Microbial loop carbon cycling in ocean environments studied using a simple steady-state model

Abstract

A simple steady-state model is used to examine the microbial loop as a pathway for organic C in marine systems, constrained by observed estimates of bacterial to primary production ratio (BP:PP) and bacterial growth efficiency (BGE). Carbon sources (primary production including extracellular release of dissolved organic carbon, DOC), cycling via zooplankton grazing and viral lysis, and sinks (bacterial and zooplankton respiration) are represented. Model solutions indicate that, at least under near steady-state conditions, recent estimates of BP:PP of about 0.1 to 0.15 are consistent with reasonable scenarios of C cycling (low BGE and phytoplankton extracellular release) at open ocean sites such as the Sargasso Sea and subarctic North Pacific. The finding that bacteria are a major (50%) sink for primary production is shown to be consistent with the best estimates of BGE and dissolved organic matter (DOM) production by zooplankton and phytoplankton. Zooplankton-related processes are predicted to provide the greatest supply of DOC for bacterial consumption. The bacterial contribution to C flow in the microbial loop, via bacterivory and viral lysis, is generally low, as a consequence of low BGE. Both BP and BGE are hard to quantify accurately. By indicating acceptable combinations of parameter values for given BP:PP, the model provides a simple tool for examining the reliability of BP and BGE estimates. KEY WORDS: Microbial loop · Bacterial production · BGE · Models · DOC · Extracellular release Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 26: [37][38][39][40][41][42][43][44][45][46][47][48][49] 2001 However a recent literature review of BGE in open ocean areas by del Giorgio & Cole (2000) indicates a mean value of 0.15. Replacing the BGE of 0.45 with a BGE of 0.15 in the above calculation gives a BCD:PP of 1.33, which at face value would suggest that in excess of 100% of PP passes through the microbial loop.A number of factors should be taken into consideration when analyzing C flows in this way. Firstly, BP, and hence also BCD, are not constrained by PP, because each C atom can be cycled any number of times around the microbial loop before being respired. Indeed it is possible to construct food-web scenarios in which BP exceeds the input of organic C if growth efficiencies of bacteria and zooplankton are sufficiently high (Strayer 1988). In contrast bacterial respiration (BR), in conjunction with zooplankton respiration (ZR), cannot exceed the supply of organic C. Secondly, alternate sources of organic C should be considered. Most routine 14 C-primary production measurements do not include estimates of the extracellular production of organic carbon (EOC) by exudation or leakage (Bjørnsen 1988), which is an additional source of C for bacteria. There is little consensus on the magnitude and factors controlling EOC in aquatic systems. Expressed as a percentage of extracellular release (PER), this source of DOC typically accounts for about 10% of total pr...

Reconciliation of the carbon budget in the ocean’s twilight zone

“…A similar phenomenon exists when (incorrectly, as has often been the case) comparing bacterial carbon demand with primary production, the correct ratio being bacterial respiration to primary production 34, 35 .…”

Carbon‐nitrogen coupling and algal‐bacterial interactions during an experimental bloom: Modeling a 13C tracer experiment

“…These two autotrophic DOM sources differ in their C : N ratio and governing factors. Although passive leakage is proportional to biomass or production (as we assumed in our model), active exudation depends strongly on algal physiological status, in particular light and nutrient availability (Obernosterer and Herndl 1995;Anderson and Williams 1998;Carlson 2002).…”

“…It is instructive to distinguish between the release of DOM by autotrophic and heterotrophic mechanisms, because the governing factors differ. The heterotrophic release of DOM is caused by the dissolution of particulate detritus (Smith et al 1992), zooplankton sloppy feeding, and the incomplete ingestion (Jumars et al 1989) and lysis of bacteria (Anderson and Williams 1998). Phytoplankton release may either occur passively (leakage and algal lysis, Bjørnsen 1988) or may consist of active exudation of carbon-rich DOM (Biddanda and Benner 1997;Carlson 2002).…”

“…The direct link from algae to bacteria is via DOM, the main source of bacterial carbon and the preferred source of bacterial nitrogen (Wheeler and Kirchman 1986;Kirchman 1994). Algae produce DOM through lysis, passive leakage, or exudation of carbon-rich material (Anderson and Williams 1998), but there are other sources of DOM as well that are either indirectly linked to or independent of algal dynamics. These include enzymatic hydrolysis of particulate material through bacteria (Smith et al 1992), sloppy feeding and incomplete digestion by grazers (Jumars et al 1989), and bacterial mortality after viral lysis (Cotner and Biddanda 2002).…”

“…These formulations have proved their worth in simulations of culture (Geider et al 1998) and seasonal (Lancelot et al 1991;Soetaert et al 2001;Flynn and Fasham 2003) data. Similarly, a suite of models exists that explicitly deal with C and N acquisition in bacteria (Anderson and Williams 1998). Although these formulations provide an idealized outline as to how algal and bacterial organisms function, we still need to progress in our understanding and prediction capacity of how both work together in an ecosystem context.…”