Phytoplankton transport system

Iron interactions with N sources are not limited to phytoplankton. Kirchman et al. (2003) found that the growth rate, respiratory electron transport system activity, and growth efficiency of a marine gamma proteobacterium (Vibrio hatveyi) were much lower in Fe-limited cultures grown with NOs" than when NH4+ or amino acids were suppHed as N sources. They suggested that these results may help to explain why natural bacterial communities in nitrate-rich, iron-poor HNLC areas also typically exhibit reduced growth rates and efficiencies. 

Phosphate transporters have been characterized in many model organisms, though relatively little mechanistic work has been done in marine phytoplankton. Phosphate transport is elfected by high and low affinity transporters and dependent on ATP, Na, and Mg " " in several diatoms (Cembella et al., 1984). These observations are found to be consistent with the well known active transport system of yeast (Raghothama, 1999). The dependence of phosphate transport on Mg " " in diatoms and yeast suggests that eukaryotes may transport an uncharged cation phosphate complex (MeHP04, where Me may be Ca +, Mg +, Co +, Mn ) as has been observed in heterotrophic bacteria (van Veen, 1997). 

Since the vertical distribution of zooplankton and bacteria in the ocean is determined by the available total OM, the calculation of OD for separate depth intervals was carried out by Skopintsev (1966, 1975b). Based on an average annual value of phytoplankton production of 120 g C m with about 90% available as OM, it turned out that in 0—100 m, 100—1000 m and 1000—4100 m layers the annual OD at the in situ temperature constitutes 2.15, 0.07 and 0.003 ml O2 T or 75, 22 and 3% of the total OD of the 4000-m water column, respectively. The last value is at the limits of determination. From the evaluation of the electron transport system in the tropical regions of the Pacific Ocean it was calculated that the annual value of OD at 3000 m depths equals 0.003 ml O2 1" (Packard et al., 1971). According to Riley (1951), about 90% of the OM annually produced by phytoplankton, is consumed in the upper layer (up to 200 m depth). 

Uptake systems appear to be simplest for dissolved Mn(ii), which is taken up in phytoplankton by a single high-affinity transport system that is under negative feedback regulation. In this negative feedback, as the concentration of dissolved Mn(ii) decreases, the Vmax of the transport system is increased... 

Eukaryotic phytoplankton do not appear to produce siderophores and there is little evidence for direct cellular uptake of ferric siderophore chelates. Instead there is mounting evidence for the utilization of a high-affinity transport system that accesses ferric complexes via their reduction at the cell surface and subsequent dissociation of the resulting ferrous-ligand complexes. The released ferrous ions bind to iron(ii) receptors on iron transport proteins located on the outer cell membrane, which transport the iron into the cell. This intracellular transport involves the reoxidation of bound iron(ii) to iron(iii) by a copper protein, and thus copper is required for cellular iron uptake. The availability of iron to this transport... 

Copper occurs in cytochrome oxidase, a key protein in respiratory electron transport, and in plasto-cyanin, which substitutes for the iron protein cytochrome Cg in photosynthetic electron transport in oceanic phytoplankton. It is also an essential component of the high-affinity iron transport system of many eukaryotic algae. Because copper is needed for iron uptake and can metabolically substitute for iron, co-limitations can occur for Cu and Fe, as observed in some diatoms. 

In marine phytoplankton, cadmium uptake appears effected by a specific transport system as well as by leakage through the transport systems of other... 

The volume terminates with Chapter 16 in which also the essentiality of Cd " for certain diatoms is pointed out. The distribution of Cd " in the ocean is very similar to that of major nutrients suggesting that it may be taken up by marine phytoplankton at the surface and remineralized at depth. At high concentration, Cd is toxic to phytoplankton as it is for many organisms. However, at relatively low concentrations, Cd " can enhance the growth of a number of phytoplankton species under Zn limitation possibly Cd is taken up either by the Mn or the Zn transport system. The otdy known biological function of Cd is to serve as a metal ion cofactor in cadmium-carbonic anhydrase (CDCA) in diatoms. The expression of CDCA is regulated by the availabilities of Cd " and Zn " both Zn " and Cd can be used as the metal ion cofactor and be exchanged for each other in certain marine phytoplankton species. 

Distribution of241 Am in a dialysis system containing sediment, phytoplankton, and detrital matter established that a substantial amount of americium accumulated in all three phases both in fresh and marine waters (NRC 1981). The adsorption process was not reversible and the longer the americium was adsorbed, the more difficult the chemical was to desorb. Appreciable amounts of americium have been shown to adsorb to bacterial cells such as those found in the Waste Isolation Pilot Plant in New Mexico (Francis et al. 1998). There is a potential that americium attached to biocolloids may facilitate its transport from the waste site. 

For the internalisation of metals, many examples exist for which transport may be coupled to an energy-dependent process, of which only a few are described here. For example, the well-studied (e.g. [276]) Na+/K+ channel transports 3Na+ out and 2K+ in for each ATP molecule that is hydrolysed [242]. Mg2+ influx (but likely not efflux) is highly regulated in eukaryotes [277]. ATPases have been implicated in certain cases of Fe [278] or Zn [90] uptake by phytoplankton. Finally, although Cd internalisation by a polychaete appeared to be energy independent, accumulation was increased rather than decreased in the presence of ATPase inhibitors, suggesting that the efflux system might depend upon ATP synthesis [279]. 

Under upwelling conditions, surface water transports material out of the system and cool, clear upwelling water dilutes the suspension of detritus. Under downwelling conditions when phytoplankton-rich water from offshore is imported, phytoplankton supplements the detrital diet of shallow-water filter-feeders and their growth is faster this situation parallels that found on exposed rocky intertidal shores. 

Fig. 2. Model output showing the percentage of different food components available to filter feeders with varying water transport (0-7 water column turnovers per day). The nitrogen content of all food available to filter feeders is shown below, and is much higher when the system is "closed" (0 turnovers). Fig. 2a) shows food proportions and quantities under upwelling conditions when all food is derived from macrophytes. Fig. 2b) depicts downwelling conditions when phytoplankton is an additional component (After Wickens and Field, 1985).

Under conditions of continuous downwelling, phytoplankton imported from outside the system becomes increasingly important with faster rates of water transport (Fig. 2b) and at rates faster than one water-column turnover per day, phytoplankton contribute more nitrogen to filter-feeder food than macrophyte particulate matter or recycled faeces. 

NH4+ is recycled rapidly and repeatedly between living biomass (phytoplankton, the zooplankton that graze on them, protozoans, bacteria and archaea) and the inorganic nutrient form, which is released from heterotrophic metabohsm and grazing. Nitrate, on the other hand, is new N because it is virtually absent from the euphotic zone most of the time and must be transported into the system by physical means—mixing or upweUing from deep waters or faUing in rain — in order for phytoplankton to use it. The rate of supply can be equated with the steady... 

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