Potential uptake of dissolved organic matter by seagrasses and macroalgae

Abstract

Dissolved organic nitrogen (DON) acts as a large reservoir of fixed nitrogen. Whereas DON utilization is common in the microbial community, little is known about utilization by macrophytes. We investigated the ability of the coexisting temperate marine macrophytes Zostera noltii, Cymodocea nodosa, and Caulerpa prolifera to take up nitrogen and carbon from small organic substrates of different molecular complexities (urea, glycine, L-leucine, and L-phenylalanine) and from dissolved organic matter (DOM) derived from algal and bacterial cultures (substrates with a complex composition). In addition to inorganic nitrogen, nitrogen from small organic substrates could be taken up in significant amounts by all macrophytes. Substrate uptake by the aboveground tissue differed from that of the belowground tissue. No relationships between carbon and nitrogen uptake of small organics were found. The preference for individual organic substrates was related to their structural complexity and C:N ratio. Uptake of algae-derived organic nitrogen was of similar magnitude as inorganic nitrogen, and was preferred over bacteria-derived nitrogen. These results add to the growing evidence that direct or quick indirect DON utilization may be more widespread among aquatic macrophytes than traditionally thought.KEY WORDS: Uptake · Dissolved organic nitrogen · Stable isotopes · Zostera noltii · Cymodocea nodosa · Caulerpa prolifera · Seagrasses · Macroalgae Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 427: [71][72][73][74][75][76][77][78][79][80][81] 2011 2008). But it is not yet known to what extent macrophytes share these capabilities. Although small organic compounds were until recently not considered a significant direct nutrient source for seagrasses (Romero et al. 2006), uptake of organic matter-derived nitrogen has been demonstrated for a number of macro algae and seagrass species (Bird et al. 1998, Tyler et al. 2003. Uptake of detritus-derived compounds by seagrasses was demonstrated by Evrard et al. (2005) and Barrón et al. (2006). However, these studies did not differentiate between direct DON uptake and uptake after remineralization by the bacterial community. On the other hand, macrophyte DON uptake in axenic cultures has been observed (Bird et al. 1998, Tarutani et al. 2004, suggesting that direct utilization of DON may be more common than is generally realized.Studying the uptake of natural DON (a complex mixture of organic nitrogen compounds) is inherently complicated. Not all compounds are treated the same way by primary producers. Tyler et al. (2005) demonstrated clear differences in the processing of 2 amino acids (alanine versus glycine) by a macroalga. Harrison et al. (2007) found relationships between amino acid complexity and uptake preference by terrestrial plants. Not all primary producers are equally capable of taking up organic nitrogen. Uptake capabilities for different compounds may even vary seasonally in certain microphytobenthic taxa (Nilsson & ...

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“…Another allochthonous source relevant also for the open ocean is the DON in atmospheric deposition [63,64]. DON is actively channelled into cells via membrane transport systems [65] and seems to be a quite active component of coastal nitrogen cycling [66,67]. Better understanding …”

“…figure 2). This burial occurs mainly in vegetated systems such as seagrass meadows and mangroves [98], because of trapping of particulate nitrogen and assimilation of dissolved inorganic and organic nitrogen [67]. The present-day and future nitrogen cycling in proximal coastal systems differs substantially from that before the Anthropocene.…”

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“…In the water column, organic material can be taken up, directly or indirectly via bacterially mediated processes, by thalli of seaweeds, leaves of sea grasses, and by sponges within coral reef cavities (de Goeij, Moodley, Houtekamer, Carballeira, & van Duyl, ; Van Engeland et al., ). The uptake of dissolved inorganic and organic material by seaweeds differs from that of sea grasses, which are able to exploit nutrient sources from both the sediment and the water column (Van Engeland et al., ). Yet, both sea grass and seaweed can directly take up small inorganic ( and ) and organic (urea, glycine, leucine, phenylalanine, algae‐derived DOM, and bacteria‐derived DOM) substrates with a preference for ammonium, which can be directly incorporated into amino acids (Van Engeland et al., ).…”

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“…, Van Engeland et al . ). In addition trapped leaves will be removed by crabs into their burrows and converted into more palatable forms of nutrients for other organisms within the system.…”