It is intriguing to consider why certain bacterial species became dominant in the 17 m incubation but not the 2 m one. Heterotrophic nanoflagellates (HNF), the bacterial grazers that control bacterial abundance and community structure, may have affected peptide degradation indirectly (Pernthaler, 2005). Both abundance and activity of HNF were much higher in surface water than bottom hypoxic water in a stratified coastal water column (Park and Cho, 2002). We indeed found that protists were twice as abundant at 2 m as at 17 m (3160 vs. 1620
GRF 1-29 per mL) in the May 2012 cruise, so the growth of certain bacteria was perhaps suppressed more by HNF in the surface water than the bottom water incubation. In particular, HNF may preferentially graze bacteria with large cells, such as Alteromonas, Pseudoalteromonas and Vibrio ( Simek et al., 1999 and Beardsley et al., 2003). Alternatively, viruses may be an important ‘top-down’ force controlling the rapidly-growing bacteria in the 2 m incubation. However, viral infection may lyse the most abundant bacteria and allow growth of other bacterial species, causing fluctuations in the bacterial
community structure ( Breitbart, 2012). The fact that bacterial community structure and cell abundance remained relatively constant during the 2 m incubation suggests that viral infection did not play a major role in controlling the rapidly-growing bacteria species. It is also possible that the initial communities control the types of species that can particularly grow well through inhibitory interactions among the bacterial species ( Long and Azam, 2001). Further research is needed to decipher the factors that lead to the significant shift of the bacterial community structure in bottom water upon addition of labile substrates.
