# Module B Summary: The Organisms — From Prokaryotes to Whales ## Key takeaway This module is a tour of ocean life, organized by cell type, size, and energy strategy. The key dynamic is energy flow: **autotrophs** fix carbon (photosynthesis or chemosynthesis), **heterotrophs** eat that carbon, and decomposers (heterotrophic bacteria) and viruses recycle it back. All life started as simple prokaryotes. Complex eukaryotic cells evolved by **endosymbiosis**: cells engulfed other cells and kept them as organelles. This single idea explains mitochondria, chloroplasts, and most eukaryotic phytoplankton. From there, the module builds up in size: viruses and bacteria, phytoplankton, macroalgae and marine plants, zooplankton, nekton (fish), and marine mammals. At every level, body plan and physiology are solutions to the same problems from Module A: light, nutrients, buoyancy, oxygen, salinity, and pressure. ## Key concepts - **Prokaryotes vs. eukaryotes.** All cells have a membrane, cytoplasm, and ribosomes. **Prokaryotes** (domains Bacteria and Archaea) are small (0.2–2 µm), single-celled, have no nucleus or organelles, have circular DNA, and divide by **binary fission**. **Eukaryotes** (domain Eukarya) are larger (>5 µm), have a nucleus and membrane-bound **organelles** (mitochondria, chloroplasts), and divide by mitosis/meiosis. Archaea have ether-linked lipids and survive extreme environments. Bacteria have peptidoglycan cell walls. - **Cyanobacteria are the base of much of the ocean.** These photosynthetic bacteria include *Prochlorococcus* (the most abundant photosynthetic organism on Earth, dominates nutrient-poor tropical oceans) and *Synechococcus* (nutrient-rich coasts). They use chlorophyll-a plus accessory pigments (phycocyanin, phycoerythrin). Some, like *Trichodesmium*, do **nitrogen fixation**: they convert inert N₂ gas into usable ammonium (NH₄⁺). This is critical in tropical oceans where nitrogen limits growth. - **Three energy strategies.** Know the equations. **Photosynthesis**: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. **Respiration** (all cells, the reverse): glucose + O₂ → CO₂ + H₂O + energy. **Chemosynthesis** (some bacteria only): uses chemical energy, e.g., hydrogen sulfide at hydrothermal vents, and produces sulfuric acid. Note: chemosynthesis at vents still depends on O₂, which originally came from photosynthesis. - **Heterotrophic bacteria drive the microbial loop.** They are decomposers. They eat **DOM/DOC** (dissolved organic matter, <0.5 µm) and turn it into bacterial biomass that larger organisms can eat. They are the main consumers of oxygen in the ocean and the main recyclers of nutrients. The microbial loop is an essential part of every food web. Some dinoflagellates, coral polyps, and salps also eat DOM, but bacteria do almost all of it. - **Endosymbiotic theory.** Proposed by Margulis and Taylor. **Primary endosymbiosis**: an Asgard archaeon engulfed an aerobic bacterium → the **mitochondrion**. Later, a eukaryote engulfed a cyanobacterium → the **chloroplast** (this line gave green algae and land plants). **Secondary endosymbiosis**: a eukaryote engulfed a photosynthetic eukaryote (gave dinoflagellates, diatoms, coccolithophores). **Tertiary endosymbiosis**: this happened more than once (diatoms, brown algae). Evidence: organelles have double membranes, circular DNA, bacterial 70S ribosomes, reproduce by binary fission, and are inherited from the mother. - **Diatoms.** Eukaryotic phytoplankton with a silica cell wall called a **frustule** (two halves: **epitheca** and **hypotheca**). They do 20–25% of global primary production. They dominate in cold, nutrient-rich, coastal water. During asexual division the average cell size shrinks each generation (one daughter keeps the larger epitheca, one gets the smaller hypotheca). At a minimum size they switch to sexual reproduction, which produces an **auxospore** that restores maximum size. Auxospores can also act as resting spores to survive bad conditions. Some diatoms make the toxin **domoic acid**. - **Dinoflagellates.** Eukaryotic, single-celled, with two flagella (one for steering, one for motion). Cells are naked (**athecate**) or armored with cellulose plates (**thecate**). About half photosynthesize; most can also eat organic matter (**mixotrophy**), which is a big advantage when light or nutrients run low. They migrate vertically: up to light by day, down to nutrients below the **nutricline** at night. Sexual reproduction makes a dormant **resting cyst**. Their blooms can be toxic **harmful algal blooms (HABs)** — the correct term, not "red tide" — and toxins **bioaccumulate** up the food web. As **zooxanthellae**, they are the symbionts inside corals. Some are bioluminescent. - **Coccolithophores.** Eukaryotic phytoplankton covered in calcium carbonate plates (**coccoliths**). Their blooms look milky and are visible from space. They often bloom after diatoms use up the silica. Their CaCO₃ plates dissolve as the ocean acidifies, so they are climate-change indicator organisms. Fossil coccoliths built the White Cliffs of Dover. - **Silicoflagellates.** Minor group with an internal silica skeleton. They live in cold, nutrient-rich water and preserve well in sediments, so scientists use them as paleoclimate indicators. - **Viruses.** Not alive: they are obligate intracellular parasites (DNA or RNA in a protein **capsid**; no respiration, no growth). They are host-specific (e.g., **bacteriophages** attack bacteria). Two cycles: **lytic** (hijack the cell, replicate, burst it) and **lysogenic** (viral genes hide in the host genome for generations). Viruses are very abundant (~10⁷ per liter). They control blooms and maintain diversity by **"killing the winner"**, and cell bursting releases DOM into the microbial loop. UV light and marine snow remove viruses. - **Macroalgae (seaweeds).** Multicellular, mostly benthic, and NOT plants: no true roots, stems, or leaves. Parts: **holdfast** (anchor only, no nutrient uptake), **stipe** (stem-like), **blades** (photosynthesis, high SA/V), **pneumatocysts** (gas bladders for buoyancy). Three groups by pigment: **green algae** (chlorophyll a+b, like land plants), **red algae** (phycobilin, absorbs blue light, can grow deepest; source of carrageenan), **brown algae** (fucoxanthin; includes kelp). Kelp are large brown algae of cold, nutrient-rich water: *Nereocystis* (bull kelp, BC), *Macrocystis* (grows up to 60 cm/day, harvested sustainably), *Sargassum* (the only planktonic macroalga). Macroalgae reproduce by **alternation of generations** (diploid sporophyte ↔ haploid gametophyte). - **Marine plants.** True plants with roots, leaves, and flowers. Three groups: **seagrasses** (fully marine; roots absorb nutrients from sediment and stabilize it; among the most productive ecosystems on Earth; strong **blue carbon** storage), salt-marsh grasses, and mangroves (Module D). Few marine species exist, probably because flowering plants co-evolved with insect pollinators. - **Zooplankton.** The heterotrophic plankton. Size classes: micro- (<0.2 mm; protists, link small phytoplankton to the food web), meso- (0.2–2 mm; **copepods** — the most abundant multicellular animals on Earth — and **krill**, the short efficient link from diatoms to baleen whales), macro- (>2 mm; salps, larvaceans, jellies, chaetognaths). **Salps** are fast-growing filter feeders whose large, fast-sinking fecal pellets make them important to the **biological pump** (carbon export to the deep ocean). Residency: **holoplankton** spend their whole life in the plankton; **meroplankton** only their larval stage (this avoids parent-offspring competition). **Diel vertical migration (DVM)**: zooplankton rise to feed at night and hide deep by day to avoid visual predators. **Ontogenetic vertical migration (OVM)**: adults overwinter deep in **diapause**, then release larvae timed to the spring bloom. - **Nekton — fish.** Three classes: **Agnatha** (jawless: hagfish, lampreys; hagfish are deep-sea scavengers), **Chondrichthyes** (cartilaginous: sharks, rays; buoyancy from oily liver, many must keep swimming; oviparous or ovoviviparous), **Osteichthyes** (bony fish; most use a gas-filled **swim bladder** for precise buoyancy). Body shape, fins, and mouth parts reveal the niche. Gills extract oxygen by **countercurrent exchange**: blood and water flow in opposite directions, keeping a diffusion gradient along the whole gill. This extracts 80–90% of oxygen; parallel flow would get only ~50%. Bony fish pump water with the **operculum**; many sharks use **ram ventilation** and must swim to breathe. Fish sense vibration with the **lateral line**. **Countershading** (dark top, light belly) camouflages open-water fish. **Anadromous** fish (salmon) breed in fresh water; **catadromous** fish (eels) breed in the sea; both switch osmoregulation modes. Tuna are special: no functional swim bladder, constant swimming, and partial **endothermy** (warm body in cold water). - **Marine mammals.** Three orders: **Cetacea** (whales: toothed Odontoceti are carnivores; baleen Mysticeti are filter feeders on krill — a very short, efficient food chain), **Carnivora** (pinnipeds: seals, sea lions; plus the sea otter), **Sirenia** (manatees, dugongs; the only herbivorous marine mammals). Adaptations to the sea: streamlined body, limbs into flippers, horizontal tail flukes, blowhole on top of the head, blubber (insulation, buoyancy, energy), concentrated urine and metabolic water (no salt glands), and sound for hunting and communication (sperm whale clicks up to 230 dB; each whale has a unique **coda**; orca pods have dialects).