Until the late 1970s, relationships between organisms in the ocean were conceptualized as a linear food chain. Primary producers (phytoplankton) are a food source for zooplankton, the mid-trophic link with the top of the chain (fish, mammals). In this linear model, the role of bacteria has long been considered minor. However, from the 1970s, many discoveries have forced scientific to reconsider the role of microbes in the marine realm.
As consumer, bacteria convert organic matter into energy through respiration. These bacteria are so numerous and active that their respiration activity is more important than that of the other consumers (fish, mammals, etc.). For information, one liter of seawater contains a billion of bacteria for only a million of phytoplankton and half a million of zooplankton. Thus, it has been estimated than although small, bacteria represent more than 13% of the living mass on Earth while animals only represent 0.01%.
As they are tiny, bacteria could only consume the organic matter dissolved in their environment. That dissolved matter comes from sloppy feeding of zooplankton or their faeces, exudates from algae and hydrolysis of organic particles by the bacteria. By these processes, bacteria would consume 20-60% of the primary production. Since bacteria are a food source for Protozoa (Flagellates, Ciliates), themselves consumed by zooplankton, on the “classical” linear trophic chain model comes an other trophic pathway called “microbial loop” where heterotrophic bacteria play a key role. That trophic pathway is called loop because it expresses the return of dissolved organic matter to higher trophic levels via its incorporation into bacterial biomass.
The microbial loop is of particular importance in increasing the efficiency of the marine food web via the utilization of dissolved organic matter (DOM), which is typically unavailable to most marine organisms. The other main effect of the microbial loop in the water column is that it allows the remineralization of organic compounds into simple nutrients (carbon, nitrogen, phosphorus etc.) required in the photosynthesis process.
Impact on the open ocean and carbon sequestration
In the primary production section, we saw that a bloom could take place when phytoplanktons are stuck in a water mass rich in both nutrient and light. This phenomenon occurs mostly in Spring, when the surface waters are nutrient-rich (resulting from the mixing of water masses during winter) and laminated (preventing the algae to escape the good light and nutrient conditions). Under these conditions, the phytoplankton grows rapidly: it is the spring bloom. Phytoplankton growth is then limited by depletion of nutrients and zooplankton grazing activity. However, even in the absence of nutrients, phytoplankton activity is not zero. Indeed, the dissolved organic matter resulting from the bloom decay is actively recycled in nutrients by the bacteria in the surface layer. The primary production resulting from this recycling is called regenerated, while that resulting from the winter mix nutrients inputs is called new production. During a production period, a large amount of dissolved and particulate matter is produced. Unlike dissolved, the particulate organic matter could sink and escape the “photosynthetic layer”. Along their falls, these particles called marine snow are slowly converted into dissolved matter (zooplankton grazing etc.). Their further strong remineralization explains the nutrient-richness of deep waters and the role of winter mixing on the nutrient surface water enrichment.
One of the mechanisms by which the oceans stock the carbon dioxide (greenhouse gas derived from fossil fuel combustion) is by burying some of the primary production in the sediments. Interestingly, it is commonly admitted that less than 1% of the primary production reach the seafloor and even is buried. It means that more than 99% of that primary production is transferred into the upper trophic layer or remineralized in the water column or in sediment.