With the exception of fungi, these organisms are all photosynthetic and are therefore restricted to the well-lit upper ocean and inshore regions. Life in the oceans is almost entirely dependent on these photosynthetic organisms, which are known as primary producers. Different kinds of primary producers occur in different marine habitats. In the open ocean the producer organisms are minute, often single-celled, algae called phytoplankton, while larger attached seaweeds, seagrasses, saltmarsh plants and mangroves usually dominate the shallower waters of coastal and estuarine regions.
Some research focuses on the organisms themselves.
Examples include investigations of the evolutionary relationships and classification of marine algae and plants using techniques such as molecular biology and ultrastructural research, studies of basic physiological processes such as salt tolerance and photosynthesis, and cataloguing the biodiversity of the unique Australian marine flora. The ecological relationships of marine plants are studied to understand fundamental processes such as succession and competition, the formation of population 'outbreaks' or blooms, ecosystem productivity, the transfer of energy to higher levels in food webs, and the role of algae in nutrient cycling. Plants play particular roles in specific environments, for example seagrasses and mangroves help to stabilise shorelines, and calcareous algae are major contributors in the formation of coral reefs. Interactions between organisms such as competition, consumer interactions, symbioses, and the production of chemicals that influence the behaviour of grazers or settling larvae also attract considerable attention. At the global level there is great interest in the influence of phytoplankton on world climate and their use as indicators of climatic change.