Blue-green algae, scientifically known as Cyanobacteria, are microscopic single-celled organisms that grow naturally in fresh and salt waters.
They are not algae (eukaryotes), but are a type of bacteria (prokaryotes), possessing the ability to synthesize chlorophyll a.
Therefore, they act like plants by using sunlight to manufacture carbohydrates from carbon dioxide and water, a process known as photosynthesis.
Blue-green algae have vesicles or gas pockets inside vacuoles within their cells that they inflate with gas, thus able to regulate their buoyancy in response to environmental conditions.
This is advantageous over other algae as they have the ability to sink and rise at their will and move to where nutrient and light levels are at their highest.
Nutrient requirements of phytoplankton and N:P ratio:
In addition to light and carbon, growth of phytoplankton (all photosynthetic aquatic microorganisms including algae and blue-green algae) consumes ‘nutrients’. Every replication of an algal cell roughly demands the uptake and assimilation of a quota of inorganic nutrients similar to that in the mother cell.
In addition to carbon, the living protoplast comprises 19 other elements. The elements/nutrients most often implicated in the constraint of algal growth are: nitrogen, phosphorus, iron, and one or two of other trace elements, together with silicon - the well known constraint on diatom skeletal growth.
Since the early twentieth century (1934) it has been recognized (primarily through the late Harvard University scientist Alfred Redfield’s work on Nitrogen:Phosphorus ratios) that the elemental composition of phytoplankton was similar to that of the ocean: 16N:1P.
Scientists have accepted this as a constant called the Redfield ratio. However, the canonical Redfield N:P ratio of 16 for phytoplankton is not a universal optimal value but instead represents an average for a diverse phytoplankton assemblage growing under a variety of different conditions and employing a range of growth strategies.
The N:P ratio is not fixed in the environment and this is mainly due to the inflow of nutrients from anthropogenic sources such as fertilizers and runoff containing nutrient rich waste (e.g. effluent).
Different cellular components of phytoplankton cells have their own unique stoichiometric properties. Most notably, resource (light or nutrients) acquisition machinery, such as proteins and chlorophyll, is high in nitrogen but low in phosphorus, whereas growth machinery, such as ribosomal RNA, is high in both nitrogen and phosphorus.
Because these components make up a large proportion of cellular material, changes in their relative proportions have a marked effect on bulk cellular C:N:P stoichiometry.
During exponential growth, bloom-forming phytoplankton optimally increase their allocation of nutrients toward production of growth machinery, reducing their N:P ratio to ~8, far below the Redfield value of 16 (Figure 1). However, when nutrients are scarce, slow-growing phytoplankton that can synthesize additional nutrients acquisition machinery are favoured. This allocation of nutrients results in optimal N:P ratios ranging from 36 – 45 (Figure 1).
The optimal N:P ratio will vary from 8.2 to 45.0, depending on the ecological conditions. Nitrogen-fixing species (e.g. nitrogen-fixing blue-green algae) often have a higher N:P stoichiometry than non-fixing species. For example nitrogen-fixing, Trichodesmium blooms have N:P ratios ranging from 42 to 125. The differences in N:P ratios between phyla and super families are also significantly different. For example, green algae required N:P ~30 whereas diatom required ~10 and Dinophyceae required ~12 (Quigg et al. 2003) and red algae required N:P ~10 (Arrigo 2005).
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