Limited coupling of macrophyte production and bacterial carbon cycling in the sediments of Zostera spp. meadows

Abstract

Two approaches were used to study coupling between seagrass production and bacterial processes in the rhizosphere. In the first approach, stable carbon-isotope ratios of bacteria, sediment organic matter and plants were determined to infer sources of organic carbon used by bacteria in the sediments of 4 European Zostera marina and 2 Z. noltii meadows. Bacterial isotope ratios were derived from bacteria-specific polar lipid-derived fatty acids (PLFA), mainly methyl-branched i15:0 and a15:0. Bacterial δ 13 C ratios in the sediment from both vegetated and unvegetated sites were mostly similar and did not resemble Z. marina ratios, suggesting that seagrass material was of limited importance as a bacterial carbon source. Bacterial ratios were in most cases similar to benthic macroalgae and did correlate well with ratios of diatom biomarkers. Sediment organic matter inside the meadows had δ 13 C ratios similar to those of nearby unvegetated sites, and ratios were clearly different from the material produced by seagrasses, indicating that little seagrass material accumulated. Results from the 2 Z. noltii sites were less conclusive, as there was no clear difference in δ 13 C ratios between the potential source materials. In addition, bacterial δ 13 C ratios were highly variable at one Z. noltii site. In the second approach, cultured Z. marina was labeled with 13 C-bicarbonate to study the short-term transfer of label from plants to bacteria in the rhizosphere. However, no label was detected in bacterial PLFA after 20 h of incubation. In conclusion, a close coupling between macrophyte production and bacterial carbon cycling could not be detected in the sediment of Z. marina meadows, and benthic production by algae was probably the main carbon source for bacterial growth.KEY WORDS: Seagrass meadows · Bacterial carbon sources · Sediment · Stable carbon isotopes · Biomarkers · Zostera marina · Zostera noltii Resale or republication not permitted without written consent of the publisher

Nutrient dynamics of seagrass ecosystems: 15N evidence for the importance of particulate organic matter and root systems

“…The rapid appearance of 15 N in the seagrass plants following the addition of the labelled material suggests that bacterial mineralisation dominates over bacterial immobilisation in our system. While in temperate zones or sparse vegetations bacterial growth may not be linked directly to seagrass production (Boschker et al 2000), it seems to be the opposite in tropical zones or dense vegetations where bacterial growth can be enhanced in seagrass sediments (Holmer et al 1999, Jones et al 2003. For instance, Blaabjerg et al (1998) reported diurnal cycles in sulphate reduction in sediments with dense cover of Zostera marina.…”

Carbon sources supporting benthic mineralization in mangrove and adjacent seagrass sediments (Gazi Bay, Kenya)

“…(iii) Lastly, it has been suggested that various seagrass species may exhibit a different degree of root exudation, with some studies indicating high release rates of organic carbon from roots and others finding no evidence for root exudation (see Holmer et al, 2001 for a discussion). 13 C-tracer experiments to directly follow the transfer of photosynthetically fixed seagrass C to sedimentary bacteria with different seagrass species (analogous to those presented in Boschker et al, 2000) could provide interesting data to verify the extent to which this influences the degree of relience on seagrass C.…”

“…As an example, however, a 1‰ shift towards more enriched δ 13 C values for both end-members would raise the estimated contribution of mangrove C by approximately 7%. However, the main purpose of our simplified 2 end-member approach is to show that (i) a significant part of the seagrass sediment TOC pool is likely to have been imported to the seagrass beds, and (ii) that the relative importance of these allochtonous C sources to mineralization is largely proportional to its availability in the TOC pool A comparison of the PLFA isotope data gathered in this study and available literature data (Boschker et al, 2000;Holmer et al, 2001;Jones et al, 2003) on seagrass systems is presented in Figs. 6A and 6B.…”

“…In some cases, production by local vascular plants appears to be the dominant source for benthic mineralization, but carbon imported from the aquatic environment and local production by microphytobenthos can overtake this role in other sites (Boschker et al, 1999;Bouillon et al, 2004). For seagrass beds, several studies using phospholipid fatty acids (PLFA) stable isotope techniques have suggested either that seagrass-derived carbon was of limited importance to local bacterial communities (Boschker et al, 2000 for Zostera marina and Z. noltii), or that seagrasses were a major contributor to carbon assimilation by bacterial communities (Holmer et al, 2001;Jones et al, 2003 for Thalassia hemprichii and Cymodocea rotundata and for T. testudinum, respectively). A recent study by Holmer et Fig.…”

Export from Seagrass Meadows Contributes to Marine Carbon Sequestration

“…These were largely derived from research focused on the ecology of deep-sea fauna (e.g., Wolff, 1979) and carbon sources to marine food webs (Baeta et al, 2009), and also included research motivated by the study of organic matter fluxes to the deep sea (e.g., Wiebe et al, 1976), surveys of marine debris (Wei et al, 2012), assessments of kelp contributions to deep-sea sediments (e.g., Harrold et al, 1998), and assessments of the sources of sediment organic matter (e.g., Boysen-Jensen, 1914;Boschker et al, 2000), among other research topics. Taken in concert, however, these reports produce strong and compelling evidence that seagrass meadows contribute to sedimentary carbon stocks in bare sediments extending from shallow littoral areas to the hadal regions of the ocean.…”