Linking physiological, genomic, and ecological functioning in the seagrass, Zostera capensis

Thesis (PhD)--Stellenbosch University, 2024.

ENGLISH ABSTRACT: In South Africa, the most abundant seagrass, Zostera capensis, occurs in predominantly open and sheltered estuaries and carpets intertidal and subtidal zones. Zostera capensis exhibit a wide yet discontinuous distributional range, limited to estuaries in the cool-temperate biogeographic region along the west coast as well as estuaries in the warm-temperate south coast and the sub-tropical east coast of South Africa. Due to its continued population decline, small area of occupancy and its extirpation status in Durban Bay and St Lucia, Z. capensis is now categorised as Endangered. In South Africa, Z. capensis serves as nursery and foraging grounds for organisms of high conservation importance such as the seahorse Hippocampus capensis, pipefish Syngnathus watermeyeri and klipfish Clinus spatulatus, among others. With over 90 years of research, however, Z. capensis remains poorly protected and severely fragmented, with continued exposure to anthropogenically induced pressures, where eutrophication (leading to agal bloom) and effects of heavy exploitation for bait considered the main pressures in systems such as the Knysna Estuarine Bay. Currently, less is known regarding intertidal and subtidal differences and their responses to global climate change. South Africa is also trailing behind in terms of restoration and rehabilitation trials. Considering these scientific gaps and the continued decline in Z. capensis, this PhD took a broad experimental approach to investigate aspects around morpho-physiological variations between intertidal and subtidal plants, their photophysiological and genotypic responses to thermal stress, as well as investigating transplantation as a mechanism for seagrass conservation. To understand the morpho-physiological responses of plants to their local environmental conditions, an in situ study was conducted in a permanently open estuary (Knysna Estuarine Bay) and a temporarily closed estuary (TCE), Klein Brak Estuary. Generally, seagrasses with ecotypes spanning across bathymetric cline exhibit intertidal ecotypes (exposed twice daily), which are exposed to fluctuating irradiance and temperature regime, and subtidal ecotypes (always submerged) which experience relatively narrower temperature changes but affected by light attenuation, especially in eutrophic systems. Therefore, both ecotypes or plants show morphological and photophysiological differences, with intertidal ecotypes expected to show narrower and shorter leaf width and length (to decrease leaf surface area for light absorption), decreased photophysiological responses (Fv/Fm) but increased shoot and leaf density and the subsequent seagrass %cover (to induce self-shading). These response mechanisms are important for seagrass meadows exposed to high temperatures and light irradiance. For subtidal ecotypes in light attenuated areas, meadows usually show increased leaf length and width as a mechanism to capture light needed for photochemistry. In addition, ecotypes show decreased leaf and shoot density to reduce competition for light, this also affect their %cover which is then decreased in these ecotypes. Seagrass responses occur at leaf-scale (Fv/Fm, and all photosynthesis-related changes), shoot-scale (leaf length and width) and meadow-scale (shoot and leaf density and %cover). To investigate these differences in Knysna, leaf-, shoot-, and meadow-scale responses were determined in each tidal zone at the upper, middle, and lower reaches. Results showed that seagrass responses are localised due to topographical attributes of the estuary and that leaf-scale responses, which are modulated in seconds, were not significantly different at upper estuary, with Fv/Fm values in intertidal ecotypes decreased due to higher light intensities measured at the time of sampling. Results also showed that both intertidal and subtidal plants at the middle estuary suffered mechanical damage, with intertidal plants at the lower estuary showing significantly lower %cover due to anthropogenic effects and sandy and mobile sediment. In Klein Brak Estuary, the same methods were used, however, the estuary was experiencing a drought event and closed estuary moth, therefore, there was no tidal influences. However, exposed plants showed higher Fv/Fm than submerged plants. Nonetheless, shoot- and meadow-scale responses were consistent with other studies. However, the seagrass in this estuary is highly variable due to drought and flooding events associated with South African estuaries, especially TCEs. Collectively, these results formed the basis and highlighted the need for spatiotemporal and seasonal monitoring of Z. capensis to identify meadows that are more permanent and can be used as donor sites for restoration trials. This PhD also investigated the use of different planting patterns (straight-line, compact, and star) and core sizes (11, 18, and 25 cm Ø) in transplanting intertidal and subtidal plants in the Knysna and Klein Brak estuaries in South Africa. Planting patterns, ecotypes and size did not influence persistence; however, smaller cores should be used as these have less impact on donor meadows. Although transplants did not persist for more than three months in both estuaries, our study demonstrated that seagrass restoration in South Africa is challenging due to limited suitable habitats (in Knysna) and strong environmental variability (in Klein Brak) in estuarine ecosystems. Results also showed that restoration trials be prioritised in predominantly open estuaries and that monitoring of donor sites is an important aspect to consider in restoration studies. This PhD also investigated the transcriptomic and photophysiological (Fv/Fm) responses of intertidal and subtidal plants to a simulated acute MHW (36 °C), with results showing that plants respond in a similar manner as no differentially expressed genes (DEGs) were detected, though Fv/Fm showed significant differences between plants during the MHW. However, DEGs were observed during the MHW and after the seven-days long recovery phase. Generally, Z. capensis show a downregulation of DEGs than upregulation following a heat stress. During the recovery phase, Fv/Fm was significantly decreased in both plants compared to control condition. This was complemented by the DEGs which showed downregulation of genes associated with Photosystem I and II, the most sensitive yet important machinery in photochemistry. These results showed that an acute 36 °C is above the tolerant threshold for Z. capensis, and that the species will disappear under these conditions.

AFRIKAANSE OPSOMMING: In Suid-Afrika word die mees algemene seegras, Zostera capensis, hoofsaaklik in oop- en beskutte riviermondings aangetref en bedek ook intergety en subgety sones. Zostera capensis vertoon ‘n wye, maar diskontinue verspreiding wat beperk is tot riviermondings in die koel-gematigde biogegeografiese streek van die wes-kus, asook riviermondings in die warm-gematigde suid-kus en subtropiese oos-kus van Suid-Afrika. Weens die voortgesette populasie afname, klein area van bedekking, en uitgewiste status in Durban Baai en St Lucia, word Z. capensis nou as bedreig beskou. In Suid-Afrika handhaaf Z. capensis biodiversiteit en dien as ‘n teelarea en weiveld vir organismes van hoë bewarings status, onder andere; die seeperdjie Hippocampus capensis, pypvis Syngnathus watermeyeri, en klipvis Clinus spatulatus. Ten spyte van oor die 90 jaar se navorsing word Z. capensis steeds swak bewaar en ernstig gefragmenteer met aanhoudende bloodstelling aan antropogenies geïnduseerde drukke. Eutrofikasie (wat lei tot alge bloei) en effekte van swaar uitbuiting van aas word as die hoof drukke oorweeg in sisteme soos die Knysna Riviermonding. Tans is daar min bekend oor die ekotipiese verskille en hulle reaksies tot golbale klimaatsverandering. Suid-Afrika is ook agter met verwysing na proewe vir herstel en rehabilitasie. Met hierdie navorsingsgapings in ag genome, en die voortgesette afname in Z. capensis, het hierdie PhD ‘n wye eksperimentele toenadering gevolg om die aspekte rondom die morfo-fisiologiese variasies tussen ekotipes, en fotofisiologiese- en genotipiese reaksies tot termiese stress te ondersoek, asook om oorplanting te ondersoek as ‘n meganisme vir seegras bewaring. Om die morfo-fisiologiese reaksie van ekotipes tot hulle direkte omgewingsomstandigede te bepaal is ‘n in situ studie uitgevoer in ‘n hoofsaaklik oop riviermonding (Knysna Riviermonding) en ‘n tydelik geslote riviermonding (TGR), Klein Brak Riviermonding. Seegrasse met ekotipes wat span oor batimetriese lyne vertoon oor die algemeen intergety ekotipes (blootgestel twee keer daagliks) wat bloogtestel is aan ‘n fluktuerende bestraling en temperatuur regime, asook subgety ekotipes (altyd onderwater) wat nouer temperatuur veranderinge ervaar, maar geaffekteer word deur lig verswakking, veral in eutrofiese sisteme. Daarvolgens wys beide ekotipes morfologiese en fotofisiologiese verskille met betrekking tot seegras met intergety ekotipes wat by verwagting ‘n nouer en korter blaar wydte en lengte vertoon (om blaaroppervlakarea te verminder vir lig absorpsie), verlaagde fotofisiologiese respons (Fv/Fm) het, maar verhoogde loot en blaar digtheid en daaropvolgende %bedekking (om self-skadu te induseer). Hierdie reaksiemeganismes is belangrik vir seegras weivelde wat blootgestel word aan hoë temperature en lig bestraling. Vir subgety ekotipes in swak beligte areas wys weivelde meestal ‘n toename in blaar lengte en breedte as ‘n meganisme om lig benodig vir fotochemie vas te vang. Daarby, wys hierdie ekotipes ‘n afname in blaar en loot digtheid om kompetisie vir lig te verminder wat ook ‘n afname in %bedekking in die ekotipes tot gevolg het. Seegras reaksies vind plaas op blaar-vlak (Fv/Fm, en alle fotosintese verwante veranderinge), loot-vlak (blaar lengte en breedte), en weiveld-vlak (loot- en blaar digtheid en %bedekking). Om hierdiedie verskille in Knysna te ondersoek is blaar-, loot-, en weiveld-vlak reaksies bepaal in elke gety sone by die boonste, middel en laer areas. Resultate wys dat die seegras reaksies gelokaliseerd is weens topografiese eienskappe van die riviermonding en dat blaar-vlak reaksies, wat in sekondes gemoduleer is, nie beduidend verskillend by die boonste area van die riviermonding was nie, met Fv/Fm waardes in intergety ekotipes verlaag weens hoër ligintensiteite soos gemeet tydens monsterneming. Resultate wys ook dat beide ekotipes by die middel riviermonding area lei onder meganiese skade en die intergety area in die laer riviermonding wys beduidend laer %bedekking as gevolg van antropgeniese effekte en sanderige en mobiele sedimente. In die Klein Brak Riviermonding was dieselfde metodes gebruik, maar die riviermonding het ‘n tydperk van droogte ervaar en geslote riviermonding gehad, daarvolgens was daar geen gety invloede nie. Die droogte blootgestelde ekotipes het egter hoër Fv/Fm as die ekotipes wat onder water was. Nogtans was loot- en weiveld- vlak reaksies vergelykbaar met ander studies. Die seegras in hierdie riviermonding is egter hoogs variërend as gevolge van die droogte en vloed gebeure geassosieer met Suid-Afrikaanse riviermondings, veral TGR’s. Gesamentlik het hierdie resultate die basis gevorm en beklemtoon dit die behoefte vir tydruimetelike en seisoenale monitoring van Z. capensis om weivelde te identifiseer wat meer permanent gebruik kan word as skenker areas vir herstel proewe. Hierdie PhD het ook die gebruik van verskillende plantings patrone (reguitlyn, kompak, en ster) en kern groottes (11, 18, en 25 cm Ø) in oorplanting ekotipes (intergety en subgety) in die Knysna en Klein Brak riviermondings in Suid Afrika ondersoek. Plant patrone, ekotipes en grootte het nie volharding beïnvloed nie, maar kleiner kerne moet gebruik word omdat dit ‘n kleiner impak op skenker weivelde het. Alhoewel oorgeplante seegras nie volhard het vir meer as drie maande in beide riviermondings nie, het ons studie gewys dat seegras herstelwerk in riviermonding ekosisteme in Suid-Afrika uitdagend is weens beperkte geskikte habitatte (in Knysna) en sterk omgewings variasie (in Klein Brak). Resultate wys ook dat herstelwerk proewe geprioritiseer moet word in oorwegend oop riviermondings en dat die monitoring van skenker areas ‘n belangrike aspek is om te oorweeg in herselwerk studies. Hierdie PhD het ook die transkriptomiese en fotofisiologiese (Fv/Fm) reaksies van ekotiepes ondersoek om ‘n akute MHG (36 °C) te simuleer met resultate wat wys dat ekotipes in ‘n soortgelyke manier reageer omdat geen differensieel uitgedrukte gene (DUG) gevind is nie, al het Fv/Fm beduidende verskille tussen ekotipes tydens die MHG gewys. DUG is wel opgemerk tydens die MHG en na die sewe-dag lang herstel fase. Oor die algemeen wys Z. capensis ‘n afregulering van DUG eerder as ‘n opregulering na die voorkoms van hitte stres. Gedurende die herstelfase was Fv/Fm beduidend verlaag in beide ekotipes in vergelyking met die kontrole kondisie. Hierdie was aangevul deur die DUG wat ‘n afregulering van gene geassosieer met Fotosisteem I en II, die mees sensitiewe dog belanrike masjienerie in fotochemie, gewys het. Hierdie resultate wys dat ‘n akute 36 °C bo die tolerantheidsdrumpel vir Z. capensis is en dat die spesie sal verdwyn tydens hierdie omstandighede.

Doctorate