Nitrate fertilisation does not enhance CO2 responses in two tropical seagrass species.

Seagrasses are often considered "winners" of ocean acidification (OA); however, seagrass productivity responses to OA could be limited by nitrogen availability, since nitrogen-derived metabolites are required for carbon assimilation. We tested nitrogen uptake and assimilation, photosynthesis, growth, and carbon allocation responses of the tropical seagrasses Halodule uninervis and Thalassia hemprichii to OA scenarios (428, 734 and 1213 μatm pCO2) under two nutrients levels (0.3 and 1.9 μM NO3(-)). Net primary production (measured as oxygen production) and growth in H. uninervis increased with pCO2 enrichment, but were not affected by nitrate enrichment. However, nitrate enrichment reduced whole plant respiration in H. uninervis. Net primary production and growth did not show significant changes with pCO2 or nitrate by the end of the experiment (24 d) in T. hemprichii. However, nitrate incorporation in T. hemprichii was higher with nitrate enrichment. There was no evidence that nitrogen demand increased with pCO2 enrichment in either species. Contrary to our initial hypothesis, nutrient increases to levels approximating present day flood plumes only had small effects on metabolism. This study highlights that the paradigm of increased productivity of seagrasses under ocean acidification may not be valid for all species under all environmental conditions.

Ocean acidification (OA) increases seawater carbon dioxide (CO2) concentration and alters the relative proportion of dissolved inorganic carbon (DIC) species in seawater1. Seawater concentrations of CO2 and bicarbonate were projected to rise by 250% and 24%, respectively, up from current levels of 8 and 1650 μmol kg−1 seawater by the end of the century2. Seagrass productivity, thought to be limited by current seawater DIC composition, could benefit from the increased availability of carbon3. Studies have shown that photosynthetic rates of most seagrasses were enhanced by elevated partial pressure of CO2 (pCO2)45, which is the preferred DIC species36. Carbon fixed in the leaves through photosynthesis has a number of sinks and therefore, under increased pCO2, growth, respiration, storage, biomass and reproductive output may be increased45789.

The paradigm that OA benefits seagrass meadow productivity assumes that other environmental parameters, such as nutrient levels are not co-limiting productivity56. In terrestrial plants, nutrient availability can affect responses to elevated CO2; they initially respond by increasing productivity and growth but photosynthesis and growth are subsequently downregulated as nitrogen becomes limited10. Coastal seagrass systems can be subjected to fluctuations in water column nutrient levels11. While strong and sustained nutrient enrichment can stimulate the growth of competing macroalgae and epiphytes and in turn inhibit seagrass growth12, moderate increases in nutrients can promote seagrass growth, which demonstrates nutrient limitation1314.

Responses to elevated pCO2 are affected by nutrient availability because carbon and nitrogen metabolism are strongly coupled15. Nitrate and nitrogen metabolites regulate processes such as photosynthesis16, organic acid synthesis and starch accumulation17; leading some authors to speculate that moderate increases in dissolved inorganic nitrogen (DIN) may augment CO2 responses in tropical seagrasses918. In marine macroalgae, productivity responses to CO2 enrichment were enhanced under increased nutrient availability when compared to non-enriched nutrient conditions192021.

Energy stored in carbon metabolites plays a role in regulating the uptake and incorporation of nitrogen22. Therefore, the demand for nitrogen could be affected by the rate of carbon assimilation, and also by CO2 enrichment10. With increased nutrient availability, marine macroalgae in enriched pCO2 conditions increased photosynthetic efficiency1921, growth19 and nitrogen uptake and assimilation20, observations consistent with an increase in the demand for nitrogen driven by enhanced productivity.

Nitrogen incorporation involves both the uptake and assimilation of nitrogen species22. Both uptake and assimilation are inducible processes that may reflect instantaneous nitrogen demand in the plant22. For seagrasses, inorganic nitrate and ammonium are considered the most significant sources of nitrogen, supplying over 90% of externally acquired nitrogen2223. Sediment pore-water can potentially supply the majority of nitrogen for seagrass as the sediment contains higher concentrations of nitrogen than the water column does, but seagrasses will rapidly absorb DIN from the water column24. Furthermore, the uptake affinity (Km) of leaves is greater than that of rhizomes, meaning that a small increase in supply to the water column will trigger rapid uptake24. Nitrogen assimilation involves the enzymatic conversion of nitrate to nitrite by nitrate reductase (NR), and ammonium to glutamine through the glutamine synthetase (GS)/glutamate synthase pathway22. The activities of NR and GS, key in amino acids synthesis25, occur primarily in leaves and to a much smaller degree, in the rhizomes and roots26. Therefore, increasing external inorganic nitrogen may promote nitrogen uptake and assimilation in seagrasses2526.

The internal partitioning of fixed carbon to sink tissues and processes is affected by nitrogen availability918 and other environmental cues2728. The flux of fixed carbon in each tissue organ is controlled by key enzymes. For example, sucrose-phosphate synthase (SPS) in mature photosynthetic leaves primes the conversion of carbon into sucrose, which is subsequently transported to sinks15. The import of sucrose into sinks is controlled by sucrose synthase (SS). Under CO2 enrichment, reduced nitrogen availability could direct more carbon into below-ground biomass for storage, reducing nutrient imbalances in the leaves29.

The effects of nutrient enrichment on response to increasing pCO2 are likely to be greatest in regions where DIN is relatively low. We hypothesized that 1) pCO2 and nitrate enrichment can have additive effects on seagrass productivity and biomass and 2) pCO2 enrichment drives nitrogen demand. To test this, we increased DIN and pCO2 levels in seawater, to simulate DIN levels in flood plumes (average 2.20 μM across the Great Barrier Reef) and predicted end-of-century levels under RCP 2.6 and RCP 8.5 CO2 emission scenarios30. To allow wider inference we examined common species with different growth and storage strategies, the fast-growing species Halodule uninervis and the slow-growing species Thalassia hemprichii31. Both species contribute to the productivity and resilience of tropical seagrass meadows over different successional stages. Assessment of growth and productivity permitted us to test the first hypothesis, and measurement of nitrogen incorporation processes (uptake and assimilation) enabled testing of the second hypothesis.

Results

Experimental parameters

Water temperature (daily range 27.8–29.8 °C) and salinity (34.6–34.9) were similar between experimental tanks and throughout the experiment (Table 1). Carbonate system parameters of the enriched pCO2 treatments remained well within the target range of 428, 734 and 1213 μatm for the three treatments (Table 1). Average ammonium (0.59 μM; S.D. = 0.28 μM) and phosphate (0.05 μM; S.D. = 0.02 μM) concentrations were similar between treatments. Nitrate concentration was 0.29 ± 0.18 μM (S.D.) and 1.91 ± 0.33 μM (S.D.) in ambient and nutrient enriched treatments respectively.

Table 1

Experimental parameters.

pCO2 treatment	Nutrient	Measured parameters					Calculated parameters				Nutrient levels
		DIC (μmol kg−1 SW)	AT (μmol kg−1 SW)	pH(NIST)	Temperature(°C)	Salinity	pCO2 (μatm)	HCO3− (μmol kg−1 SW)	CO2 (μmol kg−1 SW)	CO32−(μmol kg−1 SW)	NH4+ (μΜ)	PO43− (μΜ)	NO3−(μΜ)
Control	−	1945.8 (9.59)	2234.8 (4.71)	8.01 (0.01)	28.47 (0.55)	34.74 (0.13)	435 (12.44)	1721.7 (13.66)	11.3 (0.42)	187.3 (2.45)	0.78 (0.37)	0.05 (0.02)	0.39 (0.25)
Control	+	1937.8 (9.49)	2233.8 (5.96)	8.02 (0.01)	28.53 (0.49)	34.74 (0.13)	422 (15.07)	1710.4 (14.13)	10.9 (0.43)	192.3 (2.02)	0.67 (0.32)	0.04 (0.02)	1.98 (0.34)
Intermediate	−	2045.8 (12.22)	2238.8 (5.25)	7.83 (0.04)	28.63 (0.76)	34.74 (0.13)	731 (78.43)	1875.5(21.95)	19.0 (2.09)	129.4 (13.06)	0.55 (0.18)	0.04 (0.02)	0.24 (0.13)
Intermediate	+	2047.4 (12.88)	2238.3 (4.76)	7.82 (0.05)	28.80 (0.72)	34.74 (0.13)	738 (88.59)	1877.4 (24.40)	19.1 (2.33)	131.0 (12.94)	0.60 (0.29)	0.04 (0.02)	1.80 (0.23)
High	−	2135.0 (18.95)	2240.4 (4.89)	7.63 (0.04)	28.70 (0.61)	34.74 (0.13)	1235 (129.49)	2001.1(22.17)	32.0 (3.15)	92.9 (5.91)	0.76 (0.55)	0.05 (0.02)	0.29 (0.16)
High	+	2130.6 (14.89)	2239.9 (5.20)	7.64 (0.04)	28.73 (0.58)	34.74 (0.13)	1190 (110.60)	1994.7 (18.18)	30.8 (2.67)	87.8 (4.85)	0.54 (0.18)	0.04 (0.02)	1.71 (0.68)

Values are given as mean ± S.D. Carbonate system parameters were calculated using measured values of total alkalinity (AT), total dissolved inorganic carbon (DIC), temperature and salinity on CO2calc software52.

Productivity and growth

In H. uninervis, net primary production increased with pCO2 levels (LME: P = 0.049) (Fig. 1; Table 2). The linear model predicted an increase of 1.071 mg O2 g−1 DW h−1 in net primary production for every 100 μatm rise in pCO2. There was no effect of nitrate enrichment on primary production (Table 2). Leaf respiration was not affected by pCO2 levels, but decreased by 34% with nitrate enrichment (LME: P = 0.025) (Fig. 1; Table 2). Rhizome respiration responses to pCO2 depended on nitrate enrichment (LME pCO2 × nitrate interaction: P = 0.009) (Fig. 1; Table 2). Under ambient DIN conditions, rhizome respiration increased with pCO2; under enriched DIN, rhizome respiration decreased with pCO2 (Fig. 1).

Figure 1

Net primary production and respiratory responses of (a–c) H. uninervis and (d–f) T. hemprichii measured after 22 days exposure to treatment.

Values are average ± S.E. N = 3.

Table 2

Linear mixed effects models for measured productivity response variables.

Parameter	Source	Halodule uninervis			Thalassia hemprichii
		df	F	p	df	F	p
Net primary production	pCO2	1	4.669	0.049	1	0.184	0.675
	Nitrate	1	0.091	0.767	1	2.745	0.120
	pCO2 × Nitrate	1	0.721	0.410	1	3.648	0.077
Shoot respiration	pCO2	1	3.785	0.072	1	0.849	0.373
	Nitrate	1	5.199	0.039	1	0.226	0.642
	pCO2 × Nitrate	1	1.861	0.194	1	1.756	0.206
Rhizome-root respiration	pCO2	1	1.818	0.199	1	3.082	0.101
	Nitrate	1	8.593	0.011	1	0.584	0.458
	pCO2 × Nitrate	1	9.037	0.009	1	1.607	0.226
Growth rate (10 days)	pCO2	1	10.430	0.006	1	6.376	0.024
	Nitrate	1	3.418	0.086	1	1.427	0.252
	pCO2 × Nitrate	1	0.003	0.961	1	1.575	0.230
Growth rate (24 days)	pCO2	1	19.218	0.001	1	0.068	0.799
	Nitrate	1	1.014	0.331	1	0.870	0.367
	pCO2 × Nitrate	1	1.544	0.234	1	0.077	0.786
Sucrose phosphate synthase	pCO2	1	1.556	0.233	1	0.534	0.477
	Nitrate	1	5.109	0.040	1	0.436	0.520
	pCO2 × Nitrate	1	0.275	0.608	1	0.062	0.806
Sucrose synthase	pCO2	1	0.002	0.967	1	3.619	0.078
	Nitrate	1	1.677	0.216	1	0.389	0.543
	pCO2 × Nitrate	1	3.291	0.091	1	0.251	0.624
Total non-structural carbohydrates	pCO2	1	0.003	0.959	1	0.548	0.471
	Nitrate	1	0.053	0.821	1	0.994	0.336
	pCO2 × Nitrate	1	0.152	0.702	1	4.66 × 10−4	1.000

Variables were analysed with pCO2 as a continuous predictor and nitrate as a categorical factor. Individual aquarium tanks were included as replicates (N = 3), with two sub-replicate pots nested within aquaria. For net primary production, shoot and rhizome-root respiration, linear models were used for analysis, with aquaria as replicates (N = 3) and without nested sub-replicate pots. P-values < 0.05 are in bold.

Growth rates of H. uninervis shoots increased with pCO2 enrichment after 10 days (LME: P = 0.006) (Fig. 2; Table 2). At day 10, growth rates increased from 3.3 mm shoot−1 day−1 in control pCO2 aquaria (428 μatm) to 4.2 mm shoot−1 day−1 in high pCO2 aquaria (1213 μatm). The enhancement of growth rates with pCO2 was sustained after 24 days (LME: P = 0.001) as growth rates in control pCO2 aquaria were 4.1 mm shoot−1 day−1, while those in high pCO2 aquaria were elevated by 52% (6.2 mm shoot−1 day−1). There was no significant effect of nitrate enrichment on growth (Fig. 2; Table 2). Shoot growth of H. uninervis in the source meadow at day 13–17 of the experiment was in a similar range (7.0 mm shoot−1 day−1; S.E. = 1.24 mm shoot−1 day−1).

Figure 2

Growth rates of (a,c) H. uninervis and (b,d) T. hemprichii after 10 and 24 days exposure to treatments.

Values are average ± S.E. N = 3.

Net primary production in T. hemprichii did not increase with pCO2 or nitrate enrichment (Fig. 1; Table 2). In addition, no significant changes in leaf and rhizome respiration with pCO2 and nitrate enrichment were detected.

In T. hemprichii, at day 10, leaf growth rates responded to pCO2 enrichment and no effect of nitrate enrichment was detected (LME: pCO2 - P = 0.024; nitrate - P = 0.252) (Fig. 2; Table 2). Growth rates increased by 28% with pCO2 enrichment. By day 24, no change in growth rate to pCO2 or nitrate was detected (Fig. 2; Table 2). Overall, growth of T. hemprichii in the experimental aquaria (global average = 2.98 mm shoot−1 day−1; S.E. = 0.12 mm shoot−1 day−1) was lower than that measured in the source meadow (5.95 mm shoot−1 day−1; S.E. = 0.57 mm shoot−1 day−1).

Carbohydrates translocation and storage

For both H. uninervis and T. hemprichii, pCO2 manipulation did not affect sucrose-phosphate synthase (SPS) and sucrose synthase (SS) activity indicative of carbohydrate translocation (Table 2). Nutrient enrichment reduced SPS activity in H. uninervis leaves (LME: P = 0.040) (Table 2), but overall the effects were of limited consequence for our hypotheses (see Supplementary Fig. 1). Non-structural carbohydrates in H. uninervis and T. hemprichii rhizomes showed no change to pCO2 and nitrate enrichment (Table 2).

Nitrogen uptake and assimilation

In H. uninervis, leaf uptake of nitrate, determined by 15N incorporation, did not vary with pCO2 or nitrate enrichment in H. uninervis (10.91 μmol N g−1 DW h−1; S.E. = 1.35 μmol N g−1 DW h−1) (Fig. 3; Table 3). No significant changes in nitrogen assimilation (enzymatic activity) in H. uninervis with pCO2 and nitrate enrichment were detected (Table 3). Furthermore, there were no changes in leaf tissue nutrient content (means ± S.E.: C–41 ± 0.2%; N–2.5 ± 0.03%; C:N–16.3 ± 0.2) (Table 2).

Figure 3

Nitrate incorporation (uptake and assimilation) in leaves of (a,b) H. uninervis and (c,d) T. hemprichii across a range of pCO2 concentrations.

Values are average ± S.E. N = 3.

Table 3

Linear mixed effect models for all nitrogen uptake and metabolism variables.

Parameter	Source	Halodule uninervis			Thalassia hemprichii
		df	F	p	df	F	p
Nitrate uptake	pCO2	1	0.157	0.698	1	1.014	0.331
	Nitrate	1	0.000	0.984	1	0.156	0.698
	pCO2 ×Nitrate	1	0.380	0.548	1	7.392	0.017
Nitrate reductase	pCO2	1	3.076	0.101	1	1.144	0.303
	Nitrate	1	2.523	0.135	1	8.092	0.013
	pCO2 ×Nitrate	1	0.526	0.480	1	4.061	0.064
Glutamine synthetase	pCO2	1	0.142	0.712	1	0.089	0.769
	Nitrate	1	0.376	0.550	1	0.289	0.600
	pCO2 ×Nitrate	1	3.443	0.085	1	0.466	0.506
Carbon content	pCO2	1	1.420	0.253	1	0.928	0.352
	Nitrate	1	1.390	0.259	1	0.094	0.764
	pCO2 ×Nitrate	1	0.330	0.576	1	0.094	0.764
Nitrogen content	pCO2	1	0.310	0.584	1	2.180	0.162
	Nitrate	1	1.350	0.266	1	4.363	0.056
	pCO2 ×Nitrate	1	0.510	0.487	1	3.113	0.099
C:N ratio	pCO2	1	1.571	0.231	1	1.436	0.251
	Nitrate	1	3.227	0.094	1	4.846	0.045
	pCO2 ×Nitrate	1	0.282	0.604	1	3.258	0.093

Variables were analysed with pCO2 as a continuous predictor and nitrate as a categorical factor. Individual aquaria were included as replicates (N = 3), with two sub-replicate pots nested within aquaria. P-values < 0.05 are in bold.

In T. hemprichii, nitrate uptake was increased with pCO2, but only in the nitrate enriched treatment (LME pCO2 × nitrate interaction: P = 0.017) (Fig. 3; Table 3). Nitrate uptake rates increased by 117% at the highest pCO2 relative to ambient levels. In T. hemprichii leaves, NR activity was higher with nitrate enrichment (linear model P = 0.019) but was not affected by pCO2 levels (Table 3). NR activity in T. hemprichii leaves in ambient seawater (0.61 μmol NO2 g−1 FW h−1; S.E. = 0.14 μmol NO2 g−1 FW h−1) was ~50% that in enriched nitrate conditions (1.12 μmol NO2 g−1 FW h−1; S.E. = 0.25 μmol NO2 g−1 FW h−1). GS activity in T. hemprichii leaves did not change significantly with pCO2 or nitrate (69.50 μmol g−1 FW h−1; S.E. = 5.06 μmol g−1 FW h−1) (Table 3). There were no significant changes in leaf carbon content (39 ± 0.3%), but there were marginal increase in leaf nitrogen (LME: P = 0.056) and significant reduction in C:N ratio (LME: P = 0.045) with nitrate enrichment in T. hemprichii (Table 3). Under ambient nitrate levels, nitrogen content and C:N were 2.7 ± 0.06% and 14.7 ± 0.27 respectively; with nitrate enrichment, nitrogen content was 2.8 ± 0.08% and C:N was 14.0 ± 0.41 (means ± S.E.).

Discussion

This study aimed to test whether seagrass productivity is affected by pCO2 and nitrate () enrichment, and whether pCO2 drives the demand for nitrogen in seagrasses. In H. uninervis, net primary production (NPP) and growth rates increased with higher pCO2 but were not affected by nitrate enrichment. However, in T. hemprichii, NPP and growth were not affected by either pCO2 or nitrate enrichment. In H. uninervis, pCO2 enrichment did not increase nitrate uptake or assimilation while nitrate uptake was higher in CO2-enriched (simulating end of century RCP 8.5 emission scenario)30 T. hemprichii. In addition, nitrate enrichment (1.9 μM compared to 0.3 μM in ambient) raised leaf nitrate reductase (NR) activity in T. hemprichii. Therefore, productivity responses to pCO2 and nitrate enrichment varied between species with different growth strategies.

H. uninervis and T. hemprichii differed in productivity responses to pCO2 enrichment after 24 days exposure. In H. uninervis, NPP increased by 1.1 units for every 100 μatm rise in pCO2, an increase slightly higher than the 0.9 units measured in the same species by Ow et al.4. Other fast-growing seagrass species that have increased photosynthetic rates with pCO2 enrichment include Z. marina (250% increase at pH 6.2, relative to 338 μatm pCO2)5 and Z. noltii (34% increase at pH 7.9, relative to 360 μatm pCO2)32. Leaf growth rates in H. uninervis were also enhanced in pCO2 enriched treatments, with the highest leaf growth rates [6.2 ± 0.40 (S.E.) mm shoot−1 day−1] being slightly lower than that measured in the field [7.0 ± 1.24 (S.E.) mm shoot−1 day−1]. Aquaria experiments may impose potential artefacts on leaf growth due to transplantation stress, which were minimised by allowing for acclimation prior to experiments. However, as described below, light levels within experimental tanks, which were lower than that of nearby shallow reef systems, most likely explained the lower growth rates in aquaria.

In T. hemprichii, pCO2 enrichment had no effect on NPP and growth rates after three weeks, in contrast to previous work on this species47. Jiang et al.7 studied T. hemprichii from a nutrient-enriched meadow (0.8–4.6 μM NO3− + NO2−)33 and exposed to much higher CO2 concentrations (25–1005 μM) compared to the present study (19–31 μM). T. hemprichii grown under high nitrogen might have utilised its pre-existing nutrients store22 to supplement a rapid growth increase during strong CO2 enrichment7. In the present study, T. hemprichii productivity did not appear to be nitrogen-limited (discussed below), indicating that light levels in experimental tanks, or phosphate availability in carbonate sediments34 could have limited its growth response. Interestingly, leaf growth of T. hemprichii showed a transient rise with pCO2 at day 10, but subsequently stabilised. This growth response to initial (short-term) pCO2 exposure has been reported for T. hemprichii after 14 days of exposure4. However, NPP measured at the end of the experiment (22 days) suggest a downregulation in response to pCO2 over time.

Nitrate addition did not increase NPP in H. uninervis. This was despite respiration rates of the rhizome-root complex in enriched pCO2 being lowered with nitrate enrichment. Given the relatively large proportion of below-ground biomass for this species35, a reduction in rhizome-root respiration could be substantial for improving carbon use36. In the present study, lower sucrose phosphate synthase (SPS) activity1527 in H. uninervis exposed to nitrate enrichment suggested a decline in the export of fixed carbon from leaves, potentially due to reduced metabolic demand in the rhizome-root biomass8. Further quantification of nitrate uptake rates and of the activities of the key enzymes in the nitrogen assimilation pathway, nitrate reductase and glutamine synthetase2225, revealed no effect of nitrate enrichment on nitrogen incorporation in H. uninervis.

Productivity in T. hemprichii did not increase with nitrate enrichment, even though nitrate enrichment increased nitrate uptake at high pCO2 and assimilation in the leaves of T. hemprichii. Increased nitrate uptake and assimilation under water-column nitrate enrichment could be advantageous for seagrasses acclimatised to growing in a low-nitrogen environment15. This allows the plant to sequester and store nitrogen rapidly when it becomes available. Higher nitrogen content and a lowered C:N ratio were observed in nitrate-enriched T. hemprichii leaves. Therefore, nitrate enrichment appeared to have a greater influence on nitrogen incorporation in T. hemprichii than H. uninervis.

Overall there was no evidence in the present study that nitrate enrichment enhanced productivity responses to pCO2 for either species. This was surprising as nitrogen had been suggested32 and shown to limit the productivity of marine macrophytes to pCO2 enrichment21 in subtidal rocky habitats. The experiment duration might not have been long enough for pCO2 enrichment to induce a significant change in nitrogen demand (24 days vs 5 months32), which may still be covered by pre-existing nitrogen-resources. Previous work reported increases in leaf tissue carbon-to-nitrogen (C:N) ratios in CO2 enriched seagrasses737, which suggested nitrogen limitation in these plants. However, C:N ratios in both H. uninervis and T. hemprichii here revealed no evidence that pCO2 enrichment led to the seagrasses requiring more nitrogen. In the Great Barrier Reef (GBR) region, seagrass growth was limited by nitrogen at some sites1338. In the present study, leaf nitrogen content and C:N ratios of H. uninervis (N = 2.53%; C:N = 16.3) and T. hemprichii (N = 2.75%; C:N = 14.4) were similar to previous values measured in GBR seagrasses39. These were well above the values assumed to indicate nitrogen limitation (N = 1.8%; C:N = 20)3940 and suggest that the two species were not nitrogen limited. DIN levels in sediment pore-water and that adsorbed to sediments were not quantified here, but typical concentrations can be 200 times higher than in the water column24. Thus sediment pore-water may have supplied sufficient DIN to maintain productivity rates measured here. Another possible explanation for apparent nutrient sufficiency (C:N < 20)41 is that light levels during the experiment, averaging 9 mol m−2 d−1, were low compared to longer-term monitoring from shallow seagrass meadows in far north Queensland which typically reach 15–20 mol m−2 d−1. Furthermore light levels dropped in the region of the study site (Cape York) in early 201439. Lowered levels of natural light, relative to the typical levels available39, may also explain the limited productivity responses to pCO2.

Carbon dioxide enrichment did not drive nitrogen demand in H. uninervis and T. hemprichii. In other marine macrophytes, CO2 enrichment was shown to increase nitrate reductase activity3242. Here, increased CO2 availability did not affect nitrate uptake and assimilation (measured as nitrate reductase and glutamine synthetase activity) in H. uninervis, whereas the effect was dependent on nitrate enrichment in T. hemprichii. This is interesting as water column DIN concentrations at northern mid-shelf GBR (e.g. Lizard Island) are typically lower than that at inshore reefs43, where the majority of seagrass grows44. Perhaps experiments on longer time-scales are needed to evaluate the effects of nitrogen availability on productivity, as seagrasses possess mechanisms to improve nitrogen-use efficiency, likely through recycling or re-allocation of nitrogen within the plant24. At natural CO2 seeps with elevated pCO2, no difference in tissue nutrients were found between seagrasses growing around, and away from the CO2 seeps, suggesting CO2-induced nitrogen limitation was not present45. Continual flux in nutrients in coastal habitats, supplemented by nitrogen fixation in the sediments46, may enable seagrasses to be more productive without facing nitrogen limitation with future OA.

In conclusion, the tropical seagrasses, H. uninervis and T. hemprichii, did not appear to be strongly nitrogen limited despite being collected from a mid-shelf reef where ambient water column nitrogen concentrations were low (0.13 μmol DIN). Consequently, nitrate fertilization of the water column did have some effect on nitrate uptake rates, but did not enhance seagrass productivity or leaf growth rates. Furthermore, in contrast to our initial hypothesis, responses to pCO2 enrichment, simulating future ocean acidification scenarios, were also unaffected by nitrate fertilisation. To better reconcile the effects of nutrient enrichment on seagrass CO2 responses with previous studies, there is the need to account for differences in background light, nutrient levels and durations between experiments. This helps to circumvent the current experimental limitations in expanding our findings to a wider environment. Ocean acidification can also promote the growth of epiphytic filamentous algae, outweighing the influence of nutrient addition on seagrass epiphytes47. Nutrient enrichment could encourage a shift in the dominance of submerged vegetation, from seagrasses to fast-growing macroalgae and phytoplankton, such as that observed in habitats exposed to eutrophication48. Hence, while seagrass meadows may potentially flourish in a future where the oceans are enriched in CO2, ecological effects of ocean acidification and nutrient fertilisation, such as competition from macroalgae and epiphytes, may outweigh gains to seagrass productivity.

Methods

Plant collection and experimental setup

The experiment was carried out at Lizard Island, GBR, Australia, in March 2014. Halodule uninervis was collected from an intertidal meadow and Thalassia hemprichii from the subtidal zone (2–3 m depth) of One Tree Coconut beach (14° 41.370’S, 145° 27.392’E) following protocols described in Ow et al.4. Seagrasses were potted up within 48 h of collection in the same sediments from their source meadows (H. uninervis in 20:80 carbonate sand:site mud mixture, T. hemprichii in carbonate sand). Potted seagrasses were stored in outdoor flow-through aquaria (50 L) for three to six days prior to the initiation of the experiment. Experimental treatments consisted of three pCO2 levels (ambient ~428 μatm, moderate ~734 μatm and high ~1213 μatm pCO2) and two nitrate treatments (ambient ~0.3 μM and enriched ~1.9 μM) crossed in a fully factorial design. Each treatment comprised of three replicate 25 L aquaria leading to a total of eighteen aquaria, supplied with seawater at 24 L h−1 directly from the adjacent lagoon. Two sub-replicate pots of each species were placed in each aquarium. The aquaria were situated outdoors under a solid translucent roof, which attenuated 50% of down-welling light. 2π light loggers (Odyssey, New Zealand) were randomly allocated to aquaria to record photosynthetically active radiation (PAR). Over the course of the experiment, the net daily PAR in aquaria ranged from 1.2–5.2 mol m−2 d−1, averaging 3.8 mol m−2 d−1. Mid-day maximum PAR averaged to 480 μmol m−2 s−1. Treatments were randomised between the aquaria to eliminate any potential environmental effects within the set-up area. The experiment ran for 24 days before it had to be terminated due to an approaching cyclone.

pCO2 concentrations were manipulated by injecting different amounts of CO2 gas into sump tanks. pH levels in the sump tanks were monitored with six potentiometric sensors (±0.01 pH unit) calibrated on the NIST (National Institute of Standards and Technology) scale as a proxy to control for CO2 input. The sensors provide feedback to a control system that regulates pH levels via CO2 gas injection (AquaMedic, Germany)4. We recognise that over natural seagrass meadows, seawater pH fluctuates and does not have a set point. However, such fluctuations are hard to emulate while controlling for pCO2 concentrations with our current set-up. Hence, pCO2 concentrations were controlled using fixed pH levels instead. Seawater pCO2 concentrations in mid-shelf reefs, such as Lizard Island in the GBR averaged about 380 μatm (1 S.D. = 15 μatm)49 during the dry season from 2011–2013. During the wet season, when the present experiment was conducted, pCO2 concentrations tend to be higher (460 μatm; 1 S.D. = 33 μatm) than during the dry season49.

Across the Great Barrier Reef (GBR), DIN (nitrate, ammonium and nitrite) levels in the water column over seagrass meadows are relatively low, averaging 0.13 μM11. However, terrestrial run-off into coastal areas can deliver DIN loads that are an order of magnitude or more higher (1.54 to 7.02 μM, or 2.20 μM averaged across the GBR)50. Nitrate enrichment was achieved by dripping sodium nitrate solution (Sigma-Aldrich, Australia) into individual aquaria. Peristaltic pumps (Cole Palmer, USA) delivered 2 mM of NaNO3 solution into the individual aquaria at a rate of 0.5 ml min−1. Small aquaria pumps (Hailea, China) in each aquarium provided mixing.

Seawater chemistry

pHtotal in treatment tanks were monitored by spectrometric determination of m-cresol absorbance51, and additionally checked against TRIS seawater standard (A. G. Dickson, Scripps Institute of Oceanography, Batch 106). Weekly water samples were analysed for total alkalinity (AT) by gran titration with 0.5 M HCl on a Metrohm 855 titrosampler (Metrohm, Switzerland), and for total dissolved inorganic carbon (DIC) by acid titration on a VINDTA 3C. Carbonate system parameters were calculated using measured values of AT, DIC, temperature and salinity on CO2calc software52. Duplicate water samples for dissolved inorganic nutrient analysis were filtered through 0.45 μm cellulose acetate filters and stored at −20 °C before determination of seawater ammonium, nitrate, and phosphate concentrations according to standard procedures outlined in Ryle et al.53. Temperature in the treatment tanks was logged by HOBO tidbit loggers (Onset, USA) every 5 min. Salinity readings were taken from an IMOS weather buoy (Integrated Marine Observing System; www.aims.gov.au) situated in the lagoon.

Productivity

After 22 days, photosynthetic and respiration rates were measured using the second youngest leaf of a shoot from each sub-replicate pot using optical oxygen sensors (“optodes”, PreSens, Germany) and a fiber-optic oxygen meter (PreSens Oxy 4, Germany). Respiration rates of below-ground rhizome with associated roots (~2.5 cm) from each pot were quantified similarly. Measurements were conducted in 70 mL chambers at constant 28 °C water temperature following procedures described in Ow et al.4. Respiration of the leaves and below-ground rhizome-roots were measured separately over a 20-min period in the dark while photosynthetic rates were measured on the same leaf at 400 μmol m−2 s−1 PAR over 30 min. Plant material was dried (60 °C for 48 h) and weighed after incubation. Photosynthetic and respiration rates were normalised to the dry weight of the leaf and rhizome. Optodes were calibrated according to protocol described in Collier et al.54.

Growth rates were measured according to the method described in Short and Duarte55. At day 0 and day 14 of the experiment, all shoots were marked at the top of the bundle sheath with a needle. Length of new tissue growth was measured with vernier callipers regularly throughout the experiment, totalled and normalised to the number of shoots and days since marking. Growth rates of plants, from three separate plots in each source meadow, were also obtained using the same method from day 13 to day 17.

Nitrogen uptake

Leaf nitrate uptake rates were estimated at the end of the experiment. Seagrass shoots were incubated in seawater enriched with 15N labelled potassium nitrate (atom% = 98; Novachem, Australia), and the final 15N in the leaf tissue was used to calculate the uptake of 15NO3−. Incubations were carried out on individual shoots in their pots, in their respective treatment tanks, via a method similar to that described in Prado et al.56. Individual shoots were enclosed within a plastic bag (~250 mL volume) fitted with a filter cassette and a plug that could be sealed. No leakage was detected when tested using a food dye. Potassium nitrate solution was injected into the chambers to achieve around 20% 15NO3− enrichment of the initial ambient DIN concentration57. The shoots were incubated for one hour at ambient mid-day temperature (28 °C) and light (450 μmol m−2 s−1). After one hour, the shoots were excised from the rhizomes and rinsed with deionized water to remove excess adherent label. Non-incubated leaf samples were collected from each tank to provide background leaf 15N levels for each species. Leaf material was processed and measured for total nitrogen content and atom% 15N according to method described in Takahashi et al.45. Uptake rates (μmol N g−1 dry weight h−1) of 15NO3− were calculated following equations outlined in Nayar et al.23. The atom% 15N of 15N enriched seawater was calculated based on the amount of atom% added and background DIN concentrations (assumed to reflect 15N concentration of atmospheric N ~ 0.37 atom% 15N).

Nitrogen assimilation and carbon translocation

Plant material used for measuring nitrogen assimilation and carbon translocation (i.e. enzyme analyses), except for nitrate reductase (NR), were collected at the end of the experiment and stored in liquid nitrogen until analysis.

NR activity in fresh shoot tissue was determined using the in vivo assay described for Zostera marina58. The in vivo technique was shown to yield consistently higher activity than the in vitro assay, which often gave negligible readings15. Extraction and assay for glutamine synthetase (GS) activity in new and fully extended leaf tissue was carried out following the method developed for Z. marina25, except that the incubation was carried out nearer to the aquaria temperature (30 °C).

To study carbon translocation, sucrose-phosphate synthase (SPS) from young but fully extended shoot tissue and sucrose synthase (SS) from the root–rhizome complex were extracted using a technique described in Brun et al.27 and assayed according to the protocol outlined in Zimmerman et al.28. The sucrose produced was quantified colorimetrically using anthrone assay59.

Shoot and rhizome-root biochemistry

Shoot tissue nutrients (carbon and nitrogen) of ashed samples were analysed using an elemental analyser (Elementar Vario EL, Germany) interfaced to an isotope-ratio-mass-spectrometer (PDZ Europa 20–20, Sercon Ltd; Cheshire UK), as described in Takahashi et al.45. To study carbon storage, ground rhizome-roots samples were analysed for non-structural carbohydrates content according to procedure described in Collier et al.35. The summed amount of soluble carbohydrates and starch gave total non-structural carbohydrates (TNSC) content, expressed as milligrams dry weight−1 of tissue.

Statistical analysis

Parameters were analysed using linear mixed effects models with pCO2 as a continuous predictor, and nitrate (ambient and enriched) as a categorical factor. Individual tanks were included as replicates, with sub-replicate pots nested within tanks. The nested factor was omitted for parameters without sub-replicate measurements (T. hemprichii: net primary production, respiration; both species: 15NO3− uptake). For these parameters, measurements were terminated prematurely due to an unforeseen evacuation of the research station caused by a cyclone, and therefore the second sub-replicate could not be measured. Assumptions of normality and homogeneity of variances were tested with Shapiro-Wilks’ and Bartlett’s tests, respectively. Percentage data (%C and %N) were arcsine square-root transformed to meet the assumptions60. All statistical tests were assessed at α = 0.05 and analysed using R statistical software (R Development Core Team).

Additional Information

How to cite this article: Ow, Y. X. et al. Nitrate fertilisation does not enhance CO2 responses in two tropical seagrass species. Sci. Rep. 6, 23093; doi: 10.1038/srep23093 (2016).