Studies on the Exophysiology and Biochemistry of Porphyra Dioica Brodie ET Irvine in Culture

The genus Porphyra is one of the most important seaweeds in aquaculture, both in value and biomass. According with the Food and Agriculture Organization of the United Nations, 1.132 million metric tons of Porphyra were produced in 2001, valued at 1.2 billion USD. Production of Porphyra represents 16.3% of the world’s seaweed mariculture, which in turn corresponds to 28.4% of the world’s total mariculture production including fish, aquatic plants, molluscs and crustaceans. Porphyra is mainly used for human consumption and as source of the red pigment r-phycoerythrin.

This study was thought to increase the knowledge of the biology of Porphyra dioica, one of the most common Porphyra species in the North of Portugal. The first objective was to characterize the life cycle of the species, both in the field and in the laboratory. The monitorization of the percentage cover of the population in the field showed that P. dioica can be found throughout the year. Field studies showed higher percent cover, from 23 to 66%, during February through May. In order to follow the lifecycle in the laboratory, the effects of temperature, photon flux density (PFD) and photo period on growth and reproduction were tested. The zygotospores germinated faster at 15ºC, and at 25 μmol photons m-2s-1. Growth rate of the conchocelis was affected by temperature and not by photoperiod. In the three photoperiods tested, growth rate was always higher at 15ºC, under 25 to 75 μmol photons m-2s-1.Conchosporangia formation was higher in 15ºC, short-day, 8:16 h, L:Dcycle and 25 to75 μmol photons m-2s-1 and was almost non-existent in free floating conditions. Optimal conditions for conchosporangia maturation also promoted spore release after 18 weeks. Aeration is crucial for normal blade development. No archeospores or any other kind of asexual spores were observed.

The second main objective was to determine the optimal conditions for the growth of the gametophytes and to assess its potential for use in integrated aquaculture, as nutrient removers. The influence of stocking density and PFD on the growth, production and nutrient removal was tested. Maximum growth rates, up to 33% per day, were recorded with 0.1 g fw l-1at 150 and 250 μmol photons m-2s-1. Growth rate decreased significantly with increasing stocking density, while productivity had an inverse trend. At 150 μmol photons m-2s-1and with 1.5 g fw l-1, 1.4 g fw l-1week-1 were produced. At this PFD, there was no significant difference in production between 0.6 to1.5 g fw l-1. Nitrogen (N) content of the seaweeds decreased with increasing stocking densities and PFDs. The maximum N removal was recorded at 150μmol photons m-2s-1,with 1.5 g fw l-1 stocking density (1.67 mg N day-1). However, the N removed by thalli at 50 μmol photons m-2s-1was statistically equal to that at 150 and 250 μmol photonsm-2s-1, at a stocking density of 1.0 g fw l-1.

The influence of temperature and photoperiod on growth and reproduction was also assessed. Growth rates of Porphyradioica were significantly affected by temperature and photoperiod. The highest growth rate, 27.5% fw day-1, was recorded at15ºC and 16:8 h, L:Dcycle. Formation of young bladelets in the basal portion of unfertilized female thalli was observed after 4 weeks in culture. After 7 weeks all biomass produced was solely due to these vegetatively propagated young thalli, growing 22.4 to 26.1% day-1.

The species ability to uptake and accumulate high quantities of N was assessed for increasing concentrations of two forms, nitrate (NO3-) and ammonium (NH4+). The growth rate of Porphyradioica increased linearly with the increase of the N concentration up to 500 μM NO3-. Porphyradioica was able to grow equally well using NO3- or NH4+ as source of N and NH4+ was not toxic at concentrations as high as 300 μM. The results of the diel uptake experiment showed that Porphyradioica prefers ammonium to nitrate when both forms are present. The amount of NH4+ removed during the light period was the double than during the night period. The amount of NO3- increased slightly during the night period and the amount of PO43- remained constant during the 24 hours.

The results obtained during this work showed that Porphyradioica is a good candidate for application in integrated aquaculture systems. Evidences for this are: high growth rates, achieved over a range of temperatures and photoperiods; ability to uptake and accumulate high amounts of N, under high concentrations and at least in two different forms (NO3-and NH4+); possibility of vegetative propagation of the blades. Application of this species will depend on future studies on large scale systems and assessment of the quality/value of its biomass.