Nitrogen and seaweed thallus colour

Deborah V. Robertson-Andersson and D. T. Wilson
Botany Department, University of Cape Town

It’s always amazing that in research when trying to find the answer to one question you come up with several other questions, and if you’re lucky, a possible answer to a question you never asked in the first place. This was certainly true of research done between 2000 and 2003 on integrated seaweed and abalone mariculture by researchers from UCT, UWC, MCM and Stockholm University.

Upon investigating the food quality of the seaweeds (Gracilaria and Ulva) cultivated in a variety of aquaculture effluent media we noticed that the colour of the seaweeds changed according to their tissue nitrogen content. Past studies on Gracilaria showed that levels of pigment proteins are often closely correlated with nitrogen content (Lapointe & Ryther, 1979). This is because the pigment protein phycoerythrin is largely responsible for determining the red colour of the thalli, and the concentrations of this pigment change according to nitrogen content, causing lightening or darkening of seaweed’s thallus.

A number of algal researchers have tried to quantify colour of seaweeds, mainly for descriptive purposes (e.g. Chamberlain & Keats, 1994), using the Methuen Handbook of colour. To our knowledge no one, other than Wilson (1999), has tried to quantify the relationship between tissue nitrogen and thallus colour for either Gracilaria or Ulva. Wilson (1999) quantified this relationship for raft cultivated Gracilaria using the Methuen handbook (colours converted to Pantone^®). He found a relationship between thallus colour and tissue nitrogen, as well as a transition between green-yellows and yellow-browns which occurs between 0.8 - 1.6 % (Figure 1) of the total nitrogen with the green yellow colour indicating nitrogen starved material and the yellow browns indicating nitrogen rich material.

While this colour change has been shown clearly for Gracilaria (Lapointe & Ryther, 1979; Wilson, 1999), the authors were unable to find literature relating this relationship for Ulva, although chlorosis (yellowing of the thalli due to pigment destruction) has been well documented (Turpin, 1991; Floreto & Teshima, 1998). In Ulva the main pigment proteins are chlorophylls, and changes in the concentration of chlorophylls would theoretically lead to changes in thallus colour.

Instead of using the Methuen book of colour which provides a glossy guide that is difficult to match to seaweeds, the Pantone^® colour print guide for matt colours was chosen as there is a greater selection of greens. Furthermore, each colour has a printer guide for the reproduction of the exact colours either on their PC or at the printer interface. Also, the guide is easier to use than the book format.

After many hours in the lab correlating seaweed colour to a Pantone^® colour guide and analyzing protein content of Ulva, an interesting result was observed. When tissue nitrogen of U. lactuca is plotted against tissue colour from thalli obtained from 2 abalone farms in a number of different effluent treatments, a broad relationship between thallus colour and nitrogen content is clear to see (Figure 2). Although the colour reproduction is not perfect, the darker colours indicate more nitrogen rich material than paler colours. The transition between green-yellows and green appears to occur between 25 - 35 mg N per g tissue and is indicated by bars labeled with Pantone^® matt colours 585u and 583u. Laboratory experimentation would be useful to check the validity of these results. As the accumulation of pigment occurs directly in response to the availability of N in excess of that required for growth, then green would indicate nitrogen rich material while yellow-green would indicate nitrogen starved material.

The colour relationship shown in these figures could be used by mariculture farmers to assess the nutrient value of seaweeds as a food source for abalone and thus has important benefits for seaweed aquaculture. More laboratory work, however, needs to be done to find the exact “switch over” point as is represented by the colours in the graphs.

• Chamberlain, Y.M. & Keats, D.W. 1994. Three melobesioid crustose coralline red algae from South Africa: Leptophytum acervatum (Foslie) comb. nov., L. foveatum sp. nov. and L. ferox (Foslie) comb. nov. Phycologia 33(2): 111 - 133.

• Floreto, E.A.T. & Teshima, S. 1998. The fatty acid composition of seaweeds exposed to different levels of light intensity and salinity. Botanica Marina 41: 467 - 481.

• Lapointe, B.E. & Ryther, J.H. 1979. The effects of nitrogen and seawater flow on growth and biochemical composition of Gracilaria foliifer var. angustissima in mass outdoor cultures. Botanica Marina 22: 529 - 539.

• Turpin, D.H. 1991. Effects of inorganic N availability on algal photosynthesis and carbon metabolism. Journal of Phycology 27: 14 -20.

• Wilson, D.T. 1999. Some aspects of the nitrogen nutrition and growth of Gracilaria gracilis grown by suspended cultivation in Saldanha Bay, South Africa. Honours Thesis, Unpublished, University of Cape Town, Rondebosch. 31pp.

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