Urea as a source of nitrogen to giant kelp (Macrocystis pyrifera)

Abstract

Nitrate concentrations routinely fall below levels required to sustain growth of giant kelp (Macrocystis pyrifera) during summer and autumn in the Santa Barbara Channel, yet growth continues. We found urea to be consistently present at concentrations of 0.48–1.82 μM, accounting for greater than 20% of the dissolved fixed nitrogen pool during summer (14% overall). Field experiments indicate direct uptake of urea by giant kelp at a rate of 0.19 μmol N g dw−1 h−1, comparable to rates for ammonium (0.18 μmol N g dw−1 h−1) but lower than for nitrate (0.39 μmol N g dw−1 h−1). Co‐occurring phytoplankton took up nitrate, urea, and ammonium, 2‐, 15‐, and 39‐fold faster than giant kelp; however, the nitrogen uptake advantage of phytoplankton varies by substrate and season. Together, our results suggest that urea is readily used by giant kelp and may help to sustain growth throughout the year.

Spatial Variability in the Resistance and Resilience of Giant Kelp in Southern and Baja California to a Multiyear Heatwave

“…Disentangling the effects of heat stress and nutrient limitation on giant kelp during heatwaves is challenging because temperature and nitrate concentration are strongly negatively correlated in the California Current system (Zimmerman and Kremer, 1984). Other sources of nitrogen are unrelated to temperature (e.g., ammonium and urea) and help to sustain kelp during warm periods when nitrate concentrations are low (Brzezinski et al, 2013;Smith et al, 2018). These other forms of nitrogen help account for the relatively high growth rates maintained by giant kelp (>2% dry mass per day) that we observed at our long-term study sites near Santa Barbara during the 2014-2016 heatwave (Rassweiler et al, 2018) despite positive temperature anomalies >4 • C. Thus, it seems reasonable that heat stress rather than nutrient stress caused giant kelp to display its lowest resistance at locations with the highest temperatures.…”

Seaweed nutrient physiology: application of concepts to aquaculture and bioremediation

“…From an aquaculture perspective, however, such 'fouling' by invertebrates can be detrimental to offshore production (Buck et al 2017;Stévant et al 2017). Urea is also an important source of nitrogen for seaweeds: in southern New Zealand urea provided c. 30% of nitrogen to four species of intertidal seaweed in summer (Phillips & Hurd 2003), and in California, USA, urea represents an important year-round supply of nitrogen to M. pyrifera (Smith et al 2018). Seaweeds can utilise organic phosphorous via the external enzyme alkaline phosphatase, which breaks down organic matter on the seaweed's surface into PO 4 3− , which is then taken up (Schaffelke 2001).…”

Sea Surface Temperature Imagery Elucidates Spatiotemporal Nutrient Patterns for Offshore Kelp Aquaculture Siting in the Southern California Bight

“…Ambient seawater nitrate accounts for a large portion of readily available inorganic nutrients and is a necessary ion for tissue building and photosynthesis, where frond elongation rate declines dramatically when nitrate concentrations are <1 µmol L −1 (Zimmerman and Kremer, 1984;Rodriguez et al, 2016). While seawater nitrate concentration is closely related to kelp frond elongation and biomass accumulation in natural kelp forest systems (Zimmerman and Kremer, 1984;Bell et al, 2018), other forms of nitrogen, such as ammonia and urea, have been proposed for the maintenance of photosynthetic processes during periods of low nitrate availability (Brzezinski et al, 2013;Smith et al, 2018). However, the benthic sources of these reduced forms of nitrogen (Brzezinski et al, 2013;Burkepile et al, 2013;Peters et al, 2019) suggest they will be less important in offshore areas.…”