Nitrogen acquisition and resource allocation strategies in temperate seagrass Zostera nigricaulis: Uptake, assimilation and translocation processes

Abstract

The dominant seagrass in Port Phillip Bay (PPB), Australia, Zostera nigricaulis, declined between 2000 and 2011, coinciding with the ‘Millennium drought’ that ended in 2009. These seagrasses are nitrogen-limited, underpinning the need to develop nitrogen budgets for better ecosystem management. Environmentally realistic measurements of specific uptake rates and resource allocation were undertaken to develop nitrogen budgets and test the hypothesis that the above-ground and below-ground compartments are able to re-mobilise ammonium and nitrate through uptake, translocation and assimilation to adapt to varying levels of nitrogen in the ecosystem. Uptake of 15N labelled ammonium and nitrate by above- and below-ground seagrass biomass, epiphytes and phytoplankton was quantified in chambers in situ. Preferential uptake of ammonium over nitrate was observed, where the uptake rate for nitrate was about one sixth of that for ammonium. Epiphytes and phytoplankton also registered an increased affinity for ammonium over nitrate. Translocation experiments demonstrated the uptake by both the above-ground and below-ground biomass, respectively from the water column and pore water, and subsequent translocation to the opposite compartment. Acropetal translocation (below- to above-ground biomass) was more prevalent than basipetal translocation. This is a unique outcome given basipetal translocation has been widely reported for Zostera by other researchers.

Tolerance to Nitrogen ions enrichment in a Gulf of Mexico's freshwater submerged grass

“…2) which correspond to the oligotrophic environmental limit, and were found to be similar to the control experimental of local tap water at the root with NO 3 . This maximum cumulative response coincides with N de cit in aquatic plants roots [33] to the extent of limiting the growth of aquatic grasses by N in the oligotrophic environment, supporting the need to develop an endogenous pool of N [29]. Under oligotrophy conditions, the mean of NO 3 accumulation registered in local Vallisneria seedling was approximate to those reported on submerged species (root plus leaf), such as Berula erecta,…”

“…2). The order of descent of this ion was irregular with respect to NO 3 ion.DiscussionSubmerged grasses have been found sensitive to inorganic N excesses into the water column in shorttime, under both natural and semi-controlled environmental conditions[4,14,28,29]. In this contribution, a controlled environment (i.e.…”

Effect of DIN and DON sources on the nitrogen uptake of the seagrass Zostera japonica and the macroalgae Ulva pertusa previously grown in different light levels

“…The preferential uptake of ammonium by seagrasses may be due to the physiological demands associated with nitrate uptake (Nayar et al, 2018 and references therein). Furthermore, seagrass tissues require far less energy than nitrate to transform ammonium into organic nitrogen (Nayar et al, 2018). Sandoval-Gil et al (2015) found that seagrass roots showed reduced capacity to absorb ammonium compared to leaves because of the very high availability of this nutrient in sediments.…”

Florida manatees Trichechus manatus latirostris actively consume the sponge Chondrilla caribensis

“…While manatees may be eating C. caribensis because local food resources might be diminished, possibly due to global warming (e.g., O'Shea, 1986), some scientists propose that manatees eat invertebrates for the nitrogen within. Manatees living in marine habitats eat mostly seagrasses, which take-up nitrogen mostly through their leaves and are thought to be nitrogen limited (Duarte, 1990;Nayer et al, 2018). Up to 50% of nitrogen requirements of seagrasses are supplied by epiphytes living on the leaves, and seagrasses provide 8-22% protein per dry weight equivalent to corn and wheat (Walsh & Grow, 1972).…”