Is pink your favourite colour? How about gold?

How do you like your salmon – nice and pink, or do you prefer a whiter shade of pale? And when you go to the zoo, do you like your flamingoes off-white or a nice rosy shade? If you prefer pink, you should know that farmed salmon and captive flamingoes only go that colour when the diets they are fed are supplemented with astaxanthin, a red ketocarotenoid pigment. More recently, astaxanthin has started to be used for human consumption because of antioxidant, anti-ageing, anti-inflammatory and immune-stimulating properties.

At present, the global demand for this pigment is satisfied mainly by synthetic astaxanthin produced by chemical companies. The global market for astaxanthin is worth more than US$200 million per year. The estimated production cost of synthetic astaxanthin is approximately US$1000 per kilo, and the market price is approximately US$2000 a kilo. That makes astaxanthin nearly as expensive as gold. So if you could get algae to grow gold for you on the cheap, would you do it?

Haematococcus pluvialis is a unicellular green alga able to accumulate large amounts of astaxanthin (4 % dry weight) under stress conditions. The life cycle of H. pluvialis is complex and involves at least four types of cells. Under stress conditions, astaxanthin biosynthesis is accompanied by morphological changes of the motile vegetative (green) cells into non-motile cysts (red), which represent a resting stage with a heavy resistant cellulose cell wall. Astaxanthin is accumulated in the cytoplasm of cyst cells, providing protection against photo-inhibition and oxidative stress. Although H. pluvialis is one of the richest sources of astaxanthin, its massive culture for commercial purposes has been little exploited because of its slow growth rate and complex life cycle.

A recent paper in AoB PLANTS sets out to standardize and apply a genetic improvement programme to H. pluvialis in order to improve its carotenogenic capacity and to evaluate the performance of a selected strain in large commercial-sized open ponds. Improved astaxanthin productivity of the selected strain was maintained even when grown on a large scale and holds promise as the basis for viable commercial production of this valuable biochemical by natural means.

From genetic improvement to commercial-scale mass culture of a Chilean strain of the green microalga Haematococcus pluvialis with enhanced productivity of the red ketocarotenoid astaxanthin. (2013) AoB PLANTS 5: plt026 doi: 10.1093/aobpla/plt026
Astaxanthin is a red ketocarotenoid, widely used as a natural red colourant in marine fish aquaculture and poultry and, recently, as an antioxidant supplement for humans and animals. The green microalga Haematococcus pluvialis is one of the richest natural sources of this pigment. However, its slow growth rate and complex life cycle make mass culture difficult for commercial purposes. The aims of this research were (i) to standardize and apply a genetic improvement programme to a Chilean strain of H. pluvialis in order to improve its carotenogenic capacity and (ii) to evaluate the performance of a selected mutant strain in commercial-sized (125 000 L) open ponds in the north of Chile. Haematococcus pluvialis strain 114 was mutated by ethyl methanesulfonate. The level of mutagen dose (exposure time and concentration) was one that induced at least 90% mortality. Surviving colonies were screened for resistance to the carotenoid biosynthesis inhibitor diphenylamine (25 µM). Resistant mutants were grown in a 30-mL volume for 30 days, after which the total carotenoid content was determined by spectrophotometry. Tens of mutants with improved carotenogenic capacity compared with the wild-type strain were isolated by the application of these standardized protocols. Some mutants exhibited curious morphological features such as spontaneous release of astaxanthin and loss of flagella. One of the mutants was grown outdoors in commercial-sized open ponds of 125 000 L in the north of Chile. Grown under similar conditions, the mutant strain accumulated 30% more astaxanthin than the wild-type strain on a per dry weight basis and 72% more on a per culture volume basis. We show that random mutagenesis/selection is an effective strategy for genetically improving strains of H. pluvialis and that improved carotenogenic capacity is maintained when the volume of the cultures is scaled up to a commercial size.