The microbial loop in the Red Sea. dynamics of pelagic bacteria and heterotrophic nanoflagellates

Abstract

Growth and grazing experiments with free-liv~ng bacteria and nano-and microzooplankton were conducted in February-March 1987 in the central Red Sea and the Gulf of Aden using metabolic inhibitors. Results demonstrate the existence of a dynamic 'microbial loop' which dominates the pelagic system in terms of biomass and production. In the epipelagial, bacterial abundance ranged from 5.2 to 8.8 X 105 cells ml-l, and heterotrophic nanoflagellate (HNF) abundance from 0.6 to 1.2 X lo3 cells ml-l. Bacterial growth rates varied between 0 014 and 0.097 h-', grazing rates between 0.010 and 0.108 h-' Population grazlng rates corresponded to 2.0 to 5.4 x 104 bacteria consumed ml-' h-' Bacterial biomass, growth and grazing rates were higher in the central Red Sea than in the Gulf of Aden.Bacterial growth was inversely related to grazing pressure HNF were the major bacterial consumers with average ingestion rates of 29 bacteria HNF-l h-' HNF production was controlled by microzooplankton predabon. Net growth rates of bacteria and HNF were also negatively coupled and exhibited considerable temporal variation. There was also some indication for diurnal rhythms of bacterial growth and grazing loss rates. At night, grazing Impact on bacteria was reduced to less than half the noon value. The temporal variations observed caution against extrapolations derived from routine experiments run at fixed hours.

Grazing during early spring in the Gulf of Aqaba and the northern Red Sea

“…For heterotrophic bacteria and medium-sized algae, the system appears highly dynamic in spite of low nutrient levels and low phytoplankton biomass. In fact, calculated bacterial growth rates were in the middle of the range estimated for the same season in the slightly more eutrophic central Red Sea using metabolic inhibitors (0.34 to 2.3 d -1 : Weisse 1989). In order to balance grazing losses, a high production:biomass ratio of phytoplankton is required.…”

Diel changes in phagotrophy by Cryptomonas in Lake Biwa

“…in Lake Biwa may use the day to assimilate carbon with nutrients and use the night to catabolize biochemicals for cell division by ceasing assimilation (bacterial ingestion). Several studies showed that HNF decreased feeding activity at night in marine environments (Fuhrman et al 1985;Weisse 1989). This diurnal change in feeding rate of HNF is believed to relate to cell division cycles (Weisse 1989).…”

“…Several studies showed that HNF decreased feeding activity at night in marine environments (Fuhrman et al 1985;Weisse 1989). This diurnal change in feeding rate of HNF is believed to relate to cell division cycles (Weisse 1989).…”

“…However, no previous study has examined diel changes in the bacterial ingestion rate of mixotrophic algae in situ. Such knowledge is essential to assess strength in the trophic interaction between bacteria and their consumers (Weisse 1989).…”

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

“…Los Angeles, California 90089, USA from a phytoplankton-based food web through the bacterioplankton. Nanoplanktonic (2 to 20 pm) protists play a pivotal role in this scheme as conduits for the reintroduction of bactena to pelagic food webs by acting as major consurners of bactenal biomass in most planktonic environments (Weisse 1989, Sanders et al 1992, Shiah & Ducklow 1995. Phagotrophic nanoplankton also ingest photosynthetic microorganisms in the picoplankton size class (0.2 to 2 pm) including chroococcoid cyanobacteria and eukaryotic algae (Campbell & Carpenter 1986, Caron et al 1991, Sherr & Sherr 1991, Reckermann & Veldhuis 1997.…”