Iron co-limitation of Sargassum fluitans

In recent years, global distribution of holopelagic Sargassum spp. (sargassum) has extended from the subtropical Sargasso Sea and Gulf of Mexico into the tropical Atlantic. Climate and current patterns drive seasonal and year-to-year fluctuations of biomass in the ocean, but the underlying drivers of sargassum growth are poorly understood. Previous experimental studies showed that nitrogen (N) and phosphorus (P) can be limiting to sargassum. However, iron (Fe) also limits primary production in large parts of the ocean. We therefore (1) conducted a mesocosm experiment studying the effects of N+P and Fe addition on the growth rate and nutrient content of Sargassum fluitans, and (2) compiled literature on Fe tissue levels in sargassum throughout its distribution area. The Fe levels in collected experimental specimens (Mexican Caribbean) were like those previously reported near coastlines with low terrestrial nutrient runoff, and in the open ocean. The addition of Fe greatly boosted growth, averaging 0.13 doublings day⁻¹, 40 % faster than our controls, and maximum growth rate (doubling biomass in 5½d) was 46 % above previously reported maximal value. While oceanic Fe is relatively abundant in the tropical North Atlantic during rain episodes in the summers due to Saharan dust deposition, its availability is likely more limiting during other parts of the year, particularly in the western Caribbean. However, the true limiting potential of Fe depends on many factors. Our study suggests Fe co-limitation might occur widely and urges to include Fe availability in future sargassum forecasting models.