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O rápido aumento das concentrações atmosféricas de CO2, devido à atividade antropogénica, está a alterar o clima da Terra e a transformar a química oceânica, com implicações profundas para os ecossistemas marinhos. Deste modo, para cumprir as metas estabelecidas no Acordo de Paris de manter o aumento da temperatura global abaixo dos 2°C (idealmente 1.5°C), não podemos depender exclusivamente da redução de emissões, sendo necessária a remoção ativa de dióxido de carbono da atmosfera. Entre as estratégias emergentes de remoção de carbono (CDR), o aumento da alcalinidade do oceano, denominado Ocean Alkalinity Enhancement (OAE), tem ganho destaque devido ao seu potencial para aumentar a capacidade de tampão do oceano de forma a reforçar a captura de CO2 a longo prazo. Contudo, a implementação em larga escala exige uma compreensão clara das consequências biogeoquímicas e das respostas ecológicas das comunidades microbianas marinhas, que desempenham um papel central na produtividade oceânica e nos ciclos biogeoquímicos globais.
Esta tese investigou os impactos da OAE em comunidades microbianas marinhas naturais, com particular ênfase nas interações entre fitoplâncton e bactérias e no funcionamento do ciclo marinho do enxofre. Ao longo de uma série de estudos experimentais realizados no Atlântico Norte (Açores) e no Mar do Norte (Helgoland), examinou-se como diferentes formas de adição de alcalinidade, regimes nutricionais e tipos de aplicação influenciam a composição e a biomassa das comunidades microbianas, a atividade enzimática extracelular, os hidratos de carbono e a produção e tranformação de dimetilsulfoniopropionato (DMSP) e dimetilsulfureto (DMS). Os resultados forneceram informações sobre a forma como a OAE pode alterar a dinâmica das redes tróficas microbianas e os processos biogeoquímicos em condições atuais e sob cenários de alteração na disponibilidade de nutrientes. De forma a interpretar plenamente estas respostas ao nível do ecossistema, é essencial caracterizar os grupos taxonómicos envolvidos. Nesse sentido, esta tese incluiu também o isolamento e a descrição taxonómica de Brachybacterium atlanticum, uma nova espécie bacteriana recolhida ao largo da Ilha Terceira.
Em conjunto, estes estudos oferecem uma avaliação abrangente das respostas microbianas à OAE e contribuem com conhecimento ecológico essencial para a avaliação da segurança, viabilidade e limites ambientais das abordagens de CDR baseadas em alcalinidade.
ABSTRACT: The rapid rise in atmospheric carbon dioxide (CO2) driven by human activities is altering Earth’s climate system and reshaping ocean chemistry, with far-reaching implications for marine ecosystems. Thus, to meet the targets established in the Paris Agreement of maintaining the increase of temperature below 2°C (ideally 1.5°C), we cannot only rely on emission reduction but have to actively remove CO2 from the atmosphere. Among emerging CO2 removal (CDR) strategies, Ocean Alkalinity Enhancement (OAE) has gained attention due to its potential to increase the ocean’s buffering capacity and enhance long-term CO2 sequestration. However, large-scale implementation requires a clear understanding of the ecological responses of marine microbial communities, as these responses underpin and ultimately shape the biogeochemical consequences of OAE. This thesis investigated the impacts of OAE on natural marine microbial assemblages, with a particular focus on the interactions between phytoplankton and bacteria and the functioning of the marine sulfur cycle. Across a series of experimental studies conducted in the North Atlantic (Azores) and the North Sea (Helgoland), it was examined how different forms of alkalinity addition, nutrient regimes, and deployment approaches influence microbial community composition, biomass, extracellular enzymatic activity, carbohydrate content and the production and cycling of dimethylsulfoniopropionate (DMSP) and dimethyl sulfide (DMS). Results provided insights into how OAE may alter microbial food-web dynamics and biogeochemical processes under present and nutrient-altered conditions. To fully interpret these ecosystem-level responses, it is essential to characterize the microbial taxa involved. Accordingly, this thesis also included the isolation and taxonomic description of Brachybacterium atlanticum, a novel bacterial species recovered offshore Terceira Island. Together, these studies offer a comprehensive evaluation of microbial responses to OAE and contribute essential ecological knowledge toward assessing the safety, feasibility, and environmental thresholds of alkalinity-based CDR approaches.
ABSTRACT: The rapid rise in atmospheric carbon dioxide (CO2) driven by human activities is altering Earth’s climate system and reshaping ocean chemistry, with far-reaching implications for marine ecosystems. Thus, to meet the targets established in the Paris Agreement of maintaining the increase of temperature below 2°C (ideally 1.5°C), we cannot only rely on emission reduction but have to actively remove CO2 from the atmosphere. Among emerging CO2 removal (CDR) strategies, Ocean Alkalinity Enhancement (OAE) has gained attention due to its potential to increase the ocean’s buffering capacity and enhance long-term CO2 sequestration. However, large-scale implementation requires a clear understanding of the ecological responses of marine microbial communities, as these responses underpin and ultimately shape the biogeochemical consequences of OAE. This thesis investigated the impacts of OAE on natural marine microbial assemblages, with a particular focus on the interactions between phytoplankton and bacteria and the functioning of the marine sulfur cycle. Across a series of experimental studies conducted in the North Atlantic (Azores) and the North Sea (Helgoland), it was examined how different forms of alkalinity addition, nutrient regimes, and deployment approaches influence microbial community composition, biomass, extracellular enzymatic activity, carbohydrate content and the production and cycling of dimethylsulfoniopropionate (DMSP) and dimethyl sulfide (DMS). Results provided insights into how OAE may alter microbial food-web dynamics and biogeochemical processes under present and nutrient-altered conditions. To fully interpret these ecosystem-level responses, it is essential to characterize the microbial taxa involved. Accordingly, this thesis also included the isolation and taxonomic description of Brachybacterium atlanticum, a novel bacterial species recovered offshore Terceira Island. Together, these studies offer a comprehensive evaluation of microbial responses to OAE and contribute essential ecological knowledge toward assessing the safety, feasibility, and environmental thresholds of alkalinity-based CDR approaches.
Descrição
Tese de Doutoramento, Ciências Agrárias, 22 de junho de 2026, Universidade dos Açores.
Palavras-chave
Effects Global Changes
Contexto Educativo
Citação
CASTRO, Inês Manuel de Sousa Martins de. (2025). Effects of global change on phytoplankton-bacteria interactions and exudates. Ponta Delgada: Universidade dos Açores, 2025. 149 p. Tese de Doutoramento em Ciências Agrárias. Disponível em http://hdl.handle.net/10400.3/9030