Uptake of dissolved organic and inorganic nitrogen in microalgae-dominated sediment: comparing dark and light in situ and ex situ additions of 15N

Assimilation and short‐term processing of microphytobenthos nitrogen in intertidal sediments

“…The contribution of MPB to N uptake from NH 4 + in the current study (accounting for 87.6% of the 15 N in microbial biomass at 1.5 d; Fig. 2) was comparable to that previously observed in the Richmond River (90%; Riekenberg et al 2017) and in MPB-dominated temperate sands (100%; Hardison et al 2011). However, the contribution of MPB to N uptake in the current study was considerably higher than the contribution of MPB to N uptake reported for NO 3 − in subtidal sediment in the nearby Brunswick River (40%; Eyre et al 2016) and NH 4 + Fig.…”

“…where L*0 and D*0 represent the original concentrations of L-and D-ala, L* and D* are the measured concentrations after hydrolysis, and 0.6 represents the racemization factor associated with acid hydrolysis of free alanine (Kaiser and Benner 2005). This racemization factor was further supported by the results of in-house acid hydrolysis of the L-ala standard (Riekenberg et al 2017).…”

“…The study site, an intertidal shoal in the lower Richmond River estuary, on the east coast of Australia (28 52 0 25.01 00 S, 153 33 0 20.43 00 E), has been used in a number of previous studies (Oakes and Eyre 2014;Riekenberg et al 2017). This includes a complementary study of MPB-C processing, which was based on data also collected during the sampling campaign described here (Oakes and Eyre 2014).…”

“…The excess 15 N content of bacteria and MPB was estimated based on the excess 15 N within the AAs D-and L-alanine (for assumptions and uncertainty associated with this approach, see Veuger et al 2005Veuger et al , 2007b. The concentrations of D-and L-alanine in the sediment were calculated based on the peak area of the added L-norleucine standard, and were corrected for creation of D-ala from racemization during acid hydrolysis as described by Riekenberg et al (2017):…”

Short-term fate of intertidal microphytobenthos carbon under enhanced nutrient availability: a <sup>13</sup>C pulse-chase experiment

“…For diatoms, a mixing model was used to correct the concentration and δ 13 C value of 16 : 1(n−7) for any contribution from non-diatom sources. Due to the scarcity of cyanobacteria observed using light microscopy (1000×), low sediment D-/ L-alanine ratios measured previously at this site (as low as 0.0062, Riekenberg et al, 2017), and lack of the characteristic 18 : 2(n − 6) peak (Bellinger et al, 2009), cyanobacteria were assumed to make a negligible contribution to the 16 : 1(n−7) peak. A two-source mixing model was applied to correct the concentration and δ 13 C value of the 16 : 1(n − 7) peak for the contribution of gram-negative bacteria, based on a typical ratio of 18 : 1(n − 7) to 16 : 1(n − 7) for gramnegative bacteria of 0.7 (Edlund et al, 1985) as previously applied in Oakes et al (2016).…”