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Seagrasses and their epiphytes: Characterization of abundance and productivity in tropical seagrass beds
Stockholm University, Faculty of Science, Department of Botany.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seagrass beds cover large intertidal and subtidal areas in coastal zones around the world and they are subjected to a wide variety of anthropogenic influences, such as nutrient enrichment due to sewage seepage. This study was undertaken to address specific questions focusing on whether near shore tropical seagrasses that receive a constant influx of groundwater nutrient inputs, would exhibit a higher productivity and to what extent epiphytic algae reflect the impacts of nutrient inputs. An additional aspect of study was to determine the prevalence of “acid zones” in tropical seagrasses. The productivity of the seagrasses Cymodocea rotundata, Thalassia hemprichii and Thalassodendron ciliatum was compared in two sites along the Kenyan coast; Nyali (a high nutrient site) and Vipingo (a low nutrient site). Of the three seagrasses T. hemprichii showed the most distinct differences with higher growth and biomass in the nutrient rich site whereas the growth of C. rotundata was similar in the two sites. A high epiphytic cover was found on the shoots of T. ciliatum found in the high nutrient site Nyali.

Morphological and genetic characterization of bacterial and cyanobacterial epiphytes showed specific associations of nitrogen fixing cyanobacteria on the seagrass C. rotundata in the low nutrient site (Vipingo). At this site, shoots of C. rotundata had a higher C:N ratio compared to shoots in the high nutrient site (Nyali) indicating that the association with nitrogen fixing cyanobacteria is a strategy, for this species, to meet its nutrient needs. Bacterial epiphytes belonging to the group Cytophaga-Flavobacteria-Bacteroides (CFB) were found on T. ciliatum and T. hemprichii from the two sites. CFB bacteria are characteristic of waste water, particularly from livestock farming areas, thereby confirming seepage of groundwater from surrounding catchment areas. These prokaryotic associations were specific for the different seagrasses and it appears that the establishment of epiphytic associations may not be a random encounter but a specific association that meets specific needs.

The seagrass T. ciliatum in the high nutrient site had an abundance of macroalgal epiphytes and the impact of the epiphytic coverage was assessed using Pulse Amplitude Modulated (PAM) fluorometry. The photosynthetic activity of seagrass parts that were covered by epiphytes was suppressed but the productivity of the whole shoot was not significantly reduced. In the nutrient rich site, epiphytes were found to contribute up to 45% of the total estimated gross productivity, during the SE monsoon season, while epiphytic contribution in the nutrient poor site, was 8%. Epiphytic abundance and contribution to productivity decreased during the NE monsoon. The photosynthetic activity of T. ciliatum shoots was similar in the two study sites with shoots in the nutrient rich site growing faster. T. ciliatum, in the low nutrient site, invested in the development of below ground root tissue which may indicate the development of a strategy to gain access to pore water nutrient pools.

Carbon uptake strategies of eight tropical seagrasses were re-evaluated to determine how common the “acid zone” mechanism is among tropical seagrasses. Six of the eight species studied showed photosynthetic inorganic carbon (Ci) acquisition based on carbonic anhydrase catalysed HCO3- to CO2 conversions within an acidified diffusion boundary layer (“acid zone”). Cymodocea serrulata appeared to maintain its carbon uptake by extracellular carbonic anhydrase catalysed CO2 formation from HCO3- without the need for acidic zones, whereas, Halophila ovalis appeared to have a system in which H+ extrusion may be followed by HCO3--H+ co-transport into the cells. These findings indicate that competition for carbon, between the host seagrass species and epiphytes, could determine seagrass-epiphyte associations.

Place, publisher, year, edition, pages
Stockholm: Botaniska institutionen , 2005. , 32 p.
Keyword [en]
Seagrass, epiphytes, cyanobacteria, prokaryotes, diversity, productivity, carbon utilization, buffer sensitivity
National Category
Biological Sciences
Identifiers
URN: urn:nbn:se:su:diva-527ISBN: 91-7155-036-4 (print)OAI: oai:DiVA.org:su-527DiVA: diva2:195050
Public defence
2005-06-02, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 13:00
Opponent
Supervisors
Available from: 2005-05-10 Created: 2005-05-10Bibliographically approved
List of papers
1. Productivity aspects of three Kenyan seagrass species in areas of different nutrient levels in Kenya
Open this publication in new window or tab >>Productivity aspects of three Kenyan seagrass species in areas of different nutrient levels in Kenya
2005 In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, Vol. 63, no 3, 407 - 420 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23868 (URN)
Note
Part of urn:nbn:se:su:diva-527Available from: 2005-05-10 Created: 2005-05-10Bibliographically approved
2. Characterization and comparison of prokaryotic epiphytes associated with seagrasses
Open this publication in new window or tab >>Characterization and comparison of prokaryotic epiphytes associated with seagrasses
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23869 (URN)
Note
Part of urn:nbn:se:su:diva-527Available from: 2005-05-10 Created: 2005-05-10 Last updated: 2010-01-13Bibliographically approved
3. Does the abundance of epiphytes affect the productivity of the tropical seagrass Thalassodendron ciliatum?
Open this publication in new window or tab >>Does the abundance of epiphytes affect the productivity of the tropical seagrass Thalassodendron ciliatum?
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23870 (URN)
Note
Part of urn:nbn:se:su:diva-527Available from: 2005-05-10 Created: 2005-05-10 Last updated: 2010-01-13Bibliographically approved
4. Growth and photosynthetic productivity of the tropical seagrass Thalassodendron ciliatum and its associated macroalgal epiphytes
Open this publication in new window or tab >>Growth and photosynthetic productivity of the tropical seagrass Thalassodendron ciliatum and its associated macroalgal epiphytes
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23871 (URN)
Note
Part of urn:nbn:se:su:diva-527Available from: 2005-05-10 Created: 2005-05-10 Last updated: 2010-01-13Bibliographically approved
5. Buffer sensitivity of photosynthetic carbon utilization in eight tropical seagrasses
Open this publication in new window or tab >>Buffer sensitivity of photosynthetic carbon utilization in eight tropical seagrasses
2005 (English)In: Marine Biology, ISSN 0025-3162, E-ISSN 1432-1793, Vol. 147, no 5, 1085-1090 p.Article in journal (Refereed) Published
Abstract [en]

Some of the mechanisms involved in inorganic carbon (Ci) acquisition by tropical seagrasses from the western Indian Ocean were described by Björk et al. (Mar Biol 129:363–366, 1997). However, since then, it has been found that an additional, buffer-sensitive, system of Ci utilisation may operate in some temperate seagrasses (Hellblom et al. in Aquat Bot 69:55–62, 2001, Hellblom and Axelsson in Photos Res 77:173–191, 2003); this buffer sensitivity indicates a mechanism in which electrogenic H+ extrusion may form acidic diffusion boundary layers, in which either HCO3–H+ is co-transported into the cells, or where HCO3 is converted to CO2 (as catalysed by carbonic anhydrase) prior to uptake of the latter Ci form. Because a buffer was used in the 1997 study, we found it important to reinvestigate those same eight species, taking into account the direct effect of buffers on this potential mode of Ci acquisition in these plants. In doing so, it was found that all seagrass species investigated except Cymodocea serrulata were sensitive to 50 mM TRIS buffer of the same pH as the natural seawater in which they grew (pH 8.0). Especially sensitive were Halophila ovalis, Halodule wrightii and Cymodocea rotundata, which grow high up in the intertidal zone (only ca. 50–65% of the net photosynthetic activity remained after the buffer additions), followed by the submerged Enhalus acoroides and Syringodium isoetifolium (ca. 75% activity remaining), while Thalassia hemprichii and Thalassodendron ciliatum, which grow in-between the two zones, were less sensitive to buffer additions (ca. 80–85% activity remaining). In addition to buffer sensitivity, all species were also sensitive to acetazolamide (AZ, an inhibitor of extracellular carbonic anhydrase activity) such that ca. 45–80% (but 90% for H. ovalis) of the net photosynthetic activity remained after adding this inhibitor. Raising the pH to 8.8 (in the presence of AZ) drastically reduced net photosynthetic rates (0–14% remaining in all species); it is assumed that this reduction in rates was due to the decreased CO2 concentration at the higher pH. These results indicate that part of the 1997 results for the same species were due to a buffer effect on net photosynthesis. Based on the present results, it is concluded that (1) photosynthetic Ci acquisition in six of the eight investigated species is based on carbonic anhydrase catalysed HCO3 to CO2 conversions within an acidified diffusion boundary layer, (2) C. serrulata appears to support its photosynthesis by extracellular carbonic anhydrase catalysed CO2 formation from HCO3 without the need for acidic zones, (3) H. ovalis features a system in which H+ extrusion may be followed by HCO3–H+ co-transport into the cells, and (4) direct, non-H+-mediated, uptake of HCO3 is improbable for any of the species.

National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-23872 (URN)10.1007/s00227-005-0019-0 (DOI)
Note
Part of urn:nbn:se:su:diva-527Available from: 2005-05-10 Created: 2005-05-10 Last updated: 2010-08-05Bibliographically approved

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