marine microbial secondary production (chapter 3 from marine ecology book)

what are marine microbes

  • marine microbes include:

    • bacteria

    • archaea

    • viruses

    • fungi

    • protists (single celled eukaryotes)

  • they form an enormous portion of marine biomass and metabolism

  • figure 3.11 shows that most ocean metabolism is carried out by organisms <100m, meaning microbes dominate global biochemical activity

why are microbes essential?

  • climate regulation: microbes produce and consume greenhouse gases (e.g., CO2, N2O, DMS)

  • oxygen production: cyanobateria + mall algal cells produce a large share of earth’s oxygen

  • nutrient cycling: only microbes can perform many metabolic trasnfromations (e.g., nitrification, nitrogen fixation, sulfate reduction)

microbial metabolism and energy sources

  • microbes use two main energy strategies:

    • phototrophy (ligth as energy)

    • chemotrophy (chemicals as energy)

      • chemoorganotrophs (use inorganic molecules like H2S, NH4+, Fe2+)

key marine microbial groups

  • SAR11 (pelagibacter)

    • most abundant marine bacteria (25-50% of bacterioplankton)

    • found form surface to >4000 m

    • extremely small, specialists in oligotrophic waters

    • use proteorhodopsin, a light driven proton pump

  • vibrio species

    • marine pathogens of humans, fish, and corals

    • thrive in coastal, nutrient-rich waters and during phytoplankton blooms

microbial biomass and size strucutre of the ocean

  • viruses = most abundant

  • bacteria = dominate biomass (15%)

  • phytoplankton = 16%

  • zooplankton = tiny share of abundance but large individuals

  • smaller organisms have more surface area per volume, enabling efficient nutrient uptake

detritus: teh microbial habitat

  • detritus are complex microhabitats where bacteria and protists cluster, graze, and remineralize carbon

  • this leads to the microbial loop concept

what is the microbial loop?

  • the microbial loop is the pathway in which dissolved organic matter (DOM is returned to higher trophic levels via microbes

  • steps:

    • DOM is produced

      • phytoplankton exudation

      • viral lysis

      • grazing sloppy feeding

      • protists egestion

    • bacteria consume DOM

      • bacteria act as perfect swimming stomachs, hydrolysing polymers into monomers

    • protozoa (HNFs, ciliates) graze bacteria

      • this transfers bacterial carbon to the food web

    • zooplankton graze protozoa

      • reconnecting microbial production to classical food chains

  • why does the loop matter?

    • recovers energy that would otherwise be lost as DOM

    • recycles nutrients (N, P, Fe) back into inorganic forms

    • generaltes particulate organic matter (POM for export

    • supports food webs in oligotrophic oceans (low nutrients)

viruses in the microbial loop

  • they infect bacteria and phytoplankton

  • causes viral lysis, releasing DOM

  • increase bacterial production

  • slow flow to higher trophic levels (viral shunt)

  • viruses are central in controlling microbial population abundance and turnover

organic matter architechture

  • microbes live and feed within this three dimensional organic matric, not in uniform water

  • teh figure in the microbial loop lecture slide (slide 13) shows DOM and POM as a continuous spectrum form monomers → colloids → gels → large aggregates

seasonal dynamics of the microbial community

  • winter

    • low bacterial activity due to low phytoplankton production

  • spring:

    • phytoplankton bloom → DOM increases → bacterial production increases

    • heterotrophic nanoflagellates (HNFs) increase in response

  • summer:

    • nutrient depletion → lower phytoplankton growth

    • recycling becomes dominant

  • autumn:

    • second bloom → same coupling repeats

microbial loop and carbon pumps

  • slide 52 from the marine microbes lecture shows THREE major carbon pumps

  • biological pump →

    • particulate eorganic matter (POM) sinks and is remineralised in depth

  • solubility pump →

    • driven by CO2 dissolving more readily in cold water

  • microbial carbon pump (MCP)

    • microbes convert labile DOM → recalcitrant DOM that persists for centuries

  • the microbial loop feeds the MCP by trasnforming DOM at all size classes

why microbes + microbial loop = foundation of ocean ecology

  • together, the two lectures show:

    • microbes dominate biomass, abundance, and metabolic activity

    • they recycle nearly all nutrients in the ocean

    • they form the base for energy trasnfer all the way up to fish

    • viruses regulate microbial communities

    • the microbial loop is essential in oligotrophic regions (most of the ocean)

    • microbes play a major role in carbon sequestration

  • understanding the microbial loop is essential for understanding:

    • primary production

    • decomposition

    • nutrient regeneration

    • climate feedbacks

  • everything else in marine biology builds on this