Influence of temperature, salinity and dissolved nutrients (N and P) on the cultivation of estuarine and coastal water microalgae
Although the advent of microparticulate commercial foods has contributed to better food management of crustacean, mollusc and fish larvae, fresh microalgae continue to play an important role, being essential for the production of good quality cultivated organisms. Microalgae have been used not only in direct feeding, but also indirectly, as in the production of zooplankton and in maintaining water quality in green water systems. From a biotechnological point of view, the use of microalgae ranges from the production of chemical and pharmaceutical substances to the food industry, in addition to studies developed to convert lipids extracted from microalgae into biofuels. Despite the high costs involved in production,microalgae have a rapid growth and a wide tolerance to extreme environmental factors, allowing intensive cultivation in small areas and with the use of salt or brackish water. However, in order to decrease production costs, research is needed that results in the use of more productive species, with good nutritional quality or a higher growth rate. In the present study, the growth of the microalgae Thalassiosira weissflogii (Grunow) Fryxell & Hasle 1977 and Nannochloropsis oculata (Droop) Hibberd 1955 was evaluated under different conditions of temperature, salinity and nutrients (N: P ratio). These species are considered highly productive and widely used in larvicultures. Other than these,a new species of Skeletonema isolated in the coastal region of the state of Rio Grande do Sul was tested, named in the present work as Skeletonema sp. The experiments were carried out with the maximum time required to determine the growth curve, using 200 mL Erlenmeyer flasks, under three different conditions of temperature (20, 25 and 30°C), salinity (10, 20 and 30) and N: P ratio (Guillard medium f/2 - 8: 1, 16: 1 and 24: 1). For the experimental design, a factorial experiment 3x3x3 was applied, resulting in 9 treatments, with 3 repetitions each. These 27 sample units were randomly arranged in greenhouses, with a 12C/12E photoperiod and optimal irradiance for each species. The responses of the tested microalgae were evaluated from the estimate of chlorophyll a (956 g/L) and cell density (nº cel x 104/mL), from which the cell growth rate, doubling time, yield and chlorophyll/cell ratio (pg Cla/cel) were calculated. Considering the cell growth results obtained, T. weissflogii grew at all temperatures tested, with better growth and chlorophyll concentration at 25ºC and 30ºC, respectively, with good Cla/cel ratio (4.73 ± 1.99 pg Cla/cel). However, it presented the lowest yield and cell growth, and a high doubling time when compared to the other species. Depending on the results obtained, it is suggested that this algae be cultivated mainly in spring and summer. N. oculata, was the species that achieved the greatest cell growth and yield in the shortest time, being indicated for studies of large-scale crops. Besides that,this species did not present great restrictions regarding the variability of temperature, salinity and nutrients, being able to be cultivated throughout the year, in a wide range of temperature, salinity and nutrients. On the other hand, Skeletonema sp., Also has great growth potential as it has a high growth rate and short doubling time, despite not reaching the same values in cell density as N. oculata. However, Skeletonema sp., Showed a restricted growth potential at high temperature (30ºC), which allows its large-scale production only in the spring, autumn and early winter.