Heterotrophic and mixotrophic nanoplankton predation on picoplankton in the Sargasso Sea and on Georges Bank

Abstract

Nanoplankton and picoplankton abundance and community grazing on picoplankton were deterrnined in surnmer and autumn at several stations in a productive coastal environment (Georges Bank. NW Atlantic Ocean) and in an oligotrophic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton (HNAN) abundance were 1.2 to 3.6 X 103 ceils rnl-' on Georges Bank, and 2.2 to 6.8 X 10' ceiis ml-' in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 X 103 and 1.3 to 4.7 X 102, respectively. Mixotrophic nanoplankton (MNAN), operationaiiy defined here as chloroplast-bearing nanoplankton that ingested fluorescent tracers, comprised an average of 12 to 17% of PNAN in surface waters in both environments during August and October. Mixotrophs at specific stations constituted as much as 38% of total PNAN abundance on Georges Bank and 30 % in the Sargasso Sea. Mixotrophs represented up to 39 % of the total phagotrophic nanoplankton abundance (MNAN/[MNAN + HNAN]). Community grazing impact was estimated from the disappearance of fluorescent prey surrogates (fluorescently labeled bacteria, FLB; cyanobacteria, FLC; and <3 pm algae, FLA). Absolute grazing rates (total picoplankton cells removed d-') on Georges Bank exceeded those in the Sargasso Sea due to the greater abundances of predators and prey. However, there was overlap in the specific grazing losses at the 2 sites (ranges = 0.08 to 0.38 d-' in the coastal ocean and 0.05 to 0.24 d-' in the oligotrophic ocean). Rates of bactenvory were in approximate balance with rates of bactenal production (3H-thymidine uptake), but production exceeded bacterivory on Georges Bank during the surnmer cruise. These data are among the first documenting the impact of grazing on picoplankton in these environments, and they are consistent with the prediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly variable, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should be considered quantitatively in microbial food web investigations.KEY WORDS: Nanoplankton/picoplankton interactions . Mixotrophy . Bacterivory . Herbivory Microbial food w e b . Flagellates . Cyanobacteria

Distribution of micro-organisms along a transect in the South-East Pacific Ocean (BIOSOPE cruise) using epifluorescence microscopy

“…Heterotrophic dinoflagellates were always much more abundant than ciliates as shown previously in the Sargasso Sea (Lessard and Murrell, 1996) and the North-East Equatorial Pacific (Yang et al, 2004). Although it is generally admitted that heterotrophic nanoflagellates are the major grazers of picoplankton (Mackey et al, 2002;Sato et al, 2007), predation by heterotrophic dinoflagellates could also be important (Sanders et al, 2000;Sherr et al, 1991).…”

The role of mixotrophic protists in the biological carbon pump

“…Now, however, it appears that mixotrophic protists are indeed nearly ubiquitous in mature ecosystems (Sanders, 1991a;Stoecker et al, 2009;Sanders and Gast, 2012;, though exceptions certainly occur (Arenovski et al, 1995;Sanders et al, 2000). The importance of mixotrophy has not been widely appreciated and embraced because traditional field and laboratory studies typically concentrate on strict phototrophs and strict phagotrophs; in addition, experimental protocols are typically suboptimal for elucidating the mixotrophic activities.…”

Seasonal variations in trophic dynamics of nanoflagellates and picoplankton in coastal waters of the western subtropical Pacific Ocean

“…growth to be controlled by temperature and nutrients. Wikner et al (1990) reported grazing to be an important removal process for the picoplankton community in aquatic ecosystems; the consumption of picoplankton by phagotrophic protists has been recognized as a major pathway of carbon flow (Nagata 1988, Dolan & 2imek 1999, Sanders et al 2000, Tsai et al 2005. Thus, top-down controls such as grazing are thought to set limits on picoplankton biomass and abundance.…”