GROWING HALOPHYTES DIRECTLY AT SEA

Required global food increases from agriculture face growing freshwater limitations. As an alternative strategy that does not require freshwater, we are researching aquatic agriculture both at sea--cultivating seaweeds and using them as food and growing land crops irrigated with water from distillation and rainwater harvesting--and on lakes, growing land crops and aquatic plants using for crop transpiration the water that is naturally anyway evaporated.

Seeking to expand plant growing capabilities, we cultivated halophytes floating at sea, potted and during rainless periods accessing only seawater. Four halophyte species, two woody (Rhizophora mangle, red mangrove and Avicennia germinans, black mangrove) and two herbaceous (Sesuvium portulacastrum, sea purslane, an edible plant, and Sporobolus virginicus, salt couch grass) that normally grow rooted in soil at the intertidal zone, were grown in coastal Pacific waters of Costa Rica. Initial trials with roots immersed directly in seawater resulted in stunted growth attributed to nutrient deficiencies, with sea purslane being the least affected. After that plants were potted using a mixture with 50 % sand in containers of 1.5 to 4 L and fertilizer was used. Containers were attached to floating rafts, with their lower end perforated and immersed (Fig.1). Free water entry and exit from bobbing with waves effectively controlled salinity build-up yet it also leached nutrients.

Growth at sea was monitored for two years, including two dry seasons, evidencing prolonged exclusive reliance on seawater. There was 100 % survival and all-year growth that peaked after fertilization, with no pests and diseases. No water deficit symptoms were observed and canopy temperature was lower than or equal to air temperature. Salinity within leaves varied between species. Suppressing rain entry into the potting mixture of black mangrove plantlets produced no significant difference on growth rate. Growth rate of sea purslane was of 3.03 g/plant/d, equivalent to a high yield of 88.5 t/ha/y. While further research is needed, this evidenced to be a viable plant production option that has many applications, including bioremediation of mariculture waters and effluents if nutrient concentration allows no fertilization and/or growth with roots immersed directly in water.