Chromium is a Period 4, d-block element.  The former classification places it within the same group of elements as potassium and calcium, the latter within the same group of elements as vanadium, iron, manganese, cobalt, nickel, copper, and zinc, all of which are depleted through biochemical reactions associated with photosynthesis.  The extent to which chromium is utilized in, and increases the rate of, photosynthesis is, at least to some extent, dependent upon bioavailability of suitable iron (in particular, that which has not become oxidized).  When abundant iron exists, presence of chromium appears to improve the rate of photosynthesis in some species of phytoplankton.  To an extent, the same appears to be true in systems with inadequate aqueous iron to satisfy the requirements of primary producers.  In either circumstance, however, Cr maintained at grossly elevated concentrations appears to interfere with some reactions associated with photosynthesis, and under these conditions Cr would be considered inhibitory.  The specific mechanisms of action which influence this relationship between iron, chromium, and photosynthesis are the subject of ongoing research.

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If chromium supplementation is to be undertaken, then recommended is to acquire an ICP analysis of the system in question and dose according to actual requirements as determined through interpretation of the data.  As noted elsewhere on this page, the standardized value for [Cr3+] in natural seawater (S=35, T=77°F) is 2.129 x10-4 mg/L (0.213 µg/L).  Strive to maintain the [Cr3+] within 10% of the standardized value, recognizing that a value in significant excess may be detrimental to critical biological processes.