Biogeochemistry - Geochemistry: An Introduction - page 172

Table 8.1 Elemental composition of the marine plankton (Martin, 1973 ).
Concentrations in gg −1 except for Na, K, Mg, Ca which are in mg g −1 μ

Na K Mg Ca Sr Ba

Phytoplankton 14 1.3 1.6 0.7 147 33

Zooplankton 11 1 1 0.2 132 16

Al Fe Mn Cu Zn

Phytoplankton 110 224 6

Zooplankton 94 199 11 6 100

8.4 Biominerals

What comes around goes around. In a world with no biological activity, sediments would
probably be deposited on the ocean floor on average at pretty much the same rate as they
are in our biological world. Alkalinity, phosphate, and dissolved silica liberated from the
continents by erosion would be transported from the continents to the oceans and create
huge carbonate, phosphate, and silica deposits. Iron and manganese would come out of
black smokers at the bottom of the ocean and precipitate as metalliferous sediments near
mid-ocean ridges. Even the most soluble compound, NaCl, has a residence time in the
ocean of 300 My: elements can only accumulate in the ocean for as long as solubility is
not exceeded or as long as they are not scavenged by detrital particles. In the end, what
precipitates on the ocean floor is what came through estuaries and other inputs. Beyond
the accumulation of organic matter discussed in the previous section, the role of biology
with respect to inorganic elements is somewhere else: it changes the site and sometimes
the mineral form under which minerals leave the ocean.
How life separates elements and recombine them into insoluble compounds to fit its own
needs may vary. One of the main functions of membranes is to use energy to temporar-
ily defeat the second principle of thermodynamics and the destructive power of diffusion
and mixing: building a hydrophobic wall between the inside and the outside of a cell, or
between different compartments of the same cell brings diffusion to a halt simply because
it locally removes the aqueous diffusing medium. Membrane proteins nailed through the
membrane take over and act as guarded gates to transfer elements individually ( Fig. 8.7 ) .
They are fueled by ATP, the standard energy source, and therefore do not have to comply
with the normal rules of diffusion. How proteins can be so selective with respect to particu-
lar elements is not a trivial question. Coordination is of course an important parameter, but
the simple ligands, such as citrate, lactate, and oxalate, have the same order of preference,
known as the Irving-Williams stability order, which for divalent ions is (Mg 2 + ,Ca 2 + )

<

Mn 2 + < Zn 2 + . This means for instance that, under sim-

ilar conditions, the Cu 2 + lactate and citrate will be more stable than the Fe 2 + lactate and
citrate, respectively. Other parameters are used by specific proteins to capture a particular

Fe 2 + < Co 2 + < Ni 2 + < Cu 2 + >