Sloppy feeding

Some is released into seawater as an extracellular exudate or as a result of viral lysis and from sloppy feeding by the grazers. Consumption by grazers entrains the organic matter in the marine food web as shown in Figure 23.2. Cells that are not consumed or lysed by a virus can sink or be otherwise carried out of the euphotic zone by currents. 

DOM Release The Viral Shunt, Algal Exudates, and Sloppy Feeding... 

Sloppy feeding by grazers also leads to lysis of microbial and eukaryotic cell membranes with a similar outcome to viral lysis, i.e., conversion of POM into DOM as the cellular matrix is released into seawater. 

Given the relatively small contribution of terrestrial organic matter to seawater, most of the DOM in the ocean is perforce of marine origin. Most of this DOM has as its ultimate source biomolecules released into seawater by excretions and exudations from microbes, plants, and animals. Equally important is the role of viral lysis in which cell membranes are ruptured, causing the DOM within the cells to be spilled into seawater. A similar effect can result from sloppy feeding by herbivorous zooplankton. 

Estimates of N2 fixation rates in the global ocean continue to rise as results emerge from studies with the main N2 fixer in the ocean Trichodesmium, the heterocystous endosymbiont Richelia, as well as more recently discovered N2 fixers including unicellular diazotrophic cyanobacteria and bacterioplankton (Capone et al, 1997 HanseU and Feely 2000 Karl et al, 1997 Lipschultz and Owens, 1996 Montoya et al., 2004 Zehr et al, 1998 and 2001). Trichodesmium is involved in N release directly, through release of amino acids, DON, and NH4 (reviewed in Table 8.2). Trichodesmium is also a source of NH4+ and DON as a result of remineralization by associated bacteria, sloppy feeding and excretion by grazers (SeUner, 1992 Sheridan et al, 2002). 

M0Uer, E. F. (2007). Production of dissolved organic carbon by sloppy feeding in the copepods Acartia tonsa,Centropages typicus, and Temora longicornis. LimnoL Oceanogr. 52(1), 79—84. 

M0Uer, E. F. (2005). Sloppy feeding in marine copepods Prey-size-dependent production of dissolved organic carbon. J. Plankton Res. 11, 21—iS. 

M0Uer, E. F., Thor, P., and Neilsen, T. G. (2003). Production of DOC by Calanus fmmarchicus, C. glacialis and C. hyperboreus through sloppy feeding and leakage from fecal pellets. Mar. Ecol. Prog. Ser. 262, 185-191. 

The loss terms in N-cycle models that transform particulate and dissolved organic nitrogen into other forms can include a variety of processes (e.g., phytoplankton exudation, zooplankton grazing, sloppy feeding, phytoplankton and zooplankton mortality, bacterial remineralization, etc.). Different models may differ substantially in terms of which of these are included and their formulation (Christian and Anderson, 2002). Many N-cycle models now include significant phytoplankton exudation loss terms. This is often parameterized by simply specifying that some fixed fraction of the DIN uptake by phytoplankton is shunted directly to the DON pool (e.g., Anderson and Williams, 1998). Sloppy feeding by zooplankton can be similarly accounted for. Many models also include linear loss terms in the phytoplankton equation that represent either natural mortality or phytoplankton respiration (e.g.. Hood et ai, 2001). 

Seawater DOM has numerous sources and sinks and a range of potential reactions. Sources of the sub-micron components include exudation from phytoplankton, microbial degradation of bioparticles, animal wastes (excretion), viral infection of bacteria, sloppy feeding by zooplankton and other animals, and input of dissolved molecules from rivers and surface sediments. Removal mechanisms include photodegradation, sorption to sinking particles, and microbial utilization. Tritiated thymidine and leucine uptake experiments indicate that up to half of the carbon formed by photosynthesis is shunted via dissolved organic molecule intermediates into bacteria. 

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