Trough, at the center of the Suruga Bay, and Stn C2700 is a time-series station on O-Line (S...