This chapter addresses the resource requirements for the assembly of photoautotrophic biomass. In addition to light and carbon, growth of phyto-plankton consumes 'nutrients' and, equally, may often be constrained by their availability and fluxes. Put at the most basic level, every replication of a phytoplankton cell roundly demands the uptake and assimilation of a quota of (usually) inorganic nutrients similar to that in the mother cell, if her daughters are to have the similar composition. Ignoring skeletal biominerals for the moment, we may recall from Section 1.5.3 that, in addition to carbon, the living protoplast comprises at least 19 other elements. Some are needed in considerable abundance (hydrogen, oxygen, nitrogen), others in rather smaller amounts (phosphorus, sulphur, potassium, sodium, calcium, magnesium and chlorine), for the assembly and production of the organic matter of protoplasm. Others occur as vital traces in support of cellular metabolism (silicon, iron, manganese, molybdenum, copper, cobalt, zinc, boron, vanadium). However, it is less the amounts in which these elements are required that constrains growth than does the ease or otherwise with which they are obtained. It is the demand (D) relative to the supply (S) that is ultimately critical, bearing in mind that a measurable presence is not a measure of availability if the element in question is not both soluble and diffusible and, so, assimilable by cells.

There is a huge literature on this topic. The purpose here is not to review the findings in detail or to give more than the sketchiest outline of the historical development of the understanding of nutrient limitation. Even the elements most often implicated in the constraint of phyto-plankton growth (nitrogen, phosphorus, iron and one or two other trace elements, together with the well-known constraint on diatom growth set by its skeletal requirement for considerable quantities of silicon) are sufficiently well and clearly known not to require any long and detailed account of how this recognition came about. The approach that I have adopted is first to consider the mechanisms of nutrient uptake and the general constraints that govern the successful assimilation and anabolism of resources by phytoplank-ton. Then, mainly by reference to the key limiting elements (N, P, Fe, etc.), I seek to show how the abilities of phytoplankton to gather the resources necessary to support cell growth and replication might impinge upon the dynamics and ecology of populations.

Uptake and assimilation of these nutrients do need to be considered in rather more detail, as impairment to these processes, mostly through resource deficiencies, is frequently implicated in the comparative dynamics and relative abundance of phytoplankters. It is important to recognise that the first impediment to be overcome is that just about all of the nutrients to be drawn from the water occur in concentrations that, relative to their effective concentrations within the cell, are extremely dilute, or rarefied. How these steep gradients are overcome is an appropriate starting point for our consideration.

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