The distribution of inorganic nitrogen and phosphorus and bioassay experiments both show that nitrogen is the critical limiting factor to algal growth and eutrophication in coastal marine waters. About twice the amount of phosphate as can be used by the algae is normally present. This surplus results from the low nitrogen to phosphorus ratio in terrigenous contributions, including human waste, and from the fact that phosphorus regenerates more quickly than ammonia from decomposing organic matter. Removal of phosphate from detergents is therefore not likely to slow the eutrophication of coastal marine waters, and its replacement with nitrogen-containing nitrilotriacetic acid may worsen the situation.

Two centric marine diatom species, Thalassiosira oceanica and Thalassiosira antarctica, were grown in batch cultures to determine the incorporation of germanium (Ge) and silicon (Si) into siliceous shells (opal). The results were modeled as Ge/Si “isotope” fractionation. During exponential growth, diatoms take up and incorporate Ge/Si from solution without major discrimination against Ge. During stationary phase growth near silica limitation, the Antarctic species (T. antarctica) discriminates slightly against Ge but integrated (Ge/Si)opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio. These results confirm experiments using radioactive 68Ge that showed absence of fractionation during diatom silica uptake (Azam and Volcani, 1981), in contrast to two‐box ocean models that invoke 50% Ge discrimination by diatoms to explain the observed “excess” surface ocean germanium concentration (Murnane and Stallard, 1988; Froelich et al., 1989) and late Pleistocene ocean sediment (Ge/Si)opal records (Mortlock et al., 1991). Runs of a 10‐box ocean Ge and Si model (PANDORA) with 50% discrimination reproduce the excess surface ocean Ge but introduces curvature into the deep ocean Ge versus Si relationship that is not observed in the oceans. Thus 50% fractionation is not supported by either cultures or models. If diatoms do not fractionate Ge/Si, then late Pleistocene (Ge/Si)opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization (Mortlock et al, 1991) but rather by whole ocean changes in (Ge/Si)seawater. The marine (Ge/Si)opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of Murnane and Stallard (1990). Glacial periods are characterized by lower continental weathering intensity, lower (Ge/Si)riv, and two fold higher dissolved silica river fluxes. Marine (Ge/Si)opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering.

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Six species of phytoplankton, representing 6 major phylogenetic groups (2 oceanic species: a cyanobacteria, Synechococcus sp., and a coccolithophorid, Emiliania huxleyi; and 4 coastal species: a prasinophyte, Tetraselmis sp., the green algae Dunaliella tertiolecta, the diatom Skeletonema costatum and a dinoflagellate Amphidinium carterae) were tested for their ability to reduce iodate to iodide in batch cultures. They all did so to varying degrees. Thus, the reduction of iodate to iodide by phytoplankton may be a general phenomenon in the marine environment. At ambient concentrations of iodate, the rates of depletion of iodate and appearance of iodide varied between 0.8 and 0.02, and between 0.3 and 0.02 nmol µg chlorophyll a -1 d -1 , respectively. E. huxleyi was the least efficient while A. carterae was the most efficient in the depletion of iodate. However, in the formation of iodide, while E. huxleyi was also the least efficient, Synechococcus sp. were the most efficient. The rates of appearance of iodide were noticeably slower than the corresponding rates of depletion of iodate, suggesting that part of the iodate might have been converted to forms of iodine other than iodide in these cases. The slight mismatch in the rank order of the rates of depletion of iodate and appearance of iodide between the phytoplankton species was traced to this variable and incomplete conversion of iodate to iodide. These rates were increased by up to over an order of magnitude upon enriching the culture medium with 5 and 10 µM of iodate. The depletion of iodate and appearance of iodide occurred in all growth phases. However, the rates might vary with growth phase and the patterns of these variations might be species-specific. Phytoplankton growth was not impeded even under unnaturally high concentrations of iodate implying that there is little interaction between iodine processing and the metabolic activity of cell growth.

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